Water Budget Balance

Water budgeting reflects a balance between the inputs and outputs of water to and from the plant root zone.  The method is similar to balancing a checkbook using withdrawals and deposits.  The withdrawals are the crop evapotranspiration (ETc), deep percolation, and runoff while the deposits are irrigations, precipitation, and capillary rise.   During the main irrigation season when irrigations are applied effectively, the only withdrawal and deposit that need to be considered are irrigation amount and ETc.

If the starting balance (current soil moisture level) is known and the date and amounts of subsequent withdrawals (ETc) are known, then the current balance can be calculated at any time.  Most importantly, the time when the account will be overdrawn can be determined, so that overdraft (plant stress) is avoided.  This water budget balance procedure can be used to answer the questions of how much to irrigate as well as when to irrigate.

The initial starting point for the water budget balance is when the soil is saturated (also known as field capacity).  This occurs after a thorough wetting of the soil by irrigation or winter rains.  If this is not the case, then the initial balance must be determined by direct measurements.  Daily quantities of water used by the crop are subtracted from this balance to the allowable depletion point or until the plant begins to show signs of stress.  This is known as the Managed Allowable Depletion (MAD) point.  At this point an irrigation should be applied with a net amount equivalent to the accumulated daily use since the last irrigation.  (This net amount is based on the efficiency of the irrigation system being used.)  The soil reservoir is thus recharged to full capacity and the depletion cycle begins again.