- TERMS USED IN IRRIGATION
In the last lecture you were introduced to several new terms concerned with water in the soil. In order to refresh your memory a summary of these terms is given below with some new terms added to the list. All these are important and you should try to remember them.
Gravitational Water: This is the maximum amount of water than can be held in the soil and happens after very heavy rain or flooding. The soil is said to be saturated and can hold no more water.
Capillary Water: This is the water held in the small pore spaces and the spaces inside the soil crumbs. It is the water available to plants for growth and moves upwards, downwards and sideways in the soil by the process of capillary attraction.
Hygroscopic Water: This is water absorbed by the soil from the air both above the soil and inside the pore spaces. Air always contains some moisture and this is transferred to the soil.
Field Capacity: This is the amount of water left in the soil after the gravitational water has drained out. The soil is full of water but it is not saturated. When land is irrigated, the object is to bring the soil back up to field capacity.
Permanent Wilting Point: This is the stage at which plants die through a lack of water. There is still some water in the soil but it is so tightly held by the soil particles that it is unavailable to the plant.
Water Holding Capacity: This is the amount of water in a soil at field capacity and will vary according to the texture of the soil. Clay soils have a higher water holding capacity (WHC) than sandy soils. (The explanation for this is given in the Soil Science module).
Available Water Capacity or Available Moisture: This is the amount of water available to the plant when the soil is at field capacity. The available moisture (AM) of a soil is not the same as the water holding capacity of that soil because some of the water is tightly held in the soil and is unavailable to the plant. In order to simplify this point, consider the following example.
Soil Holding Texture Sandy Clay
Water Holding Capacity 33% Available Moisture 13.5%
Take a cubic metre of this soil. 1 m3 = 1 000 litres
Thus 1 cubic metre of this soil at field capacity contains 330 litres of water and the available
moisture is 135 litres. The other 195 litres are held in the soil but are not available to the plant.
Another way of putting this is as follows:
In 1 metre depth of soil (1 000mm), there is 135mm ‘depth’ of available moisture for the plant. After a vigorous crop has removed all the available moisture from the root zone (1 metre deep), 135mm of water would have to be applied by irrigation or rainfall to bring the top 1 metre of the soil to field capacity.
If a very vigorous weed infestation is allowed to remain on a land and with deep ploughing or ripping operations the soil could lose more than just its available water e.g. more than 135 mm per metre of depth. Normally however, irrigation is only required to replace soil water in the AM range. (Available Moisture)
Potential Transpiration: At field capacity no water is being lost from the soil by drainage. Any water lost from the soil is by evaporation and transpiration. Potential transpiration is an estimate of this water loss and is explained in more detail in Lecture 6 of this module.
Soil Moisture Deficit: This is the amount of water required to bring a soil up to field capacity. Crops growing in clay soils can withstand higher moisture deficits than those growing in sandy soils. Crops growing in light sandy soils require more frequent rainfall or irrigation than those growing in clay soils, although the total amount over a season may not be greatly different.
Scheduling of Irrigation: Determining the exact time and amount of water to be applied to a crop or crops on a farm. You must know the principles and alternative methods of determining the level of moisture in a soil. This topic is discussed in Lecture 5 of this module.
- SOIL / WATER RELATIONS
The most important aspects to remember in the relationship between water and the soil for irrigation purposes are the following.
- Available Water Capacity or Available Moisture of the soil and should be at least 7% (70mm depth) otherwise the soil will require frequent irrigation. Soils of the low rainfall areas generally have a higher Available Water Capacity than soils of the same texture in the highveld.
- The permeability of the soil must be sufficient to allow the passage of water through the soil within a reasonable period of time. In other words, the soil must be well drained because:
- Where a soil has just been irrigated and this is followed by rain, the excess water must be able to drain out of the soil or else the land becomes waterlogged. Waterlogging causes lodging of the crop, the plants become starved of air and the structure of the soil can be destroyed.
- Sudden decreases in the permeability of soil within the root‐zone of a crop may cause temporary waterlogging even if the correct amount of water has been applied.
- Sometimes it is necessary to apply slightly more water to a crop than it needs in order to drain excess salts out of the soil profile (see Lecture 9 on Brack Soils in the Soil Science module). These salts may have come from the irrigation water applied to the soil or may have risen up into the root zone from weathering occurring further down in the soil profile. These salts can be washed out of the soil, but the soil must have good permeability to below the root zone. The permeability of soils can be improved by artificial drainage.
- Although it is important that irrigated soils should be permeable enough to allow for good drainage to prevent waterlogging of the crop, too high permeability in a soil will cause excess leaching (washing out) of the soluble plant nutrients. This can be a serious problem because very permeable soils normally have low water holding capacities and require frequent irrigation.
- The permeability of a soil will affect the type and layout of the irrigation employed and in the case of a flood irrigation scheme will govern the slopes used and the length of the run.
- The Infiltration Rate of the soil must be high enough to allow water to penetrate the soil surface fairly quickly. Some soils form a cap or crust on the surface when they dry out after rain or irrigation and this can reduce the rate of infiltration of further water. This ‘capping’ factor, or ‘t’ factor as it is called is an important consideration when designing an irrigation scheme.
- The Inherent Nutrient Holding Capacity of a soil. Applying water in the form of irrigation will always wash some nutrients out of the soil particularly in the root zone. Nutrients will be washed out of the root zone and down to the lower levels of the soil profile and will be unavailable to the plants. Some soils have a higher capacity to hold nutrients than other soils. In general, clay soils can hold more nutrients than sandy soils and clay soils are said to have a higher Exchange Capacity (EC). Soils with large reserves of weatherable minerals (High Base Saturation) will also lose fewer nutrients through leaching than those with low reserves of weatherable minerals. Soils in low rainfall areas usually have a higher exchange capacity and a high base saturation than similar soils in higher rainfall areas.
- The salinity of the soil, which is the concentration of soluble salts in the soil. Sodium salts reduce the permeability and structure of the soil whereas magnesium and calcium salts improve permeability and structure. Soils can be affected by salinity when the concentration of salts is below that which would affect plant growth directly. The salinity of the subsoil is important because when the water table rises it may affect the permeability and drainage of the top soil and bring harmful salts into the plant root zone.
- The quality of water used for irrigation is important and can affect the suitability of a soil for irrigation purposes. Natural water can carry solids such as clay and silt and dissolved materials such as minerals. Both solids and minerals can be carried in harmful quantities.
Underground water may carry larger amounts of salts borehole water often causes problems by adding salts to a soil through irrigation.
Regeneration water seeping from large irrigation schemes is often of high salinity due to the salts being concentrated through evaporation and also because of the leaching of fertilizers and natural salts.
The salinity of water is measured electrically and water is analysed by soil analysis laboratories.
Whether water is too saline for irrigation will sometimes depend on the composition of the salts dissolved in the water. In general, if magnesium and calcium salts exceed sodium salts, the water is safe to use. However, the total soluble salt content for irrigation water is as follows in table 1
Table 1: Total soluable salts content for irrigation water
| Water containing below 0.05% of soluble salts | Suitable for all irrigation |
| Water containing 0.05% ‐ 0.15% of soluble salts | Cannot be used under conditions of poor drainage and high evaporation |
| Water containing 0.15% ‐ 0.25% of soluble salts | Of very doubtful value for irrigation |
| Water containing over 0.25% of soluble salts | Too alkaline for any irrigation |
Water which contains 0,1% of soluble salts will, if used to apply 100mm of irrigation water to a soil, will deposit 1 ton of soluble salts per hectare. If these salts accumulate because of high evaporation and poor drainage the soil will rapidly become alkaline or brackish.
The use of water with a high salt content for irrigation purposes can produce alkaline or brackish soils very quickly. Where such alkaline water is used on badly drained soils, evaporation and transpiration can remove the water, leaving the salt behind in the topsoil. Brackish soils have been produced in this way in certain areas of South Africa. In parts of Egypt, continuous irrigation, since the Nile dams were built, is causing similar problems. In many commercial arable areas repeated irrigation is causing a gradual rise in the salinity of the soils being irrigated. Before embarking on any irrigation scheme it is most important to have both the soils and the water analysed.
3. LAND DRAINAGE IN RELATION TO SOIL WATER RELATIONSHIPS
Most crops will grow well only on well‐aerated soils. Waterlogging may be due either to a high water table or due to the inability of surface water to percolate quickly enough to below the root zone. Except in sandy soils water percolates rapidly only through cracks or spaces between structural units; poor drainage can be caused by insufficient coarse pores at the surface (needs proper cultivation), or by poorly permeable sub‐soils (needs ripping if not too deep or artificial drainage if beyond cultivating depth).
Soils should have non‐capillary pores of about 10% or more of their total volume or they will probably need artificial drainage. Tile drains are often placed at a depth of 1 metre, where they can remove non‐capillary water to a depth of 500mm which is a reasonable rooting depth for many crops. However, the water table will be nearer the surface between drains. The less permeable the soil the closer the drains should be placed.
Figure 1: showing drainage holes and the water table
4. SOIL TILLAGE IN RELATION TO SOIL/WATER RELATIONSHIPS
The main object of tilling the land (tillage) is to allow the rapid penetration, infiltration and retention of rainfall or irrigation water.
A thin compacted layer at the soil surface reduces the amount of infiltration and hence increases run‐off. Run‐off may exceed 80% on a cropped land if the surface is compacted or crusted (and depending on intensity of rainfall). Tillage must leave an open‐grained surface to allow entry of water and to resist ‘raindrop impact’, and hence erosion.
Minimum Tillage: Usually involves minimal soil disturbance, leaving the previous crop as a mulch on the land. Cover crops and crop rotation are important.
Soil Air: An open grained surface on a land also allows for free passage of air vital to micro‐ organisms, which in turn aid in improving soil structure and hence water relations.
Oragc Mulch: Provides a much better means of conserving moisture in the soil than soil mulch. This protects soil from direct rays of sun and wind currents, keeping soil cool resulting in less evaporation and drainage.
Vegetable mulches are usually too expensive to apply to any crop other than a perennial and of high returns per acre, e.g. coffee, strawberries. Crop residues if chopped into small pieces in the top 100mm of soil are of value.
Weeds: Extract water from the soil and hence their removal by means of tillage is important, both during crop growth to reduce competition for water and before the dry season to conserve soil moisture for next year.
Replenishment of Aquifers: This applies to underground water supplies (e.g. borehole supplies) which are generally much too deep to affect soil‐water relationships in cropping.
Soil Erosion: As already mentioned in these notes, infiltration rate, permeability and soil surface condition can influence erosion, mainly by increasing or reducing run‐off. Soil structure and vegetative cover are also important in resisting the forces of raindrop action and run‐off water.
Possible Effect of Irrigation on Soils: Whenever irrigation is carried out the farmer must watch the soil continuously and check for harmful effects.
The points to watch are:
- Water Table: This must not be allowed to rise too high. Irrigation followed by heavy rain can cause waterlogging and poor plant growth.
- Leaching: Irrigation water washes down through the soil and can cause nutrients, particularly nitrogen to be leached out of the root zone. The nutrient level of the soil must be checked by soil analysis and watching growing crops for symptoms of deficiencies of both major and minor nutrients.
- Salinity: Soils on irrigation schemes can become saline or brackish by using water with a high soluble salt content. Soils in the root zone of the plant can become saline through the water table rising and washing salts up from the lower reaches of the soil. When the water table falls the salts remain behind causing a brackish sub‐soil.
- Soil pH: Soil pH, which is the measure of the acidity or alkalinity of the soil, must be watched. Irrigation, particularly with borehole water can push the pH up over a number of years making the soil less acid and more alkaline. This is caused by the alkaline soluble salts in the water being added to the soil.
- Soil Structure: Irrigating with alkaline water and a lack of organic matter in the soil can cause the structure of the soil to break down particularly at the soil surface. This causes soils to cap or form a crust on the surface when they dry out after rain or irrigation. The capping on some soils can become so hard that any further water added to that soil cannot penetrate the surface crust but runs off the soil, causing erosion. This capping can occur on both light, sandy soils, and on heavy, clay soils and is an important factor in controlling water penetration and permeability on irrigation schemes.
- Erosion: Any water applied to the soil either as rainfall, overhead or flood irrigation can cause erosion of the soil unless conservation measures are taken to preserve the structure of the soil and avoid any physical washing away. In any scheme, conservation and irrigation go hand in hand.
- Weathering: In many soils this is taking place continuously in the C horizon of the soil, as the underlying rock is broken down to form soil. The danger is that this breakdown of rock can release many soluble salts which may be washed up into the higher levels by a rising water table. This can cause the formation of brackish pans in the sub‐soil region or even in the root zone of the soil.
5. UNITS USED IN IRRIGATION
| Measurement of Time | S.I. Unit | Symbol | Equivalents |
| Second | s | ||
| Hour | h | ||
| Day | d | ||
| Linear | Millimetre | mm | 25.4mm = 1 inch |
| Metre | m | 1m = 3.28 feet = 1.095 yards | |
| Kilometre | km | 1km = 0.62 miles | |
| Area | Square metre | m2 | |
| Square kilometre | km2 | 1km2 = 0.38 square miles | |
| Hectare | ha | 1ha = 10 000 m2 = 2.47 acres – 1 acre = 0.405 ha. | |
| Volume | Litre | l | 4.54 l = 1 gallon = 1.76 pints |
| Cubic metre | m3 | 1m3 = 220 gallons | |
| Thousand cubic metre | 103m3 | ¶ x 103m3 = 220080 galls = 0.22 million galls = 0.81 acre feet | |
| 4.54 x 103m3 =1 million gallons = 1 ac.ft = 1.2 x 103m3 | |||
| Million cubic metres | 106m3 | ¶ x 106m3 = 220 million gallons | |
| Rate of flow | Cubic metre/second | m3/s | 1 m3/h = 0.28 ¶/s = 35.31 cu secs |
| Litre/second | l/s | L ¶/s = 3.6 m3/h = 13.21 G.P.M. = 792.5 G.P.H. = 0.035 cu secs I cu sec = 28.32 ¶/s | |
| Cubic metre/hour | m3/h | 1 m3/h = 0.28 ¶/s = 3.56 G.P.M. 1 cu sec = 102 m3/h | |
| Power | Watt | W | 740 W = 1 horsepower |
| Kilo Watt | kW | 1 kW = 1.34 horsepower – 1 horsepower = 0.74 kW | |
| Pressure | Kilo Pascal | kPa | 100 kPa = 1 bar = 10 m head of H2O = 14.5 P.S.I. |
| Volume in m3 = mm x ha x 10 or Vol in 103m3 = mm x ha m3/h = mm x ha x 10 100 l/s x h = mm x ha x 2.8 |
