Journal of Irrigation and Drainage Engineering

The time it takes for deep percolation water from irrigated fields to reach underlying groundwater increases with decreasing particle size of the vadose zone material and increasing depth to groundwater. For average deep percolation rates, decades may be required before the water joins the groundwater. Due to nonuniform irrigation applications and preferential flow, some deep percolation water will reach the groundwater much faster. Dissolved salts, nitrate, and pesticides are the chemicals in deep percolation water of main concern in groundwater pollution. Movement of pesticides may be retarded in the vadose zone, but biodegradation may also be slowed due to reduced organic carbon content and microbial activity at greater depths. Because of the large area of irrigated land in the world and the real potential for groundwater pollution, more research is necessary on downward movement of water and chemicals in the vadose zone.

Deep percolation of irrigation return flow is a major source of recharge beneath most irrigated areas in California. Tile drainage, soils, water in the vadose zone, and shallow groundwater have been studied. Nitrate, salinity, and several pesticides have received the most attention. Numerous parts of the San Joaquin Valley have been investigated, as well as parts of the Sacramento Valley, Imperial Valley, Los Angeles Basin, and several other valleys. The results of the studies indicate that irrigation return flow usually exerts a substantial impact on groundwater quality. High nitrate contents in groundwater beneath irrigated areas are often a result of irrigation. In addition, extensive pollution of shallow groundwater in parts of the San Joaquin Valley have been caused by use of the pesticide DBCP.

Irrigated agriculture accounts for about 90% of all water consumption in both Arizona and New Mexico. More than 50% of this water is pumped from groundwater sources. Some portion of the applied irrigation water is returned to the groundwater supply through deep percolation. Several field studies have been conducted in these states to measure the quantity and quality of water that is recharging the aquifer. These studies indicate that groundwater quality in Arizona and New Mexico has been deleteriously affected in deep aquifers and in shallow aquifers that are hydraulically connected to surface water supplies. The magnitude and time rate of groundwater quality changes is a function of irrigation management practice, fertilizer and pesticide applications, quality of irrigation water, rate of groundwater level decline, presence of perched zones that intercept percolating water, proximity to surface water supplies, leakage through and along well casings, and soil salinity.

Groundwater contamination is a serious water quality problem since 50% of the population derives drinking water from groundwater sources. Severe water quality problems have resulted from deep percolation under irrigated areas, carrying high salt loads and increased nitrate concentrations to groundwater aquifers. This review looks at studies that have dealt with the effect of deep percolation from irrigation on groundwater quality. Recommendations are developed to minimize the undesirable effects of groundwater contamination by irrigated agriculture. Major among the conclusions is that improved and/or more efficient irrigation and fertilizer management practices are required to minimize the adverse impact of irrigated agriculture on the quality of underlying groundwater. When water and nutrient application and use efficiencies are improved, deep percolation is decreased and less downward movement of pollutants occurs. Structural improvements in irrigation systems are a necessary aid to achieving the desired level of management.

Results of field and laboratory studies performed over the past 20 years in Idaho, Montana, Nevada, Oregon, Utah, and Washington are reviewed. The studies varied in number, intensity, and breadth from state to state, but none was able to demonstrate conclusively the effects that agricultural irrigation had or was having on the quality of underlying groundwater, the effects that were sought both by these reviewers and by the original investigators. It is concluded that sampling projects to determine these effects must follow rigorous salt balance investigation schemata, that such projects are and will be expensive and time‐consuming, and that the state‐of‐the‐art techniques for gleaning true knowledge about waters within the earth are still emerging. Nonetheless, the review also leads to the inescapable conclusion that western U.S. groundwater quality has deteriorated in many places, and agricultural irrigation practices are strongly implicated as contributing sources of that decline.

A control device has been developed for canal structures, which can be used to control upstream water levels, downstream water level, or off‐take canal discharges. The system uses no electrical power or electronics. It is powered by the available drop in water level across the structure. It can control water levels to within a very narrow band [$\pm 3\mathrm{mm}$ (0.01 ft) for the systems tested], which is not feasible with electrically controlled motorized gates. Since this level of water surface control is often necessary for accurate flow rate control when weirs and flumes are used, this device, called dual‐acting controlled‐leak, or DACL, is particularly suited to control of canal flow rates. The DACL system was studied in a hydraulics laboratory to determine the relevant factors affecting control. A field test was conducted on a radial gate [1.9‐m (6.2‐ft) radius] to study the system's reliability in actual field operation.

This paper presents some results of calculations performed with a numerical model for unsaturated soil moisture flow. It shows the possibility of using different boundary conditions to obtain the daily moisture content of a soil profile located in the middle between two ditches. Results obtained with the numerical model generally show good agreement with measured field data. After determining only a few standard soil, plant, and meteorological parameters, it is possible to describe the changes in the moisture content of the soil profile during the growing season. Application of the model makes optimum water management in drainage‐subirrigation systems possible by controlling the water level in the ditches.

Discharge characteristics of sharp and broad‐crested triangular side weirs have been experimentally investigated. Relations between discharge coefficient and main channel Froude number for different apex angles have been established.

In open channel systems the phenomenon commonly referred to as clogging is caused by settling and straining of suspended sediment as water moves through the channel bed material, as well as by microbial transport, colonization, and other biological and chemical action. This paper summarizes a three‐year investigation of the clogging process due to accumulation of fine soil particles as it occurs in sediment‐laden open channel flow. Significant results include experimental relationships developed between major independent variables (i.e., flow velocity, suspended sediment concentration, and suspended sediment particle size distribution) and the infiltration reduction occurring through a sand bed filter in the bottom of a recirculating laboratory flume. Factors affecting both magnitude and temporal variation of channel bed infiltration are explained as well as observed differences in the physical nature of the clogged layer developed atop the filter bed. Significant features of sedimentary clogging in open channel flow are also correlated with traditional hydraulic and sediment transport variables and processes. In addition, a dimensional analysis of sedimentary clogging is presented as a focus for future experimental investigation.

The uniformity of irrigation systems is important to efficiency, yield, and economics. Wind strongly affects this uniformity. A method is presented for simulating the operation of a sprinkler system in wind. Equations describing the motion of airborne water droplets are shown. The trajectories of water droplets ejected from a sprinkler were numerically computed. Composite results led to predictions of application patterns. Sprinkler droplet size distribution was used to predict the pattern around a sprinkler, and patterns were superimposed to represent a set (not continuously moving) system. Coefficients of uniformity were then computed. The model was validated by comparing predictions with observed application patterns. Individual and multiple sprinkler tests were compared. The simulation system appeared to be an effective predictor of sprinkler performance in wind. Use of this type model can lead to improved sprinkler designs, although variability of the wind vector affects the accuracy of prediction.

The Sevier Desert and Sanpete Valley are two subbasins within the Sevier River System in south‐central Utah in which groundwater is extensively used in conjunction with surface water for irrigation. The conjunctive use has developed more by accident than by deliberate planning. The surface‐water supply in the two areas was basically developed by 1920. Severe droughts in the early 1920s and early 1950s focused attention on groundwater development as a supplemental supply. Well construction gradually increased between the mid‐1940s and 1960 in both basins until the problems of water rights and overdraft essentially halted further development. Groundwater pumpage in both the Sevier Desert and Sanpete Valley has fluctuated inversely with the available surface‐water supplies and climatic conditions. Overdraft has not occurred and the sustained yield of the groundwater basins seems to have been established.