Elements of Soil and Water Engineering

LECTURE NOTE:

ELEMENTS OF SOIL AND WATER ENGINEERING,

SOI 507

By

Dr. Mohammed K. Othman (MNSE, MNIAE, MISTRO, MASABE)

Visiting Associate Professor

Kebbi State University of Science and Technology,

Aliero

February, 2016

COURSE CONTENT

1.     Land Development for Irrigation and Drainage

2.    Basic Hydraulics: Water Conveyance and Control and Water Flow Measurements

3.    Pumps, Other Water Lifting Devices and their Selection criteria,

4.    Irrigation Efficiencies and factors affecting them

5.    Consumptive use of Water

6.    Principle of soil and water Conservation

7.    Reclamation of water – logged Areas

INTRODUCTION: DEVELOPMENT OF LAND- AND WATER-USE FOR IRRIGATION

1.1.    Land- and Water-Use Planning

Increase in agricultural production can be achieved by engineering principle apply to land and water resources. To improve the living conditions of people and agriculture, the use of the natural resources in a sustainable way must be focused. If land and water is used in the same way as without due consideration, the soil will be exhausted and then it will be impossible to produce food, resources for shelter, and other products that are necessary to sustain human health, safety, and welfare. It is, therefore, necessary to look well at how the earth and its resources can be used judiciously. One way to do this is to watch closely how we use the earth. Good agricultural soils are scarce, and so they have to be used in the best and most sustainable way. Land utilization need be determines how intensively a soil can be used, not only agriculturally, but also for other land use such as housing and recreation, and how much gain from that specific land use, economically and socially. A combination of different land uses should be made in a way that provides for all desired products, and whereby it will still possible to use the land in the same or any other way in the future. This can be achieved through careful planning of the use of land and water. First, it is important to explain land- and water-use planning. The phrase can be divided into three parts: land use, water use, and planning.

The planning process has to be recorded on paper, both the steps in the process and the proposed development for an area, the land-use plan. The land-use plan consists of two parts:

• A map of the area for which the development zones and other proposed changes and developments are represented;

• An accompanying text that explains the symbols used on the map.

All activities on the surface of the earth can be called land use. Almost everything can be used by people, socially or economically, and has been given a name. People have chosen to preserve it in a natural state or to try to change it for themselves. As a result, the surface of the earth is assigned a land use.

Water use is a little different from land use. The surface water is sometimes there naturally (oceans, seas, rivers) and is sometimes created by people (dams, tanks). Humans are always a part of the hydrological cycle and therefore have a function or use. A main function of streams is water drainage. Streams discharge the superfluous water from an area and this is stored in the lakes, seas, and oceans .In this way mankind does not have an active role; it happens by nature. What people can do is interfere and make an area drier or wetter (by artificial drainage or irrigation). In this sense, it can be called water use because a choice has to be made about what to do with this water. People depend a great deal on water: They use it for drinking, irrigation, and in industries. Bigger streams, seas, oceans, and lakes are used for transport and fisheries. It is also in limited supply, especially freshwater. Therefore, it is important to think about what do with the water and how do it. In many areas, there is a water shortage, which means that the area cannot function to its best abilities, in both socioeconomic and technical senses. The area cannot produce the products needed for survival of people, such as food, fuel, and shelter. This is the case in deserts and in many semiarid environments. In other areas, there is too much water which also causes problems. For example, roots of plants drown in groundwater, which causes production loses.

These examples show that the shortage or surplus of water is linked to the land use. The land use determines how much water is needed. This is why water also should be a part of land-use planning. The first concern of water-use planning is the state of the groundwater.

In addition to the water quantity, water quality is also important. If water is polluted, it cannot be used for drinking, for example. Also, a natural ground can be damaged by Polluted water. Land-and water-use planning can play important roles in the prevention and solution of these problems. One has to think carefully about proposed land uses, especially the effects on surface water and groundwater and on other land uses. Therefore, land development for irrigation must take into consideration the amount of crops water requirement throughout the season, the amount needed as leaching requirement as well as any special needs such as seedlings development. Similarly, any water excess must be removed out of the system to avoid water logging condition, which may lead to environmental degradation.

WATER CONVEYANCE AND CONTROL

2.0 WATER FLOW MEASUREMENT

2.1   METHODS OF IRRIGATION WATER MEASUREMENTS

Different methods are commonly used for measuring irrigation water in irrigation scheme. They can be grouped into four categories.

1)    Volumetric measurements

2)   Velocity-area methods,

3)   Measuring structures,

4)   Direct reading gauges.

The suitability of each of the above methods depend on the precision, availability of   instruments and the type of needs for the measurements

2.2 Volumetric Measurements

Irrigation water in small irrigation streams can be measured by collecting the flow with a container of known volume directly and stopwatch to record the time taken for the flow to full up the container.  The flow rate is calculated using the equation below:

      

-------------------------------------------------(1)

Where

Q = flow rate (m³/s, L/S, M3/h, L/h)

V_c = volume of container (m³,L)

T = time taken to full up the container

The method is mostly used in hydraulic laboratories for calibrating other measuring structures.  However, the method is also suitable for measuring water flow into reservoir the volume is calculated by direct measurement of the depth and then multiply by the surface area while noting the time the reservoir is filled to the measured depth.  Similarly, the method can be used to determine the discharge rate of pumps and other water lifting devices. However the method is difficult to use in field channel due to difficulty in collecting the flowing water into the container without loses

2.21    Velocity – Area Method

Water flow measurement can be conducted by determining the average flow velocity in the canal and then multiply by the cross sectional area of the flow section at right angle to the direction of flow in the canal. The cross sectional area is determined by direct measurements while the velocity are generally measured with an instrument called current meter or approximated with “floating material” introduce into the flowing water of the canal.

2.22 Current Meter Method

The current meter is an instrument containing a revolving wheel or vane, which is turned by the movement of water.  The principle of operation of current meter is based upon the proportionality between the flow velocity and angular velocity of the vanes.  The manufacturers usually establish a relationship between the stream flow velocity and the revolutions of the current in a form of calibration graph or table.  Such calibration table gives relationship between the number of revolutions and the velocity in m/s. The modern current meters give direct flow velocity in m/s without using calibration table/graph.

 Generally, current meters can be grouped into two types; vertical-axis meters with their rotating axis perpendicular to the direction of flow and the horizontal axis meters with their rotating axis coinciding with the direction of flow (George. 1997).  There are many current meters in existence; the choice depends solely on where it is to be used.  For example current meters meant for measuring river flows are not suitable for use in small canals used for irrigation as they will be insensitive to water flows in the canal due to low flow velocity. However, each current meter should be used according to the guidelines given by the operators manuals supplied by the manufacturers.  Generally a typical schematic diagram of current meter is shown in figure 1.

Figure 1 Cup and Propeller type current meters

Current meter is suspended by a calibrated cable for measurements in deep streams or attached to a calibrated (meter/feets) rod in shallow streams and held as close as possible to the vertical by suspended weight object mounted below it.

Measurements using current meter are generally made at metering bridges or cable ways or any convenient place accessible to the flowing stream. Velocity measurements are made on vertical lives at regular intervals across the streams see Figure 2.

 

Figure 2 showing the current meter positions

It has ben found that if only one reading is to be taken at vertical line.  The velocity at a point 0.6 the water depth is generally chosen as the mean velocity over the complete depth (Bruce and Stanely 1974).  Readings taken at 0.2 and 0.8 (as shown in figure 2) of the water depth are found to be more accurate estimate of the average flow velocity of the upper and lower levels of the canal (Michael 1978). The mean velocity of the velocities taken at 0.2 and 0.8 is the assumed to be the estimate of the water flow velocity at the particular section.

-----------------------------------------(2)

Where Vm = mean velocity of section 1

V_0.2 and V_0.8 are velocities at point 0.2 and 0.8 of section 1 respectively

To determine the flow rate the canal is divided into segment the number of segments depend on the canal width and the precision required, the bigger the number of divisions, the more the precision and the longer the time taken in their measurements.  The flow rate (discharge) is computed with the equation below. ------------------------------------(3)

Where:           q_n= flow rate at n^th layer

            d₁ = depth at section 1

            d₂ = depth at section 2

            b = interval distance between 1 and 2

             V_m = velocity taken at the middle of section 1 and 2

Figure 3 canal cross-sections for area determinations

The flow velocity at the end sections next to the canal bank is usually negligible hence assumed to be zero.  The flow rate of the canal Q_canal is therefore the summation of all segment discharges calculated.

----------------------------------------(5)

Velocity determination of flow rate with current meter is more accurate when the time of any single observation is not less than 5 minutes, a period of 6- 10 minutes are

recommended (Gibrson 1989).

2.23 Surface Float Measurement Methods

A floating material is normally introduced into the water up stream of the channel and allows it to travel with water over a predetermined distance while noting the time taken to cover the distance. The distance cover divide by the time is an estimate of the flow velocity of the canal. The product of the average velocity and the cross-sectional area is the water flow rate in the canal. The floating materials used are  a long-necked bottle partly filled with water, a block of wood stick, coconut or any floating materials that can not easily be deflected by air or surface currents. The floating material should be placed in the center of the canal and the time measurement should be taken several times to obtain the average. The stretch of canal used for measurement should be straight and uniform in order to avoid changes in the velocity and in the area of the cross-section because any such variations reduces the accuracy of the velocity estimation

Figure 4 Canal section showing the floating material from point A to B

Compute the surface velocity V

-------------------------------------------------------------(6)

L = distance between A to B.

t = time taken to distance Ds by

V= Velocity between A and B

The average water flow velocity Va in the canal is less than V because surface water flow is faster than canal flow.  Thus to obtain Va surface float co-efficiency is multiplied with V depending on the depth at the velocity is desired.  Table 1.shows the depths of water and the surface float coefficients.

Table 1: Water depth in the canal and coefficients of velocity (Source Bruce and Stanley

Water depth (cm)	10	15	23	31	38	46	70	114	150+
Coefficient (C)	0.66	0.58	0.10	0.72	0.74	0.76	0.77	0.78	0.81

  In most canals the value of 0.75 is taken as the float coefficient (FAO, 1993)

Therefore:  V = C x Vs

Flow rate: Q = A V = AC Vs------------------------------------------(7)

Where A = Average segmental area where the flow velocity is determined.

In case of the edges of the canal; the area Ae velocity of the edges Vc is given as: (Bruce and Starley, 1974).

            Ve = 0.667 Va

Flow rate in the canal edges

            Q = Vc Ae = 0.667 V Ae = 0.667 C Vs---------------------(8)

The canal flow rate is determined by taking the sum of all the segmental flow rates in the canal section.

2.3   Water Flow Measuring Structures

Flow measuring structures (hydraulic structures) are relatively permanent devices, designed and installed along the canal for water flow measurements.  The most common hydraulic structures used for this purpose are weirs, flumes and orifices. Using these devices the flow rate is measured directly by making a reading on a scale which is part of the instrument and using mathematical equation to compute the discharge or obtained the flow rate from calibrated graph or table of the structures.

2.31  Weirs

Weirs are sharp-crested hydraulic structures, which can easily be constructed and used for measuring the flow rate in the canal with a better accuracy when correctly installed.  However, they must be installed at a small drop where the water level down the stream in the canal is always below the weir crest.  The depth of water level at the up stream above the crest of the structure is measured as H at approximate distance of 4H from the structure (see fig..5)                

The canal flow rate Q is mathematically linked with H, thus, for any value of H given is computed or obtained from calibrated table.

Figure 5 Thin plated Weir used for flow rate measurements

3.33   Type of Weirs

Generally, there are three types of weir; the rectangular weir, the cipolelli trapezoidal weir and V-notch weir.  The flow rate equations linking, the discharge Q and flow depth above the weir crest H are derived by application of the Bernoulli and continuity equations (Othman, 2002, Bruce and Starlye, 1974) and is generally given below (Michael 1978).

            Q = CL H^m---------------------------------------------(9)

Where

Q = flow rate

C = coefficient, depending on the nature of the crest and approach conditions.

            L = length of crest

            H = head on the crest,

m = an exponent depending on the weir opening.

The values of c and m are determined by conducting a calibration fest experimentally. 

3.34     Rectangular Weir

 Rectangular weir has a horizontal crest and vertical sides, it can be contracted rectangular weir or suppressed rectangular weir.  End contraction are produced when crest length is less than the width of the canal/channel while a suppressed weir is when the crest length extends across the full width of the canal/channel and no contractions are produced. The equations for flow rate of rectangular weir using francis as given by Micheal (1978).

Q = 00184 LH ^3/2  suppressed rectangular weir---------(10)

Q = 0.0184 (L-0.2)H ^3/2 Contracted rectangular weir.—(11)

Where Q = discharge in L/s

L = length of crest (cm)

H = head over the weir

Rectangular weir is used for large flow rate in canal.

Fig.6  Contracted Rectangular Weir

3.34    Trapezoidal Weir (Cipolatti)

 It is trapezoidal in shape where each side of the notch has a shape of 1 horizontal to 4 vertical.   It is a modification of rectangular weir designed by an Italian Engineer Cesare Apoletti to correct the problems of side contraction (figure 6). The flow rate equation is below.

Q = 0.0186 LH^3/2 …………………………………….(12)

Trapezoidal weir is for medium flow rate in the canal

Fig. 7 Trapezoidal weir

3.35    V- Notch Weir

 It is weir with a triangular opening well suited for small flows with high accuracy.

Fig. 8 V-notch Weir

 The flow rate equation passing through a v-notch weir is given bellow

Q = 0.0138 H ^5/2--------------------------------(12)

3.36  Flumes

Weir structures require being installed where there is considerable drop in the canal in order to function properly, which is sometimes difficult to find in relatively shallow canals with flat grades.  They are also unsuitable for the water carrying silt, which are deposits in the approach channels and destroys the proper conditions of flow measurements.  Flumes are similar water flow measuring devices like weirs but the advantage of flume over weir is the requirement of only one quarter drop in the canal compared to that of weir.  Flumes are insensitive to sand/silt in the flowing water as the transported materials do not affect its operation or accuracy.  This is why smaller flumes are used as transportable measuring devices. There are three types of flumes commonly used for water flow measurements in the irrigation schemes.  There are the Parshall flume, Cut throat flume and the RBC flume.

3.36    Parshall Flume

It consists of three sections; a converging section at the upstream, a constricted section called the throat in middle and a diverging section at the downstream.  The wall of the throat section is parallel and the flow is inclined down ward while the walls of the downstream section diverge toward the outer and the floor is inclined upward.  The floor of the conveying section is at the upstream is leveled while the wall converge toward the throat.

To measure the flow rate with Parshall flume, two gauge readings H_a and H_b are taken at upstream and downstream sections of the flume.  Under free flow condition it is only H_a that is taken while under submerged condition the reading at the two gauges need to be taken.  The submerged conditions are presented below as described by Micheal (1978)

Width of “throat”         Free flow unit

                         Ratio of H_b/H_a

2.5 -7.5cm                             0.5

15 – 22.5cm                           0.6

30 – 240m                              0.7

Fig. 9: Front elevation and plan of flume

The above limits are relatively wide range of submergence and it is important characteristics  of the flume.  However, beyond this unit of submergence, the encroaching downstream addition to the head at Ha.  Thus it may not give an accurate reading; Ha and Hb are all measured above the floor of the converging section, for smaller size flumes (less than 15 cm), Ha and Hb  can be measured directly from the wall of the flume but for flume size larger than 15cm, Ha and Hb are measured via stilling wells with tapping in the flume wall due to turbulence. 

3.37    Cut Throat Flume

  The cut throat flume is a modification of parshall flume by simplifying the design by completely removing throat. Flat metals can be used to construct cut throat flume of varying sizes between 45 cm to 3m sizes.  Use a slope of 1:6 for diversion section with a length of two-third of flume length while 1:3 for conversion section with the length of one-third of the flume length is recommended.  The relationship between flow rate Q and upstream depth H_a for cutthroat for cutthroat is given below.

Q = C 1H_aⁿ¹…………………………………………(13)

C1 = free flow co-efficient

n1 = Exponent, whose value depends only on the flume length L.

Figure 10; Cut-throat flume

Due to their small sizes, cut-throat flumes are used for on-farm water measurements.  A much smaller size of 100mm (throat width medium), was recently designed and calibrated by Abubakar (1990) and has a stream size of:

Q = 0.0525 H^2.105……………………………….(14)

Where Q = flow rate L/S

H = measured head in the flume (cm)

3.38    Orifices

Orifices are hydraulic structures with opening in circular or rectangular shape installed across the open channels for purpose of flow water measurement.  The edges of orifices are the sharp and constructed with metal.  In irrigation schemes mostly small orifices are used where the cross sectional area of the orifice is small in relation to stream cross section.  This condition allows complete contrast of the stream flow and the velocity of approach becomes negligible.  Orifices can be operated under free flow or submerged flow conditions.  The free flow orifice is the one which the discharge at downstream does not influence the water flows out of the orifice.  This type of orifice is used for measuring small streams like flow in border strips furrows or check basins as the size of the orifice usually range from 2.5 cm to 7.5 cm (Michael 1978).  The flow rate is measured with the equation below:

Q = C_da (2gH)^1/2………………………………………….(14)

Where a = area of the orifice

            H= head from the orifice

             C_d = coefficient of discharge

Submerged orifice is the one in which the flow rate at downstream level has influence over the flow velocity at the orifice.  Sluice gate is one of such orifice in which the depth of the opening is varied according to the required stream flow.  Generally, submerged orifice has flow rate equation given below.

Q = C_d A (2g[H-h]) ^½ ……………………………..(15)

Where A = cross sectional area = BD

B = width

D = depth of opening in case of sluice gate

H  = Depth from the orifice to the free water surface at up stream

h =   “                “                              “                         at down stream

In addition to water measurements sluice gate can be used to control the volume of water passing downstream and also estimate the cost of water on volume basis.

3.38   Direct Reading Gauges

A staff gauge is water-measuring device that link the flow rate with water depth in the canal.  It works on the principle that the higher the head in the canal the bigger the canal stream size.  The device is permanently installed vertically or inclines with graduation units, extending from the elevation of the lowest stage to be measured upwards to the elevation of the highest stage to be measured.  A stage of a stream is the elevation of its water surface with respect to an established datum plane usually canal bed.

The significant feature of a staff gauge is that the stage measurement is done directly in units of length without any intervening influences.  For this reason, a gauge must be installed on a straight reach of the canal where the slope and wetted perimeter are such that at all stages of the stream, the velocity at all parts of the section may be easily measurable.  The banks of the canal should be sufficiently high to prevent over-flow in times of flood and the section should be outside the sphere of influence of bridges piers.  Staff gauges can be made of wood, cast iron or steel but they should have essential characteristics as follows:

1.       They must be accurately graduated with permanent markings

2.      They should be constructed of durable materials particularly with respect to alternating wet and dry conditions, weir and fading of the markings.

3.      They should be constructed of materials with a low coefficient of expansions with respect to temperature and moisture changes.

The vertical staff gauges are mostly used in irrigation scheme due to their simplicity in design construction and installation.  They are designed to be mounted on a vertical plate which is securely one anchored to a foundation extending below the ground surface.  A typical graduated staff gauge is shown in the figure below.

Figure 11: A typical graduated staff gauge

3.39  Calibration of Staff Gauge

Readings on the staff gauge give depth of water level from a reference point, generally the bed of the canal.  The water level depth can only give the flow rate when the section of the canal is properly calibrated where the flow rate is link with the water level depth. The calibration is done by releasing different stream sizes from the take-off  into the canal and then measuring the flow rate using current meter at the section where the gauge is to be installed.  A depth of water level is taken for each release made to pass through the canal section minimum of three releases from the one that will give minimum water level in the canal to the maximum canal capacity.  A calibration graph of flow rate in the Y-axis and depth of water level in the X-axis should be established.  The graph is a straight line graph with origin approximately at zero.  From the graph various depths would give corresponding flow rate in the canal.

4.0 CONSUMPTIVE USE OF CROP

In designation of water use by crops, evaporation and transpiration are combined into one term evapotranspiration (ET), as it is difficult to separate the two losses in cropped fields. The term consumptive use is used to designate the losses due to evapotranspiration and water that is used by the plant for its metabolic activities since the water that is used in the actual metabolic processes is insignificant (less than 1 % of ET), the term consumptive use is generally taken equivalent to ET. It thus, includes all the water consumed by plants plus the water evaporated from bare land and water surfaces in the area occupied by the crop.

 

4.1 Evaporation

Evaporation is the process whereby liquid water is converted to water vapor (vaporization) and removed from the evaporating surface (vapor removal). Water evaporates from a variety of surfaces, such as lakes, rivers, pavements, soils and wet vegetation.

Energy is required to change the state of the molecules of water from liquid to vapor. Direct solar radiation and, to a lesser extent, the ambient temperature of the air provide this energy. The driving force to remove water vapor from the evaporating surface is the difference between the water vapor pressure at the evaporating surface and that of the surrounding atmosphere. As evaporation proceeds, the surrounding air becomes gradually saturated and the process will slow down and might stop if the wet air is not transferred to the atmosphere. The replacement of the saturated air with drier air depends greatly on wind speed. Hence, solar radiation, air temperature, air humidity and wind speed are climatologic parameters to consider when assessing the evaporation process. Where the evaporating surface is the soil surface, the degree of shading of the crop canopy and the amount of water available at the evaporating surface are other factors that affect the evaporation process.

4.2 Transpiration

Transpiration consists of the vaporization of liquid water contained in plant tissues and the vapour removal to the atmosphere. Crops predominately lose their water through stomata. These are small openings on the plant leaf through which gases and water vapour pass. The water, together with some nutrients, is taken up by the roots and transported through the plant. The vaporization occurs within the leaf, namely in the intercellular spaces, and the vapour exchange with the atmosphere is controlled by the stomata aperture. Nearly all water taken up is lost by transpiration and only a tiny fraction is used within the plant.

Transpiration, like direct evaporation, depends on the energy supply, vapor pressure gradient and wind. Hence, radiation, air temperature, air humidity and wind terms should be considered when assessing transpiration. The soil water content and the ability of the soil to conduct water to the roots also determine the transpiration rate, as do water logging and soil water salinity. Different kinds of plants may have different transpiration rates. Not only the type of crop, but also the crop development, environment and management should be considered when assessing transpiration.

 

4.3 Evapotranspiration process

Evapotranspiration (ET) is a combination of two separate processes whereby water is lost on the one hand from the soil surface by evaporation and on the other hand from the crop by transpiration. Evaporation and transpiration occur simultaneously and there is no easy way of distinguishing between the two processes

In Figure 12 the partitioning of evapotranspiration into evaporation and transpiration is plotted in correspondence to leaf area per unit surface of soil below it. At sowing nearly 100% of ET comes from evaporation, while at full crop cover more than 90% of ET comes from transpiration.

Fig. 12. The partitioning of evapotranspiration into evaporation and transpiration over the growing period for an annual field crop (FAO Irr. And Drain. Paper 56)

4.5 Factors affecting evapotranspiration

Weather parameters, crop characteristics, management and environmental aspects are factors affecting evaporation and transpiration. The related ET concepts presented in Figure 2 are discussed in the section on evapotranspiration concepts.

Fig. 2. factors affecting evapotranspiration with reference to related ET concepts (FAO Irr. And Drain. Paper 56).

4.51 Weather parameters

The principal weather parameters affecting evapotranspiration are radiation, air temperature, humidity and wind speed. Several procedures have been developed to assess the evaporation rate from these parameters. The evaporation power of the atmosphere is expressed by the reference crop evapotranspiration (ETo). The reference crop evapotranspiration represents the evapotranspiration from a standardized vegetated surface.

4.52 Crop factors

The crop type, variety and development stage should be considered when assessing tile evapotranspiration from crops grown in large, well-managed fields. Differences in resistance to transpiration, crop height, crop roughness, reflection, ground cover and crop rooting characteristics result in different ET levels in different types of crops under identical environmental conditions. Crop evapotranspiration under standard conditions (ETc) refers to the evaporating demand from crops that are grown in large fields under optimum soil water, excellent management and environmental conditions, and achieve full production under the given climatic conditions.

4.53 Management and environmental conditions

Factors such as soil salinity, poor land fertility, and limited application of fertilizers, the presence of hard or impenetrable soil horizons, the absence of control of diseases and pests and poor soil management may limit the crop development and reduce the evapotranspiration. Other factors to be considered when assessing ET are ground cover, plant density and the soil water content. The effect of soil water content on ET is conditioned primarily by the magnitude of the water deficit and the type of soil. On the other hand, too much water will result in water logging, which might damage the root and limit root water uptake by inhibiting respiration.

When assessing the ET rate, additional consideration should be given to the range of management practices that act on the climatic and crop factors affecting the ET process. Cultivation practices and the type of irrigation method can alter the microclimate, affect the crop characteristics or affect the wetting of the soil and crop surface. A windbreak reduces wind velocities and decreases the ET rate of the field directly beyond the barrier. The effect can be significant especially in windy, warm and dry conditions although evapotranspiration from the trees themselves may offset any reduction in the field. Soil evaporation in a young orchard, where trees are widely spaced, can be reduced by using a well-designed drip or trickle irrigation system. The drippers apply water directly to the soil near trees, thereby leaving the major part of the soil surface dry, and limiting the evaporation losses. The use of mulches, especially when the crop is small, is another way of substantially reducing soil evaporation. Anti-transpirants, such as stomata-closing, film-forming or reflecting material, reduce the water losses from the crop and hence the transpiration rate.

Where field conditions differ from the standard conditions, correction factors are required to adjust ETc. The adjustment reflects the effect on crop evapotranspiration of the environmental and management conditions in the field.

There are several methods to determine the ETo. They are either:

a.    Experimental, using an evaporation pan, or

b.    Theoretical, using measured climatic data, e.g. the Blaney-Criddle method

4.6 Method Using Pan Evaporation Pan

Evaporation pans provide a measurement of the combined effect of temperature, humidity, windspeed and sunshine on the reference crop evapotranspiration ETo (Fig. ).

Figure 2: Pan Evaporation

Many different types of evaporation pans are being used. The best known pans are the Class A evaporation pan (circular pan) and the Sunken Colorado pan (square pan).

The principle of the evaporation pan is the following:

Ø  · The pan is installed in the field

Ø  · The pan is filled with a known quantity of water (the surface area of the pan is known and the water depth is measured)

Ø  · The water is allowed to evaporate during a certain period of time (usually 24 hours). For example, each morning at 7 o'clock a measurement is taken. The rainfall, if any, is measured simultaneously

Ø  · After 24 hours, the remaining quantity of water (i.e. water depth) is measured

Ø  · The amount of evaporation per time unit (the difference between the two measured water depths) is calculated; this is the pan evaporation: E pan (in mm/24 hours)

Ø  · The E pan is multiplied by a pan coefficient, K pan, to obtain the ETo.

Figure 3: Class A Evaporation Pan

Where:

ETo: reference crop evapotranspiration

K pan: pan coefficient

E pan: pan evaporation

If the water depth in the pan drops too much (due to lack of rain), water is added and the water depth is measured before and after the water is added. If the water level rises too much (due to rain) water is taken out of the pan and the water depths before and after is measured. When using the evaporation pan to estimate the ETo, in fact, a comparison is made between the evaporation from the water surface in the pan and the evapotranspiration of the standard grass. The water in the pan and the grass do not react in exactly the same way to the climate. Therefore a special coefficient is used (K pan) to relate one to the other.

The pan coefficient, K pan, depends on:

Ø  · The type of pan used

Ø  · The pan environment: if the pan is placed in a fallow or cropped area

Ø  · The climate: the humidity and wind speed

The values for K pan varies from 0.35 to 0.85 with average of 0.7

The K pan is high if:

Ø  The pan is placed in a fallow area

Ø  The humidity is high (i.e. humid)

Ø  The winds peed is low

 The K pan is low if:

Ø  the pan is placed in a cropped area

Ø  the humidity is low (i.e. dry)

Ø  the wind speed is high

 Details of the pan coefficient are usually provided by the supplier of the pan.

5.0 IRRIGATION EFFICIENCIES AND FACTORS AFFECTING THEM

5.1   Introduction

Worldwide, water is a major limiting factor for irrigated crop production. There is therefore always a need to make maximum use of available water for irrigation. At the level of planning and designing of an irrigation system, a critical decision is determining the value of irrigation efficiency to use in the design that can meet the requirements of soil, agronomic and social factors of the scheme.  It is essential that efficient use of irrigation water is made in any irrigation scheme. It is however difficult to achieve 100 % irrigation efficiency because of various water losses during conveyance and distribution.  Only a fraction of water taken from a source (river, well, dam etc) eventually reaches the root zone of the plants. These water losses occur in form of evaporation from the water surface, deep percolation to soil layers underneath the canals, seepage through the bunds of the canals, overtopping the bunds, bund/canal breaks, runoff in the drain, rat holes in the canal bunds and other incidental losses during application. The loss of irrigation water translates to a reduction in cultivated areas, insufficient water availability, environmental degradation, breeding of disease carrying microorganisms etc and subsequent poor crop performance.

5.2   Efficiencies on Water Allocation and Utilization

The movement of water through an irrigation system, from its source to crop, can be regarded as three separate processes; conveyance, distribution and application. The efficiencies of water use in each of these processes are the major indicators that measure the performance of water use in the scheme. The utilisation of this part is responsible for optimum crop performance and yield provided that other production inputs are made available while disease and pests are prevented.

a)     Conveyance Efficiency

Conveyance efficiency E_c is the efficiency of canal or conduit networks to convey water from the reservoir, river diversion, pumping station to the off-takes of distributary system. Ec measures the amount of water loss by the canal through evaporation and seepage during conveyance. It is defined as ratio in percentage between water received (delivered) at inlet to that released (diverted) at headwork. It is given bellow;

                                                                (1)

Where

V_d = Volume of water (l, m³) delivered to the distributory system

V_c = Volume of water (l, m³) diverted from source or pumped out from river

V₁ = Volume of water (l, m³) inflow into conveyance system from other source

q_d = flow rate (l/s, m³/s) delivered to the distributory system

q_c = flow rate (l/s, m3/s) diverted from source or pumped out from river

q₁ = flow rate (l/s, m3/s) inflow into conveyance system from other source

Recommended standard values

Recommended Ec given by International Committee on Irrigation and Drainage (ICID) is presented in Table1. The losses in conveyance are mostly due to seepage, leakage and evaporation respectively.

The recommended Ec values are given in Table 1

Table: 1 Indicative Values of the Conveyance Efficiency (Ec) for Adequately Maintained canals

	Earthen canals			Lined canals
Soil type	Sand	Loam	Clay
Canal length
Long (> 2000m)	60%	70%	80%	95%
Medium (200-2000m)	70%	75%	85%	95%
Short (< 200m)	80%	85%	90%	95%

Source: FAO Training Manual 4, 1986

In large irrigation schemes more water is lost than in small schemes, due to a longer canal system. Canals made of sandy soils loss more water than from canals made with heavy clay soils. If canals are lined with bricks, plastic or concrete, only very little water is lost. If canals are badly maintained, bund breaks are not repaired properly and rats dig holes, a lot of water could be lost.  Table 1 provides some indicative values of the conveyance efficiency (E_c), considering the length of the canals and the soil type in which the canals are constructed. The level of maintenance is not taken into consideration: bad maintenance may lower the values of Table 1 by as much as 50%.

Improvement of E_c

Periodically, E_c for main canal and secondary canals should be evaluated through inflow-outflow water measurement to determine its value, if it is less than the design or recommended ones then improvement has to be made. E_c can be improved through routine/seasonal maintenance and repairs of canal network. The maintenance works required are desilting of canal, removal of weeds, repairs of cracks, preventing of animals and machineries to cross over the canals. At the beginning of each season, canal managers have to inspect the entire canal system to identify sections that require repairs for immediate action before water is release into the system. The reliable indicator of canal maintenance, which affects E_c is the Canal Maintenance Factor (CMF). This indicator shows the level of maintenance of the canal. Manning-Strickler coefficient (n) is the measure for CMF. At the designed stage, the size and shape of soil grains forming the wetted perimeter of the canal are the only parameters that affect its roughness coefficient but improper maintenance of the canals leads to weeds infestation, which increases the roughness coefficient and thus, reduces the flow rate. The coefficient is given as:

                                                                           (2)                                          

Where:

Q = Rate of flow in the canal

R = Hydraulic radius of the canal

A = Cross sectional Area of the canal

S = Energy gradient of the canal

The method of determining n is to use inflow-outflow technique of water flow measurement within the canal section of interest.

Recommended values of n for irrigation canals are given in Table. 2

Table 2: Values of Manning Coefficients for different condition of irrigation canals

Type and condition of canal	Manning Coeff. n
Earth Channels Straight and uniform Stony bed with weeds on bank Small drainage ditches	0.023 0.035 0.04
Lined channels Concrete Masonry Metal Wooden Vegetated waterways	0.015 0.017 – 0.030 0.011 – 0.015 0.011 – 0.014 0.02 – 0.040

  Source: Michael, 1987

Example of E_c computation

26750 l/min of water is being diverted from dam into unlined canal system. The system consists of 26 furrows with average flow rate of 19 l/min and 70 furrows with average flow rate of 27 l/min. compute E_c

                                   

b)     Water Distribution Efficiency E_d

Irrigation water distribution efficiency E_d is the efficiency of the distribution canals and conduits supplying water from the conveyance network to individual field channels or fields. Ed depends on field canal efficiency E_f, which is defined as the ratio of water received at the field inlet to that diverted from the conveyance system or canal. It can be expressed as:

                                                                             

V_t = Volume of water delivered to the field

V_d = Volume of water diverted into the distributory canal

q_t = flow rate delivered to the field

q_d = flow rate diverted into the distribution canal

The amount was water delivered is the sum of all the waters diverted into the fields or field channels from the distributory canal along its entire length.

                                                                                          

Vi = volume of water diverted from distributory canal into i^th field or channel.

Similarly, as the case for E_c, the flow measurements for determination of E_t are done with flow measuring instrument or structure installed at diversion point  or canal section of interest. E_d indicates the extent to which water is uniformly distributed along the distributory canal. Therefore, Ed is particularly sensitive to the technical and organizational operation procedures (Doorenbos et al, 1992). Thus, Ed can be expressed as:

                                                                             

Equation above is used for measuring the distribution efficiency of a given canal. However, if the distribution of water over a given field along the run is the point of interest, then equation given by Modi (2000) could be applicable.

                                                                               

                                                                    

where d = average depth of water stored along the run, y = average numerical deviation from d and n = number of depths considered.

E_d evaluates the degree to which water is uniformly distributed throughout the root zone during irrigation and hence it is also called uniformity coefficient.

ICID recommended the values for Ed and Et as presented in Table 3.

Table 3: ICID recommended Ed and Et for surface irrigation

Distribution Efficiency (Ed = Ec.Et (%))		Field Canal Efficiency Et (%)
Rotational system with adequate communication	65	Blocks larger than 20 ha unlined	80
Rotational system with sufficient communication	55	Blocks larger than 20 ha lined	90
Rotational system with insufficient communication	40	Blocks up to 20 ha unlined	70
Rotational system with poor communication	30	Blocks up to 20 ha lined	80

Source: FAO Irrigation and Drainage Paper 24 (revised 1992).

Example of E_d computation

24 hrs after  plots of land were irrigated, soil moisture contents were determine at 30 cm in three plots located at upstream, midstream and downstream levels. The average depths of the moisture contents were found to be 70, 60 and 50 mm at the three levels, respectively. Determine E_d

   Solution:

Depths: d1 = 70 mm, d2 = 60 mm and d3 = 50 mm

Average d = (70 + 60 + 50)/3 = 60 mm

Apply equation 7,   

Apply equation 6,

Improvement of Ed

E_d can be improved through correct canal operation; diversion gate should be opened when water reaches required level in the canal, diversion structures should be made function correctly, effective communication among the operators, monitoring and supervision of water diversion by farmers and correct time should be respected.

.

Field Application Efficiency E_a

Field application is the movement of water from field inlet to the crop. Efficiency (E_a ) therefore accounts for the water losses, which occur during application of water to the to the crops on the field. The common losses are the surface runoff at the tail end of the field and deep percolation below the root zone depth.

Definition of Ea

E_a is defined as a ratio in percentage of quantity of water stored in the root zone to the quantity of water supplied to the field. The quantity of water stored in the root zone is the quantity that will be useful to the crops. Ea varies with irrigation method; it is as much as 80 % for sprinkler while in case of surface it may not exceed 60 %.  Ea can be expressed as:

V_m = volume of water stored as soil moisture (m³), V_s = volume of water supplied to the field (m³)

The evaluation of the Ea requires the measurement of water deliveries to each field and measurements of soil water content after each application when the soil moisture must have stabilized (a day after irrigation, the value for evapotraspiration of that day should be taken into consideration).

Ro = volume of runoff, Dp = volume of deep Percolation. (see figure 1)

Figure 1: Schematic diagram showing Deep Percolation (2) and Run off (1)

Low E_a will occur when rate of water applied exceeds the infiltration rate and excess is lost by runoff. When depth of water applied exceeds the storage capacity of the root zone, excess is lost by deep leaching or deep drainage.

Recommended Values of E_a

Ea may vary during the growing season with highest efficiency during peak water use period. The recommended values of Ea given by USDA and ICID is presented in a Table 4

Table 4: Recommended Values of Ea

Conditions	Ea (%)
Light soils	55
Medium soils	70
Heavy soils	60
Graded border	60 – 75, 53 (ICID)*
Basin & level border	60 – 80, 58 (ICID)
Contour ditch	50 – 55
Furrow	55 – 70, 57 (ICID)
Corrugation	50 - 70

Source: FAO Irrigation and Drainage paper 24, 1992

* the second values were recommended by ICID

Measurement of E_a

Ea is measured after the water has reached the field supply channel. To calculate Ea for an individual field, data have to be collected on the quantity of water delivered to the field, on the effective rainfall and consumptive use of the crop grown. These data may be summarised over a period relevant for the purpose. Suitable periods are; the irrigation interval, one month or the growing season of the crop. A table should be developed showing the period over which the data are collected. Beside the more detailed study of the application efficiency for individual fields, a more general approach can be done by studying this efficiency on monthly basis, for a large area. 

The simple approach of measuring Ea is the use the concept of Readily Available Water (RAW). It is soil moisture between field capacity and permanent wilting point, which plant can use without adverse affect. RAW depends on the crop type and its growth stages.

where:

MAD = Maximum Allowable Deficiency

Rz = Root zone depth

Fc =  Soil moisture at field capacity

Pwp =  Soil moisture at permanent wilting point

MAD is a fraction of water for which irrigation has to be done before the moisture reaches permanent wilting point. MAD for most crops is about 0.65 (James, 1993). For a good design, moisture stored after irrigation is equivalent to RAW. Appendix 1 shows how to determine soil moisture for use in evaluating Ea.

Example of Ea Computation

0.6 ha of irrigated field was used for corn cultivation and the field has 26 furrows supplying water at rate of 19 l/min/furrow. If the RAW for corn was determined to be 8 cm after the field was irrigated for 24 hrs. Determine the water application efficiency

Solution

Moisture stored = RAW = 8X0.6X10,000 = 480 m3

Moisture supplied =  =67.5 %

Improvement of E_a

After evaluation, determine the factors (runoff or deep percolation or both) responsible for more water loss than necessary. If it is runoff, which can be noticed through observation of excess water at the tail end of the field, then improve the land levelling or change the basin or boarder size to appropriate size in relation to soil type as given by design guidelines. If it the problem of deep percolation, then reduce irrigation duration or stream size and if the two factors are both responsible for low Ea then the solution required reviewing the values used for the design.

Irrigation Efficiency

Irrigation efficiency is the overall effect of all the three efficiencies; Ec, Ed and Ea and hence the Irrigation efficiency is computed as a product of the three efficiencies.

Irrigation efficiency = EcXEdXEa

PUMPS & PUMPING

a. Considerations in Planning a Pumping System

1.     Source of water

2.    Pumping rate

3.    Total dynamic suction head

- vertical suction lift

- length and size of suction pipe

- number and kind of bends

- foot valve and strainer

4.    Total discharge head

- vertical lift

- size and length of pipe

- number and kinds of bends

- head required at delivery point (sprinkler system)

Total head = total static head + pressure head + friction head + velocity head

Total static head = vertical distance that the pump must lift the water

Pressure head = pressure required at sprinkler.

Friction head, H_f = energy loss due to friction in pipe lengths, fittings, bends, valves
Velocity head = small amount of energy given to the water to get it in motion; usually negligible

Maximum Practical Suction Lift =

where:

Ht = atmospheric pressure at water surface correct Ht for altitude by reducing it by 1.2 m per 1000 m elevation

es = saturated vapor pressure of the water

NPSH = net positive suction head = head required to move water into impeller

Fs = safety factor equal to 0.6 when all other units in meters

b. Types of Pumps

The most common pumps use rotating impellers to transfer energy to the fluid. Impeller pumps are classified according to the direction of flow through the pump with respect to the axis of the pump:

Radial flow: axial flow: mixed flow

Common name: centrifugal - propeller - turbine
suction head: medium - low - low-medium
total head: high, > 100ft - low, < 10ft - 20-80 ft
capacity: low - high - medium
specific speed: low - high - medium

Specific Speed = an index of pump type

where ns = specific speed

N = impeller speed (rpm)

q = pump discharge (m3/s, cfs)

h = total head (m, ft)

C = 51.5 if q in m3/s and h in m; 1.63 if q in l/s and h in m; 1.0 if q in gpm and h in ft

c. Performance Characteristics

When selecting a pump, must determine the relationship between head and pump capacity at different speeds. These relationships are shown on a characteristics curve, obtainable from the pump manufacturer. A pump should be selected that has a high efficiency for a wide range of discharges. Each pump has its own characteristics curve.

Centrifugal Pump:

Propeller Pump:

d) Pump Efficiency The power imparted to water as it moves through a pump is calculated from:

where: WHP = water horsepower

q = discharge (gallons per minute, gpm)

TDH = total dynamic head (ft)

Power requirements for pumping:

where: kW = (input) power delivered to pump

bhp = brake horsepower required

q = discharge rate in m³/s

h = total head (m)

Ep = pump efficiency (decimal)

Pump Laws: discharge is directly proportional to impeller speed: head varies as the square of the speed: power varies as the cube of the speed:

Ex. If speed increases by 50%, what will be the relative increase in discharge, head, and power?

discharge, q, will increase by 50%

head, H, will increase by (1.5/1)² = 2.25 times = 225%

power, bhp, will increase by (1.5/1)³ = 3.38 times = 338%

LAND RECLAMATION

Sources / Causes of soluble salts in Soils

Salinization is a major problem associated mostly with arid lands. It is estimated that 10 to 50 per cent of all irrigated land in the world is adversely affected by salinity. Many processes, both natural and man-induced, result in salinization. Salinization requires a source of salt, a process which concentrates the salt and impaired drainage. Sources of salts include the natural weathering of minerals and redeposition of the salts at another location, the importation of salts in water used for irrigation, the deposition of rainfall with low concentrations of marine salts, and the transportation of salts in airborne dusts from dry salt beds. Natural weathering of geological salt deposits is a major source of salts in arid regions. The weathering of non-saline minerals can also be important when combined with processes which concentrate the salts. The main origin of salts in the soil is from the weathering of parent material of soil or rock which include hydration, oxidation, carbonation etc. however, hydrological conditions and poor managements of irrigation schemes contribute substantially to the development of soil salinity and alkalinity. (Excess exchangeable salt and excess exchangeable sodium)

These are

1                      Use of saline water in irrigation

2                     Deposition of salts on soil surface from high water table

3                     Arid region (high evaporation)

4                     Poor drainage (non leaching)

5                     Water back flow or intrusion of sea water  in coastal areas

All the above factors either singly or in association with each other are responsible to salt accumulations in soils

Definition of Salinity

The salinity of irrigation water is the sum of all the ionized dissolved salts in the water without reference to the specific ions present. The electrical conductivity EC of irrigation water is used for characterisation of salinity, since the ability of water to conduct electricity is directly related to the number of ions present. EC has unit of decisiemens per meter (dS/m) or milimhos per centimetre (mmhos/cm) but dS/m is the SI unit and 1dS/m = 1 mmhos/cm. The term salinity used herein refers to the total dissolved concentration of major inorganic ions (i.e. Na, Ca, Mg, K, HCO3, SO4 and Cl) in irrigation, drainage and groundwater. For reasons of analytical convenience, a practical index of salinity is electrical conductivity (EC), expressed in units of deciSiemen per metre (dS/m). An approximate relation (because it also depends upon specific ionic composition) between EC and total salt concentration is 1 dS/m =  700 mg/l. Electrical conductivity values are always expressed at a standard temperature of 25 °C to enable comparison of readings taken under varying climatic conditions. With all its obvious shortcomings, this custom of using EC as an index of salinity emphasizes the concept that, as a good first approximation, plants respond primarily to total concentration of salts rather than to the concentrations or proportions of individual salt constituents. A similar usage of EC for expressing soil salinity has evolved, where the parameter of primary interest is the total salt concentration, or EC, of the soil solution. However, the content of water in the soil is not constant over time nor is the composition of the soil solution. For this reason, soil salinity is not an easily defined, single-valued parameter. In an attempt to standardize measurements and to establish a reasonable reference for comparison purposes, "soil salinity", is commonly expressed in terms of the electrical conductivity of an extract of a saturated paste (ECe; in dS/m) made using a sample of the soil.

In addition to total salt concentration, sodium and pH can adversely affect soil properties for irrigation and cropping. At high levels of sodium relative to divalent cations in the soil solution, clay minerals in soils tend to swell and disperse, especially under conditions of low total salt concentration and high pH. The permeability of the soil is reduced and the surface becomes more crusted and compacted under such conditions of swelling or from clay dispersion, Thus, the ability of the soil to transmit water can be severely reduced by excessive sodicity.  Since high total salt concentration tends to increase a soil's stability with respect to aggregation and permeability, distinction is made between saline soils and sodic soils. With respect to sodicity, it is the proportion of adsorbed exchangeable sodium relative to the cation exchange capacity (often expressed as the exchangeable sodium percentage, ESP), rather than the absolute amount of exchangeable sodium, that is relevant along with the total salt concentration of the infiltrating and percolating water and the soil pH. The Sodium Adsorption Ratio (SAR) of the saturation extract. SAR is commonly used as a substitute for ESP and as an index of the sodium hazard of soils and waters

Classification of Salt-affected Soil

Soils	ECe dS/m	SAR	ESP	Comments
Normal Soil	< 4	< 13	<15	Salinity affect on yield negligible
Saline Soil	> 4	< 13	<15	PH is below 8.5
Sodic soil (Alkaline)	< 4	> 13	>15	PH>10, toxic effect on crops
Saline-sodic	> 4	> 13	>15	CEC >15% occupies by Na, PH<8.5

Effect of salinity and alkalinity on soils

The high concentration of sodium in soils and irrigation water causes deterioration of soil structure, resulting in decrease of water infiltration and hydraulic conductivity. Permeability becomes a problem when the rate of soil water infiltration is reduced to the point that the crop is adequately supplied with water but the water is not available to crops.

In saline soils the main problem is high soluble salt concentration which reduces water availability to crops. The presence of salt in soil water increases the energy needed to remove water from the soil

A  typical relationship between crop yield and salinity level

Water content and energy for extracting water as the concentration of salt increases

Reclamation Procedures

Reclamation of a soil on temporary basis can be done through;

·                                 Removal of salt crust from the surface of the soil

·                                 Ploughing salt-surface crust deep into the soil

·                                 Neutralizing the effects of certain salts by adding other salts

However, permanent reclamation can be done through;

·                                 Lowering of the water table

·                                 Improving the soil infiltration rate

·                                 Leaching of salts in saline soils and providing adequate subsurface soil drainage

·                                 Replacing excessive exchangeable Na by Ca salts and removing the replaced products

·                                 Suitable management practices

Reclamation of saline and sodic soils

There are three general methods in which saline and alkaline lands may be handled in order to avoid injurious effects to plants. The first is eradication, the second is a conversion of the salts to less injurious forms and the third is the control of the effects of the salts in the soils. The third has been already been discussed. The first is the leaching out of the soil with water with effective subsurface drainage system of the land. The first is only possible for the saline soil while the second is possible for the alkaline soil as discussed below.

Reclamation of saline Soil

To reclaim a saline soil, it is necessary to reduce the soluble salt concentration to acceptable limits and this can be done by leaching provided there is adequate drainage either natural or artificial. The simplest method of procedure to reclaim is to flood the field with water after making ridges or levees at the boundaries. The movement of water through the soil profile will carry the dissolve salts into the lower layers below the root zone. These salts are drained away if adequate subsurface drainage is provided. The quantity of water needed for leaching depends on the degree of the salinity problem and the level of reclamation desired. The effectiveness of leaching varies from soil to soil and depends largely on the quantity of water passing through the soil  The rate of salt removal is fast at the beginning but the process becomes slow after some times.

The quantity of water required for leaching a particular soil can be determined using leaching requirement method

Reclamation of Alkali Soils

The reclamation of the alkali soils requires the replacement (conversion) of excessive exchangeable sodium by calcium; it is necessary that the replaced sodium ions are leached down to lower layers. Excessive exchangeable sodium can be replaced by salts of calcium or magnesium. Example, by using soluble salts of magnesium and that of calcium, the salt of sodium can be converted to leach able salts that can be washed down. The use of gypsum on sodic soils which change the caustic alkali carbonates into sulphates. Several tons of gypsum per hectare are required for the chemical reactions to take place. The soil must be kept moist to facilitate the reaction, and the gypsum should be cultivated into the surface, not plowed under. The treatment is supplemented later by through leaching of the soil with water to free it of its sodium sulfate. The gypsum reacts with both sodium carbonate and the adsorbed sodium as follows:

 

Sulphur is similarly used with advantage. When sulphur oxidizes, it yields sulphuric acid, which not only changes the sodium carbonate to less harmful sulphate but also tends to reduce the intense alkalinity. The reaction of the sulphuric acid is given below:

   

Not only is the sodium carbonate changed to sodium sulphate, a mild neutral salt, but the carbonate radical is entirely eliminated. When gypsum is used, however, the carbonate remains as a calcium salt The quantity of gypsum required depends on the quantity of exchangeable sodium present on the clay surface. However, it is estimated that 4.1 tones of gypsum and 0.78 tones of sulphur per hectare for 30 cm depth for each milliequivalent of sodium to be replaced by sodium is required

Reclamation of saline-alkaline soils

In certain cases, it is possible to reclaim these soils by straight leaching without any structural deterioration whether this possible can be assessed by doing leaching test. If the average soil infiltration rat over 7 to 14 day period during the test is similar to minimum infiltration rate during the first few hours, then this is good indication that little damage result from straight leaching. Otherwise, it is necessary to add chemical amendments to replace to exchangeable sodium first. The combination of leaching and conversion are used for the treatment of this type of soils. It is essential to leach out the excessive salts up to an EC value of 6-8 mmhos/cm before any attempt is made to remove excessive exchangeable sodium by gypsum application. However, some excess salts can be leached by flooding the field after gypsum application, for removing the released sodium from the soil exchange phase. It is essential to note that maintenance of an adequate drainage is essential for successful reclamation of saline, sodic and saline-alkaline soils, as leaching out of the salts or removal of the reaction products is necessary for the success of reclamation. In case the subsoil has lime or clay hard pan it should be broken by deep ploughing or sub-soiling.

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