The scheduling of irrigation from Climatic Data relies on the relationship that exists between the amount of water lost from the soil by the transpiration of a growing crop, the Evapotranspiration which has the symbol Et, and the water lost by evaporation from an open water surface in the same area as the crop. This evaporation has the symbol Eo. An evaporation pan is used to measure the Eo every day and in this way a check is kept on the water being lost by Et from the soil. Obviously, in the early stages of the crop most water is being lost from the soil by direct evaporation (Eo), because the small plants are not transpiring much water. At the other extreme, when the crop is fully developed and covering the ground completely most of the water is being drawn up from the soil and transpired by the plants.

The facts required for irrigation scheduling by this method are:

  • The Et/Eo Ratio
    • The Root Extraction Depth for the crop being grown
    • The Available Moisture of the soil where the crop is grown
    • Climatic Data for the area
    • The Maximum Permissible Moisture Depletion Level for the crop
    • Tabulation of the Data

The ratio of the evapotranspiration from a crop and its soil to the evaporation from an open water surface (i.e. an evaporation pan) varies with the stages of growth. The table below shows the  general Et/Eo ratios at different stages of growth for most crops. The ratio is correlated with percentage crop cover.

Table 1: ET/EO Ratios which are correlated to a percentage of crop cover.

 Et/Eo Ratio

Figure 1: Graph of the Et/Eo Ratio and Time after Planting

This graph is for an annual crop with a growing season from planting to maturity of 22 weeks. Crop water use as represented by the Et/Eo ratio has a value of 0.22 at planting and this increases to a maximum of almost 1.0 between 12 and 16 weeks and then decreases slowly to maturity. The mean Et/ Eo ratio for the curve is 0.72 and the curve applies to most annual crops grown in this region. A crop such as wheat grown in a Highveld area will have a longer curve than wheat grown in a Lowveld region, but the general form will be the same. The following graphs show the actual curves for maize, cotton and both Highveld and Lowveld wheat.

Figure 2: Maize Et/Eo Curves

Figure 3: Cotton Et/Eo Curves

Figure 4: Lowveld and Highveld Wheat Et/Eo Curves


This varies with stage of growth, soil type and from crop to crop. It is the depth at which 90% of water withdrawal by the plant occurs. Generally, maximum root depth is reached at the same stage as maximum Et/Eo Ratio, e.g. in maize, at about 10 weeks, and wheat at 7 – 10 weeks after planting.

Table 2: The accepted maximum root extraction depths of several crop plants are listed below:

CropMaximum Root Extraction Depth
Vegetables300 mm
Potatoes500 – 600mm
Maize600 – 800mm
Field Beans600 mm
Sorghum600 – 800mm
Wheat600 – 800mm
Soya Beans600 – 800mm
Fruit Trees800 mm
Tobacco900 mm
Cotton900 mm
Groundnuts900 mm
Lucerne1200 mm

These figures are approximate and will vary according to the depth of the soil, type of soil and the nutrients and water available to the crop. Figures are available for the root depth of different crops and at various stages of growth. The root development of crops can be plotted onto a graph, and Figure 5 below shows the root development typical of most farm crops

Figure 5: Root development of most farm crops

Graphs are available for the Effective Root Depth and Stage of Growth for most crops, although it is  a good exercise to collect the necessary information for yourself during a season and draw a graph  of your own.


The Available Moisture and the Water Holding Capacity of your soil should be determined by the mechanical analysis of the soil (see the Soil Science module). This can be done by sending soil samples to the local laboratory. The results are expressed as a percentage of water by volume of  soil. Figures for the common textural classes of soils are given in the table in Lecture 4. From that table you can see that the average Available Moisture in a Coarse Sandy Soil is 6% and that for a Heavy Black Clay soil is 20%.

The Available Moisture of a soil can change with the soil depth and samples should be analysed from every 150mm layer down to 1 metre in the soil profile. If the value for AM does  not change as the depth increases then scheduling is much simpler.


In order to carry out accurate scheduling of his irrigation the farmer must keep daily records of rainfall and evaporation. A farm situated close to a meteorological station might use their

figures but this is never really satisfactory, because both rainfall and evaporation can vary very much over a distance of a few hundred metres. The farmer requires the following:

  • A rain‐gauge, protected against vandalism and damage by animals. The bottle should be checked daily and readings taken of any rainfall.

Figure 6: A rain‐gauge (left) and a measuring cylinder (right)

  • An evaporation pan. Ideally one should be placed near each land being irrigated but normally one will suffice on a farm. The standard evaporation pan used is the United States Weather Bureau Class ‘A’ pan. Some irrigators start off by using a Landsberg pan which is a simple farm‐type pan made from an oil drum. It has an evaporation rate fairly consistent with that of a U.S.W.B. pan provided that the reading is multiplied by 0.85 but on the whole it is not recommended for accurate scheduling.

The Class ‘A’ pan consists of a circular tank made from iron sheeting with an outside diameter of

1.207 metres and a depth of 0.254 metres. It is mounted on timber slats which allow the air to circulate below the pan and covered with a standard 25mm diameter wire netting screen. Although the screen is designed to prevent birds and animals from drinking from the pan, its presence does prevent some evaporation by cutting down on the movement of air over the surface of the water and because of this it is important to keep to the standard size of screen

Readings are made in a stilling well made of brass with a scale screwed onto the outside. The Class ‘A’ pan showing the timber slats, the stilling well and wire screen is shown below.

Figure 7: A class ‘A’ Evaporation pan

Source: cimel

The pan must be carefully sited, protected from animals and must be cleaned out at least every 2 weeks. The pan readings should be taken every day at the same time, preferably early morning (08h00) and the readings to be recorded on a chart in the farm office. Rainfall figures should be recorded from as near the land as possible.