- THE NEED FOR IRRIGATION SCHEDULING
The objective of any irrigation scheme is to provide the optimum amount of water for plant growth. This means that a growing crop is allowed to use up the water in the soil and just before the crop is about to suffer from a lack of water the crop is irrigated to bring the soil back up to field capacity. This needs skill and judgement on the part of the farmer as allowing the water reserves in the soil to fall too far will damage the crop and, irrigating the soil beyond field capacity will waste water and money. Irrigation scheduling simply means applying the correct amount of water to a crop at the right time and is important for the following reasons.
This is very important in a country where little or no rain fails for half of the year. It can be achieved by means of dams, proper crop and veld management and by efficiency in irrigation, and preventing water from being wasted. This can be done in two ways; preventing leakage from dams, canals, pipes, etc., and the accurate application of water. The proper application of irrigation water is vital to keep costs down, prevent the waterlogging of the soil, avoid the leaching of valuable nutrients and stop water being wasted.
If a farmer applies 80mm depth of water to a mature crop, when the root zone can hold only an extra 60mm of water, then 20mm of the water will pass below the effective range of the roots. This is over‐irrigation of 25%. If a farmer over‐irrigates by 25% throughout the season and the crop requires 800mm of irrigation water, then the water wasted is 2 000 cubic metres per hectare.
By scheduling his irrigation the individual farmer can keep his costs down and save a large amount of water.
OPTIMUM GROWTH CONDITIONS
For the crop to produce the best possible yields means no stress for the crop, no leaching of nutrients and no waterlogging. If the application of irrigation water is not accurate then the increased yield of the crop over dryland farming may not pay for the increased cost of the irrigation.
Under‐irrigation by 25% could result in a drastic yield reduction, depending on the stage of growth of the crop at which stress occurs and in such a case it may be more profitable not to irrigate at all.
At certain times, it may be necessary to stress a crop slightly but by careful scheduling the farmer is able to obtain the best use of his water by applying it in the most important place at any critical time.
By using a proper scheduling plan for irrigation, the times of all water applications to all irrigated crops are known in advance for the whole season. This allows the forward planning of other crop tasks; the application of insecticides, herbicides, machine and hand cultivations, etc.
The Requirements of a scheduling system should be:
- The system should not be too time‐consuming and complicated. Keep it simple;
- It should not be too expensive, although the cost of a scheduling scheme can be recouped quite early on in the season, simply by the amount of water saved and the better performance of the crops; and
- It should enable the advance planning of all irrigation and crop operations
- METHODS OF SCHEDULING
The successful scheduling of irrigation depends on being able to measure or estimate the amount of moisture in a soil, particularly at field capacity and at the 50% level of available moisture. There are a number of methods that can be used to measure soil moisture and these are given below. Most of these methods are complicated and expensive and used only for very high value crops. However, two methods, Estimation by Feel of the Soil and Scheduling using Climatic Data are commonly used on farms and are everyday practical methods.
METHODS OF MEASURING SOIL MOISTURE GRAVIMETRIC METHOD
Soil samples are weighed, dried and then weighed again to determine the amount of water in the soil by difference. This is done in a laboratory and is used to analyse the Water Holding Capacity of soils.
The Tensiometer is attached to ceramic (pottery) cups which are buried in the soil at various depths and positions in the land. When the soil is moist, a higher reading is obtained on the scale than when the soil is dry. The scale has to be calibrated for that particular soil and these machines are used only for research or high value crops. For a high value crop the tensiometer would be calibrated and be a permanent fixture in the soil.
An instrument is used to measure the electrical resistance between two terminals separated by nylon. These ‘nylon blocks’ are buried at various points in the land with the wires leading to the surface, but is only used in research and an illustration is shown below.
A very expensive instrument used in some countries to give an instant soil moisture reading. Accurate calibration is essential for each soil type.
ESTIMATION BY FEEL OF THE SOIL
In this method, the farmer uses a soil auger to obtain soil samples at various depths in the land. Samples are taken from between the plants and the rows. The farmer handles the samples and makes an estimate of the moisture content of the soil. A farmer can become very efficient at judging when to irrigate on his own soil and many of them rely entirely on this method. After several seasons’ experience, it is possible for him to judge to within 10mm ‐ 20mm of how much water is required to restore his soil profile to field capacity. Every farmer with irrigation should become proficient in estimating the moisture content of his soil from the feel of samples obtained with an auger. This method can be used in conjunction with climatic data to provide a very accurate scheduling system for irrigation.
This is a simple tool used to obtain soil samples from different depths in the soil profile. Several types of augers are available. The spiral type is similar to a carpenter’s bit and is 20mm ‐ 30mm in diameter, and a larger model, the post‐hole digger type is 50mm to 75mm in diameter. Spiral augers tend to clog when they are used on heavy clays, therefore on this type of soil a more open version called the Dutch Auger should be used. Both types of auger are shown below.
Figure 1: A soil auger (left) and a Dutch auger (Right)
Having obtained a number of soil samples using the auger, the farmer has to estimate the moisture content of the soil by feeling, squeezing or rolling a small piece into a ball and generally looking closely at the sample. The following table is a guide to the water content of different soils. However, a farmer is dealing only with his own soil and can concentrate on estimating the moisture content for that particular soil texture.
Table 1: Guide for Determining when to Irrigate by “FEEL” of Soil
Feel and appearance of the following soils
|% of Available moisture remaining in soil||Loamy sands and sandy loams (light)||Very fine sandy loam and silt loams (Medium)||Silty clay and loams and clay loams (Heavy)||Action required|
|0||Dry, loose, flows through fingers||Powdery, sometimes slightly crusted, but easily broken down into powdery condition||Hard, baked cracked; difficult to break down into powdery condition.||Irrigate now. Soil is too dry and should have been irrigated several days ago|
|50 or less||Appears to be dry, will not form a ball with pressure||Somewhat crumbly, but will hold together from pressure.||Somewhat pliable; will ball under pressure||Irrigate. Before the field is covered, some spots will be too dry|
|50 to 75||Tends to ball under pressure but seldom holds together when bounced in the hand||Forms a ball, somewhat plastic; will stick slightly with pressure||Forms a ball; will ribbon out between thumb and forefinger; has a slick feeling||Irrigate today or tomorrow. If you wait any longer you will not be able to irrigate the whole crop until some of it is|
|75 to 100||Forms a weak ball; breaks easily when bounced in the hand; will not slick||Forms a ball; very pliable; slicks readily||Easily ribbons out between thumb and forefinger; has a slick feeling||Soil moisture OK. Check again in2 or 3 days|
|100 % (Field capacity)||Upon squeezing, no free water appears on soil but wet outline of ball is left on hand. Soil will stick to thumb when rolled between thumb and forefinger||Same as sandy loam||Same as sandy loam||Soil very wet. Check again in 5 or 6 days|
Farmers with irrigation should be familiar with the feel of their particular soil at the 50% level of available moisture. One method of doing this is to mix equal amounts of soil at field capacity and soil dried by a wilting crop and leave for 24 hours in a plastic bag. Remove from the bag and try puddling, squeezing, rolling, throwing at a wall, etc., to develop tests for this exact stage of moisture content.
Some farmers rely on the appearance of the plant to tell them when to irrigate. The obvious signs of water shortage in plants are curling up of the leaves and wilting, but with some crop plants changes do occur before this stage. Wilting of plants is a sign of serious water shortage and irrigation should be carried out before this happens.
Crops such as wheat and beans change from a bright green to a bluish colour as the available water diminishes. Under normal conditions the growing tip of the cotton plant is light green in colour over the top 100 ‐ 200mm. When the plant is suffering from water stress this light green area is reduced to less than 100mm. Another indicator is looking at a field of cotton at the full flower stage. With plenty of water the impression is of a dark green background with a few flowers visible but, if the crop is suffering from water stress you see a lot of flowers against a pale green background.
Experienced fruit and coffee growers often have what they call their ‘indicator trees’ in the orchard or plantation. These are individual trees which show signs of a lack of water long before other trees in the row. This is usually caused by the fact that these trees are growing on a lighter soil than their neighbours and have less available soil moisture.
Estimating the need for irrigation from the appearance of plants or trees is a useful aid which can be used with the necessary experience but should be used together with more reliable methods of judging water stress as sometimes the signs appear too late to prevent damage to the crop.
The methods already described to estimate the moisture content of the soil are all direct methods. The soil moisture is measured directly and the farmer knows exactly how much water to apply to bring the soil back up to field capacity. Scheduling irrigation by means of climatic data is an indirect
method and relies on measuring the water lost from the soil by evaporation and transpiration by the growing crop. This evapotranspiration, as it is called, is an estimation of the water used by the plants. It is the most commonly used method of irrigation scheduling and if applied properly, it is very accurate. The method is described in detail in the following lecture.