by Gus Tolley
The Scott River is one of California’s four major undammed streams and important spawning habitat for coho (a species listed as “threatened”) and Chinook salmon. This peaceful and pastoral agricultural valley is at the center of several water-related conflicts and lawsuits. However, it is also pioneering a range of instream flow and groundwater management activities that could set the example for balanced water use in California.
At first glance, water management in the Scott Valley appears to be a story of farms vs fish, one that is common in California: A dry year results in dry stream reaches near groundwater-irrigated fields in August that persist beyond the irrigation season into September, even October. With the Chinook fall spawning migration arriving in mid-October and coho following soon after, a dry streambed raises valid concerns about how irrigation pumping and the removal of riparian vegetation may have led to warmer and drier streamflow patterns in Scott Valley.
The story behind the valley’s seasonally dry streams is complex. Irrigated pasture, alfalfa, and cattle production have been part of the socioeconomic fabric since it was settled by white people during the mid 1800s. Current dry conditions are partly due to legacy impacts from historical land management policies, flood control, and gold mining, along with natural climatic and geologic variations.
On average, the annual discharge of the Scott River is more than five times the total evapotranspiration demand from the 34,000 irrigated acres in the valley; seemingly enough water for fish and farming. But – like elsewhere in California – surface water supplies usually dry up around July when the last of the snowmelt in the upper watershed disappears. Streamflow from mid-summer until the beginning of the rainy season largely depends on baseflow from the valley aquifer. Fed by recharge from tributaries, rainfall, and excess irrigation, the aquifer acts as a large, sponge-like reservoir that provides a steady contribution to streamflow in the summer and early fall.
In the 1970s, average late-summer streamflow in the Scott River decreased markedly by about 50%, resulting in increased stream temperatures and – during dry years – more pronounced dry stream reaches along the lower valley floor. How did the Scott River get to the condition it is in today?
A driving factor for the change in average summer flows may be a switch from reliance on surface-water prior to the 1970s to a greater use of groundwater for irrigation. Following the 1977 drought, many farmers moved from using surface water for flood irrigation to pumping groundwater. Groundwater is more reliable and the preferred source for more efficient irrigation methods such as wheel lines and center-pivots that are encouraged by agencies like the National Resource Conservation Service (NRCS). Access to groundwater allows for irrigation after surface water supplies are no longer available, but pumping groundwater reduces discharge from the aquifer to the stream.
Changing sources of irrigation water has had several major impacts in the valley, including:
- decreased groundwater recharge in the spring and summer due to increased irrigation efficiency,
- increased groundwater pumping, and
- net increase in consumptive crop water use (evapotranspiration) due to the ability to irrigate alfalfa into August or September for a third or fourth cutting instead of two cuttings through July.
Notably, with water supplies greatly exceeding water demand, there is no evidence of long-term groundwater overdraft in the Scott Valley. However, pumping near the Scott River seasonally lowers groundwater levels sufficiently to impact streamflow during the late summer, especially during the critical time between the end of the irrigation season and the beginning of the rainy season when Chinook and coho start their fall spawning runs.
Valley residents have been proactive in finding solutions to the problem of decreased late-summer streamflow. The Scott River Water Trust, the first active water trust in California, leases water from farmers with the goal of improving streamflow for salmon and steelhead at critical points during their lifecycle. The Scott Valley Groundwater Advisory Committee was established, which assists with data collection and monitoring, provides information about local farming practices, and suggests potential methods for increasing late-summer streamflow in the Scott River. Additionally, a Community Water Level Measuring Program has been monitoring about 34 wells monthly since 2006. UC Davis professor Thomas Harter and his research group have used this information to develop an integrated groundwater-surface water model of the Scott Valley that can test different management options for increasing late-summer streamflow.
One proposed solution for groundwater recharge is flooding dormant agricultural fields during the winter when streamflow is high and water is available. In January 2016, the Scott Valley received the first temporary groundwater storage permit issued in California to test this option. The goals of the groundwater recharge project, headed by UC Davis professor Helen Dahlke, are to quantify how much water can be recharged on agricultural fields, determine potential negative effects on the crop, and identify best management practices in the hopes this method can be applied to other areas in California as well.
Another management option is the conjunctive use of surface-water and groundwater involving a dual-irrigation system where more surface-water is used while it is available during the spring months to reduce groundwater pumping. Although this would require an investment in infrastructure and coordination among stakeholders, preliminary modeling results show promising streamflow increases when this management scenario is implemented.
Work will continue to improve fish habitat quality and quantity in the Scott Valley while also maintaining agriculture. There is no magic bullet, and the path forward will rely on a portfolio of management solutions, supported by active stakeholder engagement, monitoring, assessment, and modeling. Some actions may be achieved relatively easily, while others will require coordination and cooperation among stakeholders with some significant investments to successfully implement.
Gus Tolley is a doctoral candidate in the Hydrologic Sciences Graduate Group at UC Davis and 2015 UC President’s Global Food Initiative fellow. His work focuses on numerical modeling of interactions between groundwater and surface water in agricultural areas.
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