| Home | The District | Drainage | Links | Reference Library | Water Conservation | Water Supply |
Water Quality Data
IRRIGATION SCHEDULING
Previous Page Next Page
Allowable Depletion
A term in wide use by water management professionals is "allowable depletion". (You may also see it called "management allowable depletion" or "MAD" in other writings.) This is the amount of available soil moisture that is allowed to be used by the plant between irrigations.
Allowable depletions can be described as a percentage or as total inches of water use. Thus, we might say . . . "the allowable depletion for this field is 45 percent of the available water holding capacity". We are saying that we will allow the crop to use 45 percent of the available water in the effective root zone between irrigations. Or, we might say . . . "the allowable depletion for the field is 3.5 inches". We are now saying that we will allow the crop to use 3.5 inches between irrigations. Put another way, the soil moisture depletion (the moisture you want to replace with an irrigation) will be 3.5 inches before an irrigation.
The allowable depletion, AD, in inches of water use is equal to the allowable depletion percentage multiplied by the available water holding capacity, AWHC. For example, assume . . .
AWHC = 5.5 inches
AD% = 45%
thus allowable depletion in total inches of water use is .
. .
ADinches = AWHC x AD% / 100
= 5.5 x 45 / 100
= 2.5 inches
Water Into and Out of the Effective Root Zone
Modern farm managers attempt to
control all processes on the farm. When controlling the irrigation
program, modern managers first look at the effective root zone (ERZ) as a
"system". Then, they identify, measure and control the water going into
and out of the system.
FIGURE IS-1 - schematic of root
zone ins/outs
Referring to Figure IS-1, the
primary sources of water going into the effective root zone are rain
(RAIN), irrigation (IRR), and upwards movement of groundwater from a
shallow water table (GW).
The primary losses of water are deep percolation (DEEP) from excess irrigation or rainfall and crop evapotranspiration (ETc).
All of the water going into and out of the effective root zone on a daily or seasonal basis can be measured in terms of a depth of water, usually inches . . .
-
Rainfall is measured in inches.
-
Irrigations are commonly measured as acre-inches/acre applied, or just inches.
-
Upwards movement of groundwater (considered as irrigation) is measured in inches.
-
Deep percolation is water soaked into the effective root zone in excess of the soil's field capacity. Since it is either excess irrigation or excess rainfall, it too is measured in inches.
-
Finally, crop evapotranspiration (ETc) is measured in inches.
In addition to the water coming into and out of the effective root zone, the actual level of soil moisture at any time (which we've called the soil moisture reservoir) can be measured in inches. Thus all components of the system are measured in the same units, inches. This enabled scientists to develop the "water budget" equation and the water budget method of irrigation scheduling.
Previous Page Next Page Top of Page
Last updated September 2000