Summary

This case study shows how to calculate the total water requirement for a command area (irrigation blocks) under various crops, soil textures and conveyance loss conditions. In order to evaluate the required irrigation gift for the entire command area a simple water balance has to be set-up. The total water demand for each irrigation block and the crops in each block are calculated by summing the following components:

• infiltration (percolation loss) through the soil (I)
• seepage (conveyance loss) through the channel (S)
• maximum evapotranspiration of the crop (ETm)

In this exercise, the irrigation water requirement is calculated for a 10-day period during the harvest stage.

Evaluation of Percolation loss (I)

The command area is divided in irrigation blocks. First, these irrigation blocks are crossed with the soil texture map to determine the area of each soil texture class in each block.

Percolation losses differ per soil texture class so a table with the following percolation data is created:

The percolation table is joined with the cross table to get the percolation for each soil texture class in each block. The amount of water loss for each soil texture class per block is calculated with a tabcalc statement. In order to get the total percolation loss per block the results of the previous operation are aggregated.

Evaluation of Conveyance loss (S)

Conveyance losses are calculated in about the same way as the percolation losses. First, the map with the irrigation blocks is crossed with the channel distribution map. The conveyance loss per meter channel length differs per channel type and is 0.2 m³ per day for clay channels and 0.01 m³ per day for concrete channels. A new table indicating water loss per channel type is created and joined to the cross table. The amount of water loss for each type of channel per block is calculated with a simple tabcalc formula. Finally the results are aggregated to evaluate the total conveyance loss per irrigation block.

Evaluation of maximum evapotranspiration (ETm)

Crop water requirements are normally expressed by the rate of evapotranspiration (ET). The evaporative demand can be expressed as the reference crop evapotranspiration (ETo) which predicts the effect of climate on the level of crop evapotranspiration. In this case study the ETo is 8 mm/day. Empirically-determined crop coefficients (kc) can be used to relate ETo to maximum crop evapotranspiration (ETm) when water supply fully meets the water requirement of the crop. The value of kc varies with crop and development stage. The kc values for each crop and development stage are available in a table.

For a given climate, crop and crop development stage, the maximum evapotranspiration (ETm) in mm/day of the period considered is:

ETm = kc * ETo

Maximum evapotranspiration refers to conditions when water is adequate for unrestricted growth and development under optimum agronomic and irrigation management. Maximum evapotranspiration is calculated in this case study by crossing the irrigation block map with the map that shows the different crop types in the command area, joining the cross table with the kc table and by applying the maximum evapotranspiration formula with a tabcalc statement.

Water balance calculation (S+I+ETm)

The required irrigation gift for the entire command area is equal to the sum of water losses due to infiltration through the soil (I), seepage through the channel (S) and maximum evapotranspiration (ETm) for each block. The total amount of water requirement in harvest period for each block is reclassified in irrigation classes using the following table:

Finally, you will create a script to automate the calculation procedure. With the script, you can easily calculate the water requirements for other growing stages.

For more information on this case study, contact:

A.M. van Lieshout / J.A.M. de Brouwer
Department of Water Resources,
International Institute for Geo-Information Science and Earth Observation (ITC),
P.O. Box 6, 7500 AA Enschede, The Netherlands.
Tel: +31 53 4874306, Fax: +31 53 4874336, e-mail: lieshout@itc.nl