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Vertical Drainage May Improve Soil Salinity and Moisture

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Vertical Drainage May Improve Soil Salinity and Moisture tion and government programs. Statis- tical analysis of cotton revenue over time reveals price variability to be the Vertical drainage may improve primary source of income risk, while yield variability contributed very little soil salinity and moisture to income risk. The proper choice of risk-management tools then targets price risk rather than yield risk. The Abdul Karim Yusufzai D Mark E. Grismer two tools that focus on price risk are hedging and forward contracting. The survey results show that 25% of cotton growers use hedging as a risk-man- Existing drainage systems in lateral drainage systems may be exac- agement tool, while only 1.6%report many clay fields of the Imperial erbated by shallow fine-sand aquifers, using crop insurance. This illustrates Valley have failed to improve soil which are a source of artesian water that producers make appropriate salinity and to provide moisture into the clay. Lateral drainage systems choices among risk-management tools conditions favorable to crop were not designed for these condi- when the source of risk has been iden- growth. In some fields, these tions, although several drainage stud- tified and several risk-management al- problems are exacerbated by ies from the 1940s and 1950s describe ternatives exist. the widespread occurrence of the fine- saline artesian water from a sand aquifer. This artesian water re- shallow sand aquifer. This pilot- Awareness increasing sults in relatively high soil moisture in Awareness of risk issues is increas- scale field study in the Imperial the clay soil profile, and in progressive ing among California producers. Al- Valley indicates that vertical salinization of the root zone. though producers regard many drainage is more effective than Although they have been only sources of risk as relevant, it appears traditional tile systems in briefly considered as an alternative in from their responses that the attention reducing artesian water levels and the Imperial Valley, vertical drainage of lenders is making financial risk the overlying clay soil moisture, systems have been successfully in- paramount among the current risk and should over time also reduce stalled in other semiarid to arid re- concerns. The tools available to pro- the salinity of these soils. The gions such as the Patterson area of ducers to help manage particular risk cost of a widely spaced drainage Stanislaus County, the Salt River Val- sources vary in their effectiveness and well system appears comparable ley of Arizona, and parts of the Red availability, and therefore in popular- to “splitting” existing drainlines. River Valley of North Dakota. This re- ity of use. Many producers indicated port evaluates the potential for and that they would be interested in using Drainage systems are commonly used feasibility of developing a shallow ver- some of the less available tools, such in arid irrigated regions to promote tical or well drainage system for the as hedging and crop insurance, if the crop growth by controlling water-table clay soils overlying fine-sand aquifers tools were available for their crop or depth, root zone salinity and soil aera- in the Imperial Valley as a means of livestock enterprise. This indicates la- tion. The Imperial Valley is extensively reclaiming or improving these soils for tent demand for these tools. drained with both open ditches and crop production. We also compare the The limited availability of effective lateral (tile) drainage systems that are costs associated with lateral and well risk-management tools severely limits designed to provide relief from shal- drainage systems and consider some many producers’ ability to mitigate low water tables. As a result of low of the benefits and drawbacks associ- risk. At a time when California pro- permeability, the lateral drainage sys- ated with each system. ducers as a whole are becoming more tems are relatively ineffective in many aware of risk concerns, this shortage of the heavy clay soils that make up Imperial Valley setting poses obstacles to risk management. over 40% of the irrigated valley (Cali- The Imperial Valley is a highly Until tools like crop insurance and fu- fornia Agriculture, May-June 1988). Im- stratified alluvial valley with an arid tures markets are better tailored to the proving such drainage systems may be climate characterized by an average needs of producers in California, and unfeasible because of the high costs as- annual rainfall of approximately 3 until producers become better in- sociated with narrowly spaced inches, high summer temperatures, formed about managing income risk, drainlines. Nevertheless, during the low relative humidity and abundant the state’s agricultural sector will face past 3 decades growers in parts of the sunshine. The near-surface layers to unnecessarily high levels of financial valley have ”split” the original drain depths of 300 feet alternate between stress. spacing in the clay soils in an effort to sands, silts and clays that interfinger improve their efficacy, with little and are cross-bedded in formation. S.C. Blank is Extension Economist and 1. documentation that any improvement Soil boring by the USGS near the city McDonald is Research Assistant, Depart- was achieved. of El Centro indicates that the silty ment ofAgricultura1 Economics, UC In our previous work in the area, clay and clay surface soils of the area Davis. we found that the poor performance of are underlain by sands at depths rang- 12 CALIFORNIA AGRICULTURE, VOLUME 49, NUMBER 2 ing from 13 to 55 feet of land surface and thicknesses of no more than 180 feet. Development of this sand aquifer as a potential domestic or industrial source of water is limited by the rela- tively high salinity of the groundwater (5 to 10 times that of available Colo- rado River water) and the lack of need for additional water supplies. How- ever, it was found that the transmissi- bility of the sand aquifer is moderately high, on the order of 100 to 10,000 gal- lons per day per foot (gpd/ft), and that production wells could easily yield in excess of 100 gallons per minute (gpm). Similarly, in a detailed study at the UC Desert Research and Extension Center (DREC),referred to locally as the Meloland area, we found a fine-sand aquifer underlying the clay soil at depths of less than 6 feet of land surface, with a thickness in excess of 20 feet and a transmissibility of roughly 1,500 gpd/ft. The salinity of the shallow groundwater ranged from 5 to 6 dS/m (decisiemens per meter equivalent to the old unit of millimhos per centimeter). In both cases the transmissibility of the sand aquifer, although not excep- tionally large, is sufficient to develop vertical drainage systems. However, the groundwater quality is relatively poor. With careful management this groundwater could be applied to salt- tolerant crops, or blended with Colo- In many clay fields of the Imperial Valley, vertical drainage systems may be more effec- rado River water, but eventually the tive than existing lateral drainage systems. This pilot vertical drainage system was less expensive to install and its net annual cost was comparable to traditional tile drainage pumped groundwater would require systems. disposal via the surface drainage ca- nals to the Salton Sea. which to compare the performance of From the salinity control perspec- This study was prompted in part by different vertical and lateral drainage tive, it is appropriate to gauge the per- the fact that soil salinity in a clay field systems. formance of the drainage system in (area 70 of the UC DREC) has re- terms of the efficiency with which the mained practically invariant over the Drainage system Performance system extracts root zone drainage, past 30 years of record despite con- The design objective of both lateral where root zone drainage is that frac- tinuous lateral drainage at a depth of 6 and vertical drainage systems is to im- tion of the applied water not used by feet and several leaching studies in- prove root-zone soil aeration and salin- the crop or lost by evaporation. This volving continuous and intermittent ity through control of the water-table efficiency depends on several factors ponding and excess irrigations. The depth in the soil. The performance of related to the hydrogeologic setting of soil profile salinity reaches a maxi- these systems is typically evaluated in a lateral drainage system, such as mum value at a depth of roughly 5 terms of the extent to which control of drain depth and spacing, soil perme- feet, corresponding to the pressure the water table at a desirable depth for ability and aquifer depth. The drain- head (water elevation) of the artesian crop production is achieved. Unfortu- age efficiency of lateral drainage sys- aquifer. Because we had a consider- nately, the criterion of water-table tems can be very low, with the larger able data set for this site and we found depth may not adequately describe the fraction of the root zone drainage by- it to be representative of many areas in performance of the drainage system in passing the drainlines. In a vertical the valley, we used the characteristics terms of salinity control (Culiforniu Ag- drainage system, the extraction wells of this site as our base conditions with riculture, November-December 1990). control the shallow groundwater sys- CALIFORNIA AGRICULTURE, MARCH-APRIL 1995 13 tem in such a fashion as to capture the Although similar in concept to the creasing spacing, and the design prob- root zone drainage, as well as some re- lateral system design, vertical or well lem becomes one of selecting a system gional groundwater flows. Despite the drainage-system design involves more with adequate performance in terms of differences in performance, both lat- complex equations or computer mod- water table and salinity control for the eral and vertical drainage systems de- eling and the choice of either different least cost.
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