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Water Quality Data
IRRIGATION SCHEDULING
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The Water Budget Equation
Remember the different ways that soil moisture can be described volumetrically . . .
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Total moisture, the total amount of moisture in the soil.
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Available moisture, the amount of moisture in the soil above the permanent wilting point.
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Soil moisture depletion, the amount of moisture needed to take the soil up to field capacity. This would normally be the net amount of water you would apply at an irrigation.
Irrigation scheduling methods generally work with the last definition. This is because one of the questions that irrigation scheduling techniques try to answer is "how much to irrigate?". How much to irrigate is the soil moisture depletion at irrigation.
"Water budget" irrigation scheduling is the day-to-day accounting of all water going into and out of the effective root zone. The basic process is to assume a starting point, the soil moisture depletion at the start of a day. Then, the soil moisture depletion at the end of the day is calculated using the water budget equation . . .
SMDend =
SMDstart +DEEP +ETc -IRR
-RAIN -GWup
All this equation is saying is that if you start with a certain soil moisture depletion (SMDstart )
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Any irrigation is going to reduce the depletion (- IRR).
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Any rainfall that soaks into the ground is going to reduce the depletion (- RAIN).
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Any groundwater moving up into the root zone from a shallow water-table will reduce the depletion (-GWup ).
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Any crop water use (evapotranspiration, ETc) is going to add to the depletion (+ ETc ).
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And any irrigation or rainfall in excess of the field capacity will add to the depletion (+ DEEP).
For example, assume that you are starting the water budget on the day after a full irrigation. Thus the starting soil moisture depletion is 0, SMDstart = 0 inches. It didn't rain that day so RAIN = 0 inches. There was no irrigation because you just irrigated, so IRR = 0 inches. The crop water use (evapotranspiration, ET c ) for the day was estimated at .22 inches. And there is no shallow water-table, thus GWup is 0. The water budget equation then says that . . .
SMDend
=
SMDstart + DEEP + ETc -
IRR - RAIN - GWup
= 0 + 0 + .22 - 0 - 0 - 0
= .22
inches
At the end of the first day after irrigating, the soil moisture depletion is .22 inches.
As stated previously, the water budget equation is solved on a day-to-day basis. When SMDend is calculated to be at or over the allowed depletion, an irrigation is scheduled. The amount of irrigation is the SMDend (the soil moisture depletion at time of irrigation) plus any leaching requirements plus allowances for irrigation efficiency.
Effective, efficient irrigations are the result of knowing WHEN to irrigate, HOW MUCH to irrigate, and HOW to irrigate. Water budget irrigation scheduling helps to identify two of the three keys . . .
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WHEN to irrigate, when the soil moisture depletion is calculated to be more than the allowed depletion.
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HOW MUCH to irrigate, the calculated soil moisture depletion when you decide to irrigate. Detailed examples of water-budget scheduling are contained in the Appendix.
Last updated September 2000