Water balance calculations cannot begin until soil water content in the root zone is known. It may be established before or after crop emergence. From this, the soil water content of successive days can be estimated using the water balance equation. Four additional values are needed for the water balance equation. Irrigation and rain are the deposits in water balance and are measured or calculated values.
Rain is measured by using rain gauges.
Irrigation depth is calculated from the application rate of the irrigation system and the duration of application, or by dividing the total net amount of water applied by the irrigated area. This is the daily withdrawal in the equation. It is estimated from weather and crop information. KC defines the stage of growth of the crop, and KS is a function of actual soil water content. The ETr can be estimated using different ET models that relate ETr to weather conditions.