Em4885 irrigation management practices to protect ground water

CHAPTER 5
Reducing availability
The application of potentially polluting nutrients and chemicals should be minimized.
Applications should be timed and applied correctly for maximum efficiency of use.
Less persistent chemicals should be used whenever possible. Integrated Pest Management
principles should be incorporated as possible.
Reducing detachment
Check that tillage practices are appropriate when cropping erosive soils. Cover crops
should be considered where applicable. Check that water velocities in furrows are not high
enough to cause excessive erosion. Possibly there are less soluble forms of the chemicals
and nutrients used. There should not be excessive surface runoff under sprinkle or trickle
irrigation systems.
Reducing transport
Important transport agents are wind and surface runoff and deep percolation from
excessive irrigations or unexpected rainfall. If the problem is wind or surface runoff
then look to Practices that will reduce soil erosion. Minimizing deep percolation and
surface runoff requires maximizing irrigation performance. Look for good distribution
uniformity first. Then ensure that there is sufficient control over the system to be able to
control the total application of water. Practice some form of irrigation scheduling so that
there is an accurate estimate of the required irrigation application.
Determining reasonable goals
In many situations zero pollution is not an economic goal. The antidegradation policy
that is the law of Washington state is not a non-degradation policy. Note that part of this
process also involves setting reasonable yield goals. (Setting an unreasonable yield goal
may result in excessive applications of nutrients or other chemicals, thus increasing the
availability of potential contaminants.)
Determining appropriate Implementation Practices
There are certain Practices that should be a part of every water quality program.
For example, deep percolation or excessive surface runoff from inefficient irrigations
are prime detachment and transport mechanisms. A formal evaluation of irrigation system
performance should be a priority Practice to implement. Soil and plant tissue analyses to
determine fertilizer requirements is another Practice that should be implemented,
especially in areas of coarse-textured soils, shallow aquifers, and growing crops
requiring large amounts of fertilizer.
Key questions to ask when determining which Practices to implement include:
1. “Will this Practice reduce the specific aspect of availability, detachment,
or transport that is a problem?”
2. “Will implementing this Practice cause another problem which would require
another Practice to be implemented simultaneously?” A classic example would
be increasing furrow flows to increase distribution uniformity. This will increase
surface runoff which would imply the need for a runoff reuse system also.
3. “Is this Practice the most economical that can be used to reduce the problem?”
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