Farm Table says:
This PDF is part of the larger WATERpak resource – a guide for irrigation management in cotton and grain farming systems.
It covers the following topics:
Water Use Efficiency
Water Use Efficiency is a generic label that encompasses an array of performance indicators used to describe water use within a cropping system.
Different indicators that can be used are:
- Water Use Index (WUI) relates production outputs ($, bales, tonnes, etc.) to water input (ML).
Irrigation water use index is a measure that relates the total amount of production to the amount of irrigation water that was applied to produce this yield. It is a useful measure and is commonly used, particularly for internal analysis, as it only accounts for irrigation water and therefore it can reflect differences in irrigation management.
Field-scale: Irrigation water use index (applied) (%) = total yield (bales) irrigation water applied (ML)
- For example, a cotton field has a yield of 80 bales and 50 ML of irrigation water was applied to the field during a season.
- IWUI (applied) = (80 ÷ 50) = 1.6 bales per megalitre
Farm-scale: Irrigation water use index (farm) (%) = total yield (bales) irrigation water supplied to farm gate (ML)
- Across the whole farm, 450 bales of cotton were produced using 350 ML of irrigation water.
- IWUI (farm) = 450 ÷ 350 = 1.3 bales per megalitre
- Irrigation System Efficiency (%) relates water output (ML) to water input (ML).
Irrigation system efficiencies are different to indices. This is because they compare the water output (or available) to the water input (supplied) at different points of the farm or irrigation system. Efficiencies do not have units, they are expressed as a percentage (%).
- Distribution Uniformity (DU) (a measure of how even an irrigation application is
Uniformity is a measure of how evenly water has been applied to a field and is expressed as a percentage (%). It is only applicable at the field scale.
Distribution uniformity (DU) = Average of lowest 25% of infiltrated depths Average of all infiltrated depths.
Effective rainfall includes:
- water intercepted by vegetation
- soil evaporation losses
- evapotranspiration losses
- the contribution to leaching requirement.
Ineffective rainfall includes:
- Uncaptured surface runoff
- Deep drainage
- Any remaining soil moisture that is not used for subsequent crops.
Effective rainfall = total rainfall – run-off – deep drainage
Some benchmarking examples:
- Comparing the performance of different farming enterprises, for example, the growers in a local area
- Compare the performance of a single field over a period of 3 seasons, to see if management changes are having a positive effect
- Comparing the performance of your enterprise to industry averages or targets
- Comparing performance within a farm, for example between a number of adjacent fields, to determine which need additional work to increase their performance and bring them ‘up to scratch’
A water budget has two main purposes:
- To determine what area of the crop should be planted for the water resource available at the beginning of the season.
- To determine how to best utilize crop inputs during the season as water availability changes (includes determining when to plough outcrops due to insufficient water availability).