Sustainable ag research highlights differences among water conservation management practices
by Aaron Ristow, Sam Prentice, and William Horwath

	Figure 1. Project set-up at SAFS research fields in Yolo County.

The Issue

California adds over 550,000 people annually to its population, which is expected to reach 48 million by 2030. Experts project that by 2020, demand for water will exceed supply by 2.4 million acre-feet in good rainfall years and double that in drought years. Predicted trends in population growth and global climate change are raising water quality concerns for Sacramento-San Joaquin Delta inflows. Over the last 15 years, the focus of the federal Clean Water Act has turned toward Non-Point Source Polluters (NPSP) and Total Maximum Daily Load (TMDL) monitoring. All businesses that discharge into waterways are required to have a permit. However, for two decades agriculture was exempt. That changed in the Central Valley in 2004. New regulations are now holding California growers accountable for pollutants draining off their land – either from irrigation or winter runoff. Policymakers and water users have begun considering several alternatives to address future supply and demand. Options include expansion of nontraditional sources of supply, reallocation through water marketing, and using water conservation practices such as winter cover crops (CC) and conservation tillage (CT).

The Project

The objectives of the UC sustainable farming systems study are to 1) quantify discharge from research plots and farms using CC and CT compared to conventional practices, 2) quantify NPSP concentrations and loads in runoff, and 3) inform farmers, policymakers, and the general public about the usefulness of CC and CT in addressing water issues.

During the last two years, we have addressed these objectives by establishing a network of automated water samplers at the long-term UC Davis sustainable agriculture research plots and in grower fields in the Sacramento Valley. Automated samplers provide year-round monitoring of surface runoff to assess the performance of CC and CT at minimizing runoff quantity and improving runoff quality. Runoff volume and water quality parameters identified include suspended sediment, turbidity, inorganic phosphate and nitrogen, total dissolved nitrogen and phosphorous, dissolved organic carbon, and herbicides.

Our research includes conventional, low-input, and organic systems under either standard tillage (ST) or CT. The organic and low-input systems utilize winter legume cover crops (CC) as the primary nitrogen input. The CT systems incorporate practices that maintain at least 30 percent of the crop residue on the soil surface or reduces tillage passes by at least 40 percent.¹ The standard tillage systems mirror management practices typical of the surrounding area.

Measurements

At the SAFS plots, one furrow from each plot was isolated to channel runoff into a 1m by 12 in. diameter catchment (Fig. 1). At the end of each rain event, a sample was taken for analysis and the catchment emptied. In the growers’ fields, datalogger- equipped autosamplers were used to collect samples and record flow measurements taken during all runoff events.

Results

Our research team has analyzed runoff quantity and quality data from five storm events during the 2003-2004 rain season and continuously from irrigation tailwater during the 2004 growing season. Preliminary analysis of growers’ field data illustrate the effectiveness of CC at substantially minimizing discharge and NPSP loads. However, with the possible exception of sediment discharges, seasonal NPSP loading from winter fallow fields is not dramatic, suggesting that other field scale strategies (e.g., reconfiguring drainage patterns) may also be effective at meeting agricultural water quality goals.

Peak flow winter (2004-05) runoff velocities were 100% lower for CC field runoff events. That same year total discharge from grower fields was 18 times lower from the CC field. In Winter 2004- 2005 there was an average of 28 times the reduction of discharge from the grower CC fields compared to the fallow fields. It appeared that the cover crop was effective at reducing storm runoff soon after germination. However, on our research plots, CC showed higher discharge volumes NPSP loads compared to winter fallow treatments. This discrepancy between research plots and grower fields could be a result of differences in soil type or method of measurement. The results show additional research is required to understand the interplay between field size and configuration, soil type, and runoff monitoring strategies when developing predictive models for water quality concerns.

In Winter 2004–2005, discharge from the low-input CT treatment was significantly higher than other treatments except for the organic standard tillage. This was somewhat consistent with that of the growers’ CT vs. Winter Fallow comparisons. In both the research plots and the growers’ field, CT management produced greater NPSP loads in runoff water compared to non-CT management, primarily due to higher cumulative discharge. In general, concentrations of various problem materials were similar for all treatments.

The increase in runoff from CT is unexpected. Results from the Midwest, where CT promotes infiltration, suggest the opposite. One possible explanation is that California soils generally have higher clay content, and are therefore more likely to create a soil crust that inhibits infiltration. It would be expected that after many years of CT, infiltration may be enhanced, as soil near the surface accumulates organic matter. However, all CT treatments were in the first or second year of management, and therefore were still building organic matter on the surface.

Summary

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