Suitability for irrigation

1. SELECTION OF LAND FOR IRRIGATION

INTRODUCTION

The selection of land for irrigation is of primary importance. It frequently transpires that there may be no eminently suitable land within a reasonable distance of the source of water and that land of a lower order of suitability has to be considered. As a broad general principle the lower the degree of suitability of the land available, the lower the costs of laying on water. This is in order to compensate for the expense involved in overcoming or ameliorating the adverse features, which may range from factors such as inherently low fertility, low moisture retention properties to severe intractability and the need to improve the natural drainage of soils. This principle is applicable to both summer supplementary irrigation and full‐scale irrigation on a‐year‐round basis.

• SOIL REQUIREMENTS FOR IRRIGATION

The main soil requirements for suitability for irrigation are adequate available water capacity, good to moderately‐good permeability without any marked changes in permeability in the upper four feet and good inherent fertility characteristics.

AVAILABLE WATER CAPACITY

In order to avoid having to apply water too frequently the soil selected should be able to retain an adequate amount of water that will be available to the crop. The generally accepted minimum required is about 7.5%. Factors that govern the amount of available water include:

• The effective depth of soil. Obviously, the greater the effective depth up to the limits of normal crop root penetration, the greater the available water capacity.

• The texture of the soil. In general, the heavier textured or more clayey soils retain more available water per unit volume of soil than the lighter‐ textured sandy soils. There is, however, another important difference. In soils with appreciable clay content, the water is held over a wide range of tensions whereas in the sands the greater part of water is held at low tensions. Thus, in the case of the former, soil moisture tension rises gradually as soil water is removed by growing plants, whereas in the latter there is a relatively sudden rise in moisture tension when most of the available water has been removed.

• The nature of the clay fraction. The soils of the low‐rainfall areas have clay fractions that may be termed active and, per unit amount of clay present, such soils have appreciably higher moisture retention characteristics and therefore higher available water capacities than the soils of the high rainfall areas where the soils are characterised by clay fractions that are relatively inert. Typical examples are given in Table 1 on the following page.

Table 1: Examples of water capacities of some Zimbabwean soils

Place	Mean Annual Rainfall mm	Parent Material	Texture	Available Moisture 1 metre
Sabi Valley	450	Granitic Alluvium	Sandy loam	11.4%
Triangle	620	Gneiss	Sandy clay loam	12.2%
Harare	840	Gabbro	Clay	11.1%
Marondera	910	Granite	Sandy loam	7.8%

PERMEABILITY

The degree of permeability of the soil has to be considered in relation to other factors such as available water capacity of the soil (and any variations in the available water capacities of individual horizons) and the slope of the land. In general, the sandy soils are excessively permeable and have low available water capacities. The frequent irrigations necessary on these soils lead to marked leaching of the more soluble nutrients and wastage of water. In most instances, therefore, it is necessary to apply the water by overhead spray. At the other end of the scale it is necessary that the soil has a degree of permeability that enables a sufficient amount of water to be taken up by the soil within a reasonable period of time. Soils that are very poorly permeable give rise to difficulties in the layout of the irrigation system.

Finally, with regard to the question of soil permeability, the soil selected should not have any marked sudden changes in permeability in the upper metre. Sudden reductions in soil permeability particularly if the overlying soil has relatively low water‐retaining properties, will give rise to problems of temporary water‐logging following applications of water. This may lead to lodging of the crop grown and in many instances deterioration of soil structure and aeration in the water‐logged zone.

INHERENT FERTILITY

Irrigation farming usually demands a high level of productivity and this can be maintained far more easily on a soil that is inherently fertile than on a sandy relatively infertile one. The inherent fertility of soils is dependent on factors such as base saturation, reserves of weatherable minerals and the exchange capacity of the soil (i.e. its ability to retain bases and certain of the plant nutrients) which, in turn, is dependent not only on the amount of clay present but also on the nature of the clay fraction. The active clay fractions that are characteristic of the soils formed under conditions of low rainfall have much higher exchange capacities per unit of clay present than the relatively more inert clay fractions of the soils of the higher rainfall area and at the same time base saturations and reserves of weatherable minerals are also higher.

• RECOGNITION OF SUITABLE LAND

The recognition of suitable areas for irrigation will involve considerations relating to topography, the parent material from which the soil is derived (on a very broad basis) and the nature of the natural vegetation of the area.

In considering this aspect it is probably of equal importance to recognise certain adverse features, particularly insofar as vegetation is concerned, as it is to recognise favourable ones. The final selection must depend on an examination of the soil itself.

• RANGE OF AVAILABLE MOISTURE IN SOILS

Table 1 on the previous page shows how the Available Moisture in soils that have a similar texture can vary from low rainfall areas to high rainfall areas. Table 2 below shows the amounts of water in soils of different textures at Field Capacity and at the Permanent Wilting Point. The difference between these two figures is the Available Moisture for that particular soil. Remember that the Field Capacity of a soil is the same as the Water Holding Capacity.

Table 2: Amounts of water present in the different soil texture classes

TEXTURAL CLASS	FIELD CAPACITY	WILTING POINT	AVAILABLE MOISTURE
	Range	Range	Range	Range
Course sand	8 – 14%	3 – 7%	5 – 7%	6%
Fine sand Coarse loamy Sand	10 – 16%	4 – 6%	6 – 10%	8%
Fine loamy sand Coarse sandy loam	15 – 23%	5 – 10%	10 – 14%	12%
Sandy clay loam Clay loam	21 – 27%	9 – 12%	12 – 15%	13.5%
Silty clay loam Clay loam	28 – 33%	12 – 14%	15 – 21%	18%
Sandy clay Clay	35 – 41%	22 – 23%	14 – 17%	15%
Heavy Black Clay	38 – 74%	32 – 51%	17 – 23%	20%

The figures in this table are given as percentages e.g. the percentage of available moisture which a given volume, or depth, of that soil can hold. In irrigation, Water Holding Capacities and Available Moisture of soils are usually expressed in millimetres per millimetre depth of soil.

Example

A sandy clay soil has a Water Holding Capacity (Field Capacity) of 35% and Available Moisture of 14%.

• Water Holding Capacity
  • is 0.35mm per mm depth of soil,
  • or 35mm per 100mm depth of soil,
  • or 35%.

Of this water held in the soil, only part is available to the plants, so that the;

• Available Moisture
  • is 0.14 mm per mm depth of soil,
  • or 14 mm per 100mm depth of soil,
  • or 14%

(In the past, this value would have been given as about 1.7 inches of available moisture per foot depth of soil)

• WATER FOR IRRIGATION

The objective of irrigation is to transport water from where it is readily available and apply it to growing crops during summer periods of drought or winter when there is no rainfall. The main sources of water used for irrigation are:

• Rivers
  • Dams and Weirs
  • Underground Supplies
  • Sewage Effluent
  • Recycled Water
  • Harvested Rainwater

Rivers: These are one of the major sources of irrigation water in Southern and Central Africa, but only the larger rivers that have running water throughout the year can be used for direct irrigation. Rivers that dry up during the winter are of no use. There are two methods which are used to harvest water from rivers. In the first method water is pumped directly from the river into the irrigation system. In the second the water is pumped from the river into a storage dam which then feeds into the irrigation system. If the storage dam is situated on high ground no further pumping is needed as the water flows from the dam into the irrigation system by the force of gravity. This is shown in Figure 1 below.

Figure 1: Placement of storage water and pumping

Dams and Weirs: Many irrigation systems are fed from rivers which have been dammed or had  weirs constructed across the water course. These constructions form reservoirs of water which can be used up during the dry season. A dam completely stops the flow of a river or stream and the water forms a large lake. Farm dams have a spillway at one side to allow excess water to run back into the old river bed, while large dams have sluice gates which can be opened to allow water to pass through.

Figure 2: Shows the dam wall with spill ways and the path of overflow water

A weir is normally built straight across a river bed and reduces the flow of the river but does not stop it completely. Enough water collects behind the weir to form a reservoir from which irrigation water can be pumped.

Underground Supplies: The typical source of underground water supply is the natural water table which rises and falls according to the season of the year. This water is tapped by sinking boreholes down into the ground and the depth of the borehole must be sufficient to reach the lowest level of the water table at the end of winter. A number of boreholes are required for any large  irrigation scheme, each borehole having its own pump. The water is pumped either directly to the irrigation pipes or into a storage

dam. One system commonly used is to pump from the boreholes into a dam during the night and the water in the dam is used to irrigate the lands during the day.

Figure 3: Pumping water from a borehole to a storage dam

Other important sources of underground water are called aquifers. These are underground lakes or streams formed by water trapped in certain types of rock formations. Any rock formation which is capable of yielding considerable supplies of water is called an aquifer. Aquifers are tapped by sinking boreholes and pumping out the water.

In the case of a large underground lake the boreholes can be sunk anywhere in the area, but in the case of an underground stream, or secondary aquifer, the boreholes have to be placed very carefully in order to tap the narrow band of water. Aquifers occur in many areas and form a very reliable source of water.

Sewage Effluent: This is liquid which has been collected from the main drainage system of a town or city and been treated in special sewage purification plants. This liquid is disposed of by the municipal authority and, because it has a very high content of plant nutrients, cannot be allowed to run into any river or lake. The plant nutrients in the sewage effluent cause a rapid growth in aquatic plants and in particular algae. The plants and algae screen the water from sunlight and as they die off and decay, oxygen is removed from the water for the decaying process. After a comparatively short time the water reaches a stage of complete de‐oxygenation and stagnation resulting in the death by suffocation of all fish, plants and insects in the water. In order to avoid this problem of pollution, municipalities either sell the sewage effluent to farmers for irrigation or use it on their own irrigation farms. The effluent can be used for irrigated crops or pastures which can be grazed by beef or dairy cattle. The main advantage of the effluent is the high content of plant nutrients which reduces the cost of fertilizers for crops or pastures. The disadvantages of effluent are:

• The effluent has to be removed from the purification plant every day of the year. In winter this is no problem but during the rains the liquid may have to be stored for a few days during heavy rain, otherwise the land will become water‐logged. This means that storage tanks have to be built on the farm.

• Certain bacteria which are harmful to humans can be present in the effluent. Due to this, certain salad vegetables and fruits that are eaten raw cannot be grown on sewage farms. Any crop that is cooked before being eaten and grass for pastures grazed by animals can be grown and sold safely. Flood irrigation is regarded as being safer than overhead irrigation by sprinklers, due to the disease factor.

As a general rule, each 86m³ of daily outflow from the sewage works will irrigate 1 hectare of land. (8.6mm per day per ha)

Recycled Water: This means water which has passed through the soil and been removed by means of a drainage system. During the wet season this water can be stored and re‐used for irrigation during the dry months. The water would be stored in a farm dam which could cause a problem with the build‐up of salinity.

Harvested Rainwater: This is essentially a small scale operation and would be sufficient for minor irrigation e.g. hand watering. The system is to collect and store the rainfall runoff from small impervious areas such as the roofs of buildings and large rocks. Observations have shown that 80% ‐ 85% of all rain can be collected from such surfaces. In an area with an annual rainfall of 635mm, the runoff will be 500mm a year. As 1 cubic metre is equivalent to 1 000 litres, 1mm of rain falling on an area of 1 square metre will yield 1 litre of runoff water and 500mm will yield 500 litres from the same area in a year.

Water that has been harvested from roofs or rocks should be stored in tanks that are fully enclosed to prevent losses by evaporation and contamination by mosquitoes and other insects and the growth of algae. These tanks are usually partly or completely buried in the ground.

Figure 4: Harvesting rainwater into a tank

Source: Water Research Commission

Figure 5: An underground storage tank