Em4885 irrigation management practices to protect ground water

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CHAPTER 3
Salts leached from the upper root zone accumulate to some extent in the lower part,
but a salt balance is achieved as salts are moved below the root zone by sufficient
leaching. The higher salinity in the lower root zone becomes less important if adequate
moisture is maintained in the upper, more active part of the root zone and long-term
leaching is accomplished.
Restrictions on Use: The “Restrictions on Use” shown in Table 3-1 is divided into three
degrees of severity: non, slight to moderate, and severe. The divisions are somewhat
arbitrary since change occurs gradually and there is no clear breaking point. A change
of 10 to 20 percent above or below the guideline value has little significance if
considered in proper perspective with other factors affecting yield. Field studies,
research trials, and observations have led to these divisions, but the grower’s
management skill of the water user can alter them. Values shown are applicable
under normal field conditions prevailing in most irrigated areas in the arid and
semi-arid regions of the world.
Drainage
The leaching ratio equation previously presented calculates the percentage of applied
irrigation water that must pass through the root zone to maintain the soil water salinity
at a desired level. Thus, some deep percolation is required, desirable, and inevitable with
irrigated agriculture. The key question is where does this deep percolation go?
There must be sufficient internal drainage in the soil so that the required deep percolation
does not cause saturated conditions within the effective root zone. This drainage can be
natural. That is, the soil profile is such that the deep percolation continues downward, or
moves sideways, out of the effective root zone. The concern here is the effect of that deep
percolation on any ground water it reaches. Regardless of any nutrients or chemicals that it
may have leached out of the root zone, the deep percolation will always carry salts.
The other option for drainage occurs if there is insufficient internal drainage. This
situation arises when an impermeable layer of rock or clay soil occurs relatively near the
soil surface. With no other provision, deep percolation will create a saturated zone in the
soil that can “back up” into the root zone. In these situations, “tile” drain systems are
installed. These are systems of perforated, polyethylene pipe buried at various depths
and spacings. The perforations allow the deep percolation to enter the piping system.
The percolation is then gathered at collection points and pumped to the surface for
disposal. The key question in this situation is where is the endpoint of disposal?
The drainage pumped to the surface has the potential to contaminate surface waters
depending on point and method of disposal.
Again, salinity and drainage problems in irrigated agriculture can be complex. Growers
should consult experts and have laboratory tests performed to determine the best course
of action.