Em4885 irrigation management practices to protect ground water

3 CHAPTER
Specific fates of water applied to a field due to irrigation or rainfall are illustrated in Figure 3-3.
They include:
1. Immediate evaporation - some water will evaporate immediately during an irrigation or
rainfall. Evaporation losses during a sprinkle application may range from 6 to 15% or
more depending on temperature, humidity, and wind conditions.
2. Surface runoff - if applied water does not infiltrate into the soil it will run off the surface.
This runoff may go into a creek or stream, or may be picked up by a runoff reuse system
on the farm, or on another farm downstream.
3. Deep percolation - water that infiltrates the soil may be used by the crop to become
evapotranspiration, or may percolate below the root zone. The deep percolation could
end up in a usable aquifer for later pumping and reapplication to the farm.
4. Root zone storage - eventually will be used by the crop and evapotranspired to the
atmosphere.
A major Overall Management Objective for growers is to minimize the amounts of surface runoff
and deep percolation. Surface runoff can be an important detachment mechanism depending on the
erosivity of the soil. Deep percolation and surface runoff are the primary transport mechanisms
causing contamination. They move sediment, chemicals, and fertilizers from the field to surface and
ground waters.
Minimizing deep percolation and surface runoff (or its control) is the result of proper
management of the irrigation to achieve good distribution uniformity and control the total
application of water. Explanations of these two objective measures of irrigation performance,
distribution uniformity and overall application efficiency are given in the discussion of Overall
Management Objective 2.00 in Chapter 4. Discussions of the specific management concerns and
techniques for each of the major irrigation systems are also included there.
Basic Soil-Water-Plant Relationships
Processes in the hydrologic cycle include infiltration, percolation, and evapotranspiration.
An intermediate factor that is important to irrigated agriculture is the ability of soil to store water.
That is, soil can effectively stop the movement of water through the hydrologic cycle. The science
of soil-water-plant relationships provides an objective analysis of these phenomena.
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