Water-level elevations and direction of groundwater flow in Borrego Valley, California, for 2010 (2010 data are modified from California Department of Water Resources Southern Region Groundwater Basin Assessment).
From the time the valley was first settled, groundwater has been the main source of water. Before the valley was developed, long-term natural recharge and discharge were in dynamic equilibrium. Once the valley was settled, substantial changes to the amount, distribution, and type of discharge from the valley began. The valley has gradually been transformed to include farms, residential homes, and golf resorts (California Department of Water Resources, 1984b). Since the early 1950s, pumping for irrigated agriculture, golf courses, residential and commercial uses, has required more groundwater than is available through natural recharge. Groundwater-level declines during 1945-2010 were as much as 2 feet per year in wells in the northern part of the valley, where groundwater is intensively pumped for irrigation agriculture.
In an effort to address the issue of groundwater availability in the valley, the Borrego Valley Hydrologic Model (BVHM) was developed (Faunt and others, 2015). The BVHM is a tool that can be used to evaluate the effects of temporal changes in recharge and pumping and to compare the relative effects of different water-management scenarios. Overall, the development of the BVHM, along with data networks and hydrologic analysis provide a basis for assessing groundwater availability, and potential water-resource management guidelines.
Groundwater recharge to the Borrego Valley comes from both natural and human sources. The primary source of natural recharge to the basin is infiltration from the ephemeral streams and washes emanating along the western and northern boundaries of the valley. The source of water to these streams is precipitation and runoff from the San Ysidro and Santa Rosa Mountains (fig. 1). Other potential sources of natural recharge include direct infiltration of precipitation, lateral groundwater underflow from adjacent bedrock areas and groundwater basins, and groundwater flow across the Coyote Creek fault, all of which probably are small or negligible in quantity.
New, human-induced sources of recharge accompanied development in the basin, including irrigation-return flow from agricultural fields, golf courses, and municipal lawns, and infiltration of treated and untreated wastewater.
Simulated annual groundwater pumpage and climatic patterns from the Borrego Valley Hydrologic Model, Borrego Valley, California, 1945-2010, by water use.
Before the Borrego Valley was developed by settlers and farmers, groundwater discharge consisted of evapotranspiration by mesquite trees and other native vegetation, discharge from Borrego Spring (Mendenhall, 1909), and lateral groundwater underflow that left the basin across the southeastern boundary of the valley. Currently, groundwater discharge occurs in three primary forms:
Natural discharge from evapotranspiration ranges from approximately 6,500 acre-ft/yr prior to development to virtually zero in the last several decades (1990-2010) because the groundwater levels in the basin dropped below the reach of the mesquite in the basin. Underflow out the southern end of the basin is small and relatively stable over time, at about 500 acre-ft/yr. Groundwater pumpage for agriculture and recreation was estimated on the basis of irrigated acreage and consumptive-use data. Values of pumpage for municipal supply were compiled from water-use records. Estimated combined annual agricultural, recreational, and municipal pumpage peaked at around 19,600 acre-ft from 2005 to 2010.
Cumulative change in groundwater storage for two water-management scenarios simulated by using the Borrego Valley Hydrologic Model, Borrego Valley, California, 1945-2060.
Cross section showing the simulated groundwater-level tables for 1945 and 2010 and for two management scenarios projected for 2060, Borrego Valley Hydrologic Model, Borrego Valley, California
The calibrated BVHM was used to simulate the response of the aquifer to six future 50-year (2011 to 2060) pumping scenarios. Two of these scenarios (1 and 6) are summarized here. Results from Scenario 1 (continuation of current (2010) annual pumpage) indicated that the drawdown observed since pre-development would continue, with a total depletion in groundwater storage of about 1,000,000 acre-ft by 2060. Consequently, the water table declines into the middle aquifer in some areas. Because of the lower hydraulic conductivity and storage properties of the middle aquifer relative to the upper aquifer, continued pumping at these rates would result in larger, more rapid groundwater-level declines in the future and possibly a reduction in groundwater quality.
The California Sustainable Groundwater Management Act (SGMA) of 2014 requires basins to reach sustainable yield. As human activities change the system, the components of the water budget (inflows, outflows, and changes in storage) also will change and must be accounted for in any management decisions. Because there currently is little captured recharge or discharge, in this system 'sustainability' is a maximum amount of discharge that avoids future groundwater-storage depletion, and is being simplified and equated to this average recharge. In the long run, the average change in groundwater storage would be negligible when the basin is operated at the sustainable level; however, groundwater levels and storage changes would continue to fluctuate as they have historically with climatic variability.
In order to simulate a realistic approach for meeting SGMA requirements on the 20-year SGMA timeline for implementation, in Scenario 6 in the BVHM, municipal and recreational pumpages both were reduced to 50 percent of current (2010) rates, and agricultural pumpage was reduced to 40 percent of current rates. These reductions were applied linearly over 20 years and continued for the next 30 years until 2060. With these reductions, at 2060, recharge approximates discharge. Simulated drawdowns are approximately 50 feet, over a broad part of the basin. Drawdown and groundwater storage losses continue in areas where agricultural, recreational and municipal pumping occurs. In the long run, groundwater levels would stabilize and would not decline as they do in the Scenario 1 simulation which had continued significant groundwater-level and storage declines. However, changes in groundwater storage would fluctuate with climatic variability.