EVALUATION OF A MONITORING PROGRAM FOR ASSESSING
THE EFFECTS OF MANAGEMENT PRACTICES ON THE QUANTITY
AND QUALITY OF DRAINWATER FROM THE PANOCHE WATER
DISTRICT, WESTERN SAN JOAQUIN VALLEY, CALIFORNIA
INTRODUCTION
Shallow ground water that contains high concentrations of dissolved solids has adversely affected agriculture in western San Joaquin Valley, California. To help address this problem, subsurface drains were installed to lower the water table below the root zone. Discharge from these drains, which was impounded at Kesterson National Wildlife Refuge until 1985, contains high concentrations of selenium (Deverel and others, 1984; Presser and Barnes, 1984) that caused high mortality rates in waterfowl (Ohlendorf and others, 1986). Drainage-management strategies to decrease the quantity and improve the quality of discharged drainwater are described in the San Joaquin Valley management plan (San Joaquin Valley Drainage Program, 1990). Successful implementation of the plan requires monitoring of the drainflow and the response of the ground-water system to strategies in the plan. To accomplish this, the Monitoring and Assessment Subcommittee of the Interagency Technical Committee of the San Joaquin Valley Drainage Implementation Program developed guidelines for the collection and management of data to monitor ground-water and surface-water conditions in the western San Joaquin Valley (San Joaquin Valley Drainage Implementation Program, 1994). Monitoring activities recommended by the committee as well as those that have been implemented are shown in table 1. A key element of the guidelines is to ensure the efficient allocation of resources by making use of existing data-collection programs.
Several data-collection programs are being conducted by water districts in the western San Joaquin Valley, but little effort has been made to assess the usefulness of the data. Although the management decisions implemented by individual growers are unknown, changes in water management at the water-district level are documented. Deliveries of Federal and State Project water have been reduced substantially since the late 1980's because of climatic conditions and changes in project operations. The decrease in available surface water for irrigation has provided incentive to growers to improve water-conservation practices, implement new irrigation technologies, and alter the types and acreage of crops grown. However, pumping of ground water has increased in order to supplement imported water. These activities can affect the shallow ground-water system and the quantity and quality of the ground water intercepted by the drainage systems.
The purpose of this study, done by the U.S. Geological Survey (USGS) in cooperation with the California Department of Water Resources, is to evaluate the effectiveness of existing data-collection activities for assessing water-management practices and their effects on drainwater. The results of this study will help guide plans for the design, modification, and (or) implementation of data-collection and assessment programs for this and other areas of the western San Joaquin Valley. Surface-water, ground-water, land-use, and drainflow data at the water-district scale from 1986 to 1993 were used in this study.
Study Area
The study area boundary is coincident with the boundary of Panoche Water District in the central part of the western San Joaquin Valley (fig. 1). Average annual precipitation in the study area is about 230 mm/y (NOAA, 1994) measured at Los Banos, California. The 160 km water district is located on alluvial fan and interfan deposits in the vicinity of Little Panoche and Panoche Creeks. On-farm drainage systems consisting of perforated drain laterals, which underlie about 49 percent of the study area, typically are buried between 1.8 and 2.7 m below land surface, and horizontal spacing ranges from about 30 to 180 m (Fio, 1994). The topography generally slopes downward to the northeast and the drainage systems generally are located in the downslope parts of the study area. The location of the study area relative to the alluvial fans and the area containing subsurface drainage systems is shown in figure 2.
The Panoche Water district was selected for this study for four reasons: (1) The water district represents an intermediate step between the field and regional scales of observation recommended in the long-term monitoring plan, (2) it represents an effective management unit for implementing drainage-management strategies, (3) it has a good monitoring program relative to other areas of the valley, and (4) previous work has been done to evaluate the effects of the regional ground-water-flow system on drainflow (Fio, 1994; Fio and Leighton, 1994).
Geohydrologic Setting
The San Joaquin Valley is a large asymmetric trough that has been filled with as much as 9.7 km thickness of unconsolidated sediments (Page, 1986). In the western part of the valley these sediments were deposited by ephemeral and intermittent streams draining the foothills and Coast Ranges that border the valley. The resulting deposits are typically dominated by gravel and sand at the upper slopes and along stream channels, and by silt and clay at the fan margins and in areas of relatively gentle topographic relief (Laudon and Belitz, 1991).
The regional ground-water flow system in the San Joaquin Valley is divided into a semi-confined zone and an underlying confined zone separated by a thick clay layer (the Corcoran Clay Member of the Tulare Formation) (Page, 1986). The saturated thickness of the semi-confined zone is as great as 150 m in the central part of the western valley (Belitz and Heimes, 1990).
Previous studies in the same area assessed the effects of regional ground-water flow on dissolved-solids and selenium concentrations in on-farm drainflows (Fio and Leighton, 1994) and the contribution of regional ground-water flow to on-farm drainflow (Fio, 1994). These reports provide a detailed description of the geohydrologic setting in the study area.
The focus of this study, with respect to the ground-water system, is the upper 30 m of the semi-confined zone. This interval is composed of a series of discontinuous, interfingering coarse- and fine-grained deposits that dip down toward the east and the central part of the valley. These deposits are the principal source of dissolved solids and selenium in this and other areas of the western San Joaquin Valley (Tidball and other, 1986).
Acknowledgments
The technical and editorial review of this report was by Jim Baker, Tony Buono, Clark Londquist, Steve Phillips, Carol Sanchez , all of the California District, and Tim Rowe of the Nevada District. The authors thank these individuals for the special effort that went into the team-style review of the report. Also, a special thanks is made to Jerry Woodcox for his individual editorial review of the report.
