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Water Quality Data
SALINITY AND DRAINAGE
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Drainage
There are two things needed to maintain a salt balance. One is a sufficient, good-quality water supply. But there must also be sufficient internal drainage. There has to be some place for the necessary deep percolation to go.
Problems occur, and are occurring in large portions of the District, when there is no place for this leaching water to go. Unfortunately, much of the District is underlain by what is know as the "Corcoran" clay. This is a geologic formation consisting of a relatively impermeable layer of clay that lies from 50 to 200 feet below ground. Figure SD-2 is a generalized profile view of the soil in the District on an east-west line.
What has happened over a number of years is that excess water applications, including the deep percolation required for maintaining a salt balance in the soil (and thus, maintaining production), has "perched" on this clay layer until the saturated zone has moved back up into the root zone. Thus, a high water table is formed.
This causes several problems. The perched water table is usually of poor quality and salts are drawn up into the root zone from this table, thus, increasing the salinity of the soil. Root pruning occurs as the water table reduces the effective root zone. And obviously, there is no place for the required deep percolation to maintain the salt balance.
Normally, in these situations, artificial drainage (tile drains) are installed. The excess deep percolation is drawn off the field by the drains and pumped out of the ground. The resulting water table would be drawn down near the drains and thus would cause the water table to be drawn down between the drains. The drains are laid at such a depth and spacing to produce the minimum required root zone at the mid-point between the drains while providing enough drainage to carry off the deep percolation.
The problem then becomes of disposing of the pumped tile drainage. Unfortunately, this tile drainage is of very poor quality and requires the presence of a "salt sink" somewhere to put the salty drain water without harming anyone. In the Imperial Valley, which has many of the same problems as the District (additionally having to use Colorado River Water at some 1000 ppm+ TDS), that salt sink is the Salton Sea. To the north of Westlands many Districts drain to the San Joaquin River and thus, eventually to the Ocean. The Westlands Water District has no such outlet. However, with very careful irrigation management, it has been seen that enough water will percolate through the restraining clay layer to allow continued successful production. [say something else about the District programs?] This careful management consists of restricting deep percolation to the absolute minimum needed to maintain a salt balance and choosing a crop rotation and cultural management regime that minimizes the required percolation.
FIGURE SD-2: Cross-Section of Westlands
Management Techniques
Salinity problems can frequently be managed to minimize reductions in crop yield with proper crop selection and irrigation management. The following management techniques deal with existing salinity problems and conditions. Always consult a qualified agricultural scientist when analyzing and treating salinity problems.
Salinity Testing - The extent and severity of a salinity problem must be determined before proper farm management decisions can be made. The salinity problem can be identified by testing soil samples for various salinity related factors. General guidelines for collecting soil samples can be given but conditions vary and individual situations must be consider when interpreting test results.
Possible salt affected areas are generally identified by plants that appear stressed or have low production. Several sampling sites should be selected for each problem area. Soil samples should be taken from the entire effective root zone. Sampling intervals may range from one- to two-foot increments in depth and extend down six or more feet. The salinity level in the top three inches of the seed bed is critical for germination and seedling development. Sampling the field outside the problem area need not be extensive, but is necessary for comparison purposes and to see if the problem is spreading.
General salinity estimates of the field can be made by sampling 20-acre blocks, if conditions are similar over the entire area. Additional sampling sites may be necessary in fields with several different soil types or layers.
When designing a testing program be aware of the effects of irrigation distribution uniformity on salt distribution in fields. If the lower end of a field is continually under-watered, there will not be as much leaching water applied in that area and salts may build up. [comments from a Nat Dellevalle on salinity testing programs?]
Westlands Water Conservation and Management Personnel will determine the EC of the saturated extract of soil samples brought to the District's Five Points Shop and Field Office by Westlands' water users. The EC of water samples taken from perched water tables or drains will also be determined. The sample must be identified as follows: (1) water user, (2) sampler, (3) date, and (4) location.
Other measurements that would be needed to identify salt-based problems would be . . .
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Measurements of the individual salts present (how much calcium, how much magnesium, etc.).
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Measurements of the different forms of fertilizer salts present (nitrate and ammonium nitrogen, phosphate phosphorous, and potassium).
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Boron.
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pH.
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SAR, sodium absorption ratio, (calculated from the individual measurements of sodium, calcium, and magnesium salts).
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Last updated September 2000