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Efficient on-farm irrigation.

Adding the right amount of water to sustain soil productivity

In arid and semi-arid regions of the world, where crops often thirst for rain, agriculture relies on irrigation to fulfill water requirements.

Yet applying too much or too little irrigation water can increase soil and water salinization and reduce crop yield.

Irrigation infiltration that exceeds the soil's natural drainage capacity can cause a shallow groundwater table to rise. This can have adverse effects on crops due to increasing salinity and water logging in the root zone. Deficient irrigation also can increase soil salinity concentration caused by crop evapotranspiration and lack of adequate leaching.

Similar physical and chemical processes that increase salt in soil and water occur naturally over large areas of the earth and are termed "primary salinization." "Secondary salinization", is the increased salinity in soil and water resulting from inefficient irrigation.

Secondary salinization also occurs on non-irrigated lands where deforestation and reduced evapotranspiration causes a shallow groundwater table to rise.

Worldwide, the area of primary salinization far exceeds the area of secondary salinization. Yet secondary salinization adversely affects 20% to 25% of the world's irrigated agricultural land. Without appropriate management, up to 50% of irrigated land may be affected in the near future.

In some cases, soil salinity can reach levels that limit the growth of even the most salt-tolerant crops. Salt must then be removed from the soil to restore productivity.

Secondary salinization of the best agricultural land results in the shift of agriculture to less fertile land, which when irrigated may be even more prone to salinization.

The problem is not a new one. Increased soil salinity may have caused the decline of several ancient civilizations, such as those in Mesopotamia.

Sustaining existing sod productivity depends on maintaining a longterm salt balance in the crop root zone. Saline soil reclamation requires longterm leaching of salts using soil amendments, large amounts of water and effective drainage systems. which can be a costly process.

Even in areas where reclamation is otherwise feasible, increased demands for water limit water availability for leaching.

Another concern is toxic concentrations of naturally occurring trace elements, such as selenium, boron, arsenic, molybdenum and uranium, which may also occur with salt in the dram water. High concentrations of elements such as selenium in drainage can be hazardous to wildlife, as evidenced in California's San Joaquin Valley. Drain water containing selenium and other trace elements increases the challenges to irrigation and drainage management.

Salt balance in the root zone is achieved through an optimum seasonal on-farm irrigation efficiency (IE). An efficiency above or below the optimal value results in increased salinization. The amount of irrigation water needed for optimum yield and maintenance of soil productivity is determined by crop consumption, leaching requirement, and other beneficial uses such as frost protection and weed control.

Crop water consumption is the amount of water needed for plant evapotranspiration with optimum yield. The leaching requirement is additional water needed to infiltrate the crop root zone and wash out accumulated salt. The leaching requirement is dependent on the salinity of the irrigation water and tolerance of the most salt-sensitive crop in the rotation.

For example, in a cotton/tomato rotation, the more salt-sensitive tomato crop establishes the annual baseline soil salinity for calculating the leaching requirement.

In California, the growing demand for water, combined with recurring drought, puts pressure on farmers to improve irrigation efficiency and maximize beneficial use of water. Recently, public policy has focused on increasing irrigation efficiency as a tool to make agriculture water available for other uses.

The California Department of Water Resources, in cooperation with other agencies and experts, has developed a new approach for determining optimum on-farm irrigation efficiencies for California irrigated farmland. Leaching requirements, among other beneficial uses of water, were incorporated in calculations for irrigation efficiency. The new approach adds non-uniformity in water application to the irrigation requirements.

A difficulty arises over the definition of irrigation efficiency, as specialists use many different methods. Often, reported irrigation efficiencies from different sources are not comparable because different calculation methods are used. A high irrigation efficiency does not always imply good irrigation management. Currently, there are efforts to develop a standard method to estimate IEs.

Differentiation must be made between using irrigation efficiency for planning and design, and using it as a project or system evaluation tool. In planning and design, the input data are estimated based on experience, science and manufacturing specification, as in pressurized systems, to develop adequate irrigation designs for optimal production. When evaluating an operating irrigation system, the goal is to determine whether the irrigation system's performance and management has achieved planning and design numerical criteria. An important distinction is that irrigation uniformity in the planning stage is an assumed value, while in the evaluation phase it is a measured value.

In a given field, over-irrigation and under-irrigation may occur simultaneously. Growers tend to apply a volume of water over the entire field to be sufficient for the driest part. This generates optimum yield in the dry area. while other areas become over-irrigated.

In 1978. ASAE adopted the term distribution uniformity (DU) to describe the ratio of the lowest average one-quarter field depth of infiltrated applied irrigation water to the average full field depth of infiltrated applied irrigation water, expressed as a percentage. This implies that 87.5% of a field (Figure 1) would have at least adequate irrigation, while the remaining 12.5%, would be slightly under-irrigated.


The 12.5% under-irrigated area may be spatially and temporally variable, preventing localized salinization. In the case of surface irrigation, the 12.5% under-irrigated area may occur in different parts of the field during each irrigation season depending on soil type, field preparation, irrigation system layout, crop rotation, and tillage practices. Eliminating the 12.5% under-irrigated area would cause over-irrigation in the remainder of the field and decrease seasonal on-farm irrigation efficiency. The benefit/cost ratio for other agronomic parameters. such as fertilization, would also be lowered.

When an entire field is irrigated with no under-irrigation, with 80% DU and determined leaching requirement, the seasonal on-farm irrigation efficiency can not exceed 73%. However, with a 12.5% under-irrigation, and an 80% DU with the same leaching requirement, the seasonal on-farm irrigation efficiency can not exceed 80%. Figure 2 shows the relationship between IE and DU.


Based on data and experience, an IE of between 73% and 80% is the practical target to be achieved and maintained. In California, based on thousands of field evaluations of many different irrigation systems an 80% DU has been determined the most practical and attainable average value under good management. Within the 73%-80% range of IE, long-term sustainability of soil quality and the soil resource for agricultural production can be achieved.

Fawzi Karajeh is program manager for Agricultural Drainage Reduction and Reuse, Department of Water Resources, Division of Planning and Local Assistance, Office of Water Conservation, 1020 Ninth St., Sacramento, CA 95814, USA, 916-327-1828, fax 916-327-1815, fkarajeh

Baryohay Davidoff is chief for Agricultural Water Conservation Unit, California Department of Water Resources, Division of Planning and Local Assistance, Office of Water Conservation, 1020 Ninth St. Sacramento, CA 95814, USA, 916-327-1766,
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Title Annotation:putting right amount of water to soil
Author:Karajeh, Fawzi; Davidoff, Baryohay
Publication:Resource: Engineering & Technology for a Sustainable World
Date:Sep 1, 1997
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