Soil Water Balance and Irrigation Scheduling

The soil in your field is like a bucket (Fig. 1). It has a limited capacity for holding water (called soil water-holding capacity). If we put too much water in it (exceed field capacity - FC), it’s not going to hold, and it will spill out (lost to deep percolation).

Soil water balance is the math to determine how much water (irrigation or rainfall) went in and how much got out of the soil. Soil water balance can tell how much water you're supposed to put back in the bucket. It can help you with irrigation scheduling. Daily evapotranspiration (ET) amounts (i.e. crop water use) are withdrawals from this bucket (soil storage) and rainfall or irrigation are a deposit to this storage.

Account for these inputs and outputs and you know how much water is in the soil. Irrigate when soil water content drops below a minimum level (Management Allowable Deficit - MAD).

Figure 1. Soil water balance is the math to determine how much water (irrigation or rainfall) went in and how much got out of the soil.

Terms:

Field Capacity (FC): Maximum amount of water that a soil can hold indefinitely against gravity (% of volume).

Permanent Wilting Point (PWP): The amount of water remaining in the soil after plants can no longer pull water from the soil (wilt & die).

Available Water (AW) = FC – PWP

Management Allowable Deficit (MAD): percent deficit of Available Water (AW) that management will accept.