Em4885 irrigation management practices to protect ground water

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CHAPTER 4
There are two major families of irrigation scheduling techniques. One is generally called
“checkbook” or “water budget.” With checkbook scheduling, the irrigation manager first
defines the depth or volume of root zone to manage. He/she then attempts to identify,
measure, and predict all water going into and out of this root zone. A common water
budget equation is:
D 2 = D 1 + ETc + DEEP - IRR - RAIN + UP
where:
D 2 = root zone depletion at the end of a day
D 1 = root zone depletion at the start of a day
ETc = crop evapotranspiration for the day
DEEP = deep percolation out of the root zone for the day
IRR = irrigation water added to the root zone that day
RAIN = effective rainfall for the day (that portion of the gross rainfall that infiltrates)
UP = upwards percolation of water into the root zone from a high water table
These factors are seen in Figure 4-8, a schematic of the crop root zone identifying the
various types of water moving into and out of it.
All variables are defined in terms of inches of water. An irrigation is called for when the
resulting total soil water, or soil water depletion, is predicted to reach some predetermined
level, the “management allowed depletion” (refer to the discussion in Chapter 3 regarding
soil-water-plant relationships). Checkbook irrigation scheduling will result in an estimate of
both the timing and amount of irrigations.
Irrigation - IN
Upward Movement from
a Shallow Aquifer - IN
Evapotranspiration - OUT
Rain/Snow - IN
Surface Runoff - O
Deep Percolation - OUT
FIGURE 4-8. Schematic indicating the different categories of water coming into and going out
of the effective crop root zone