Newsletter - Spring 2009

Current Research - Environmental Quality

Best management practices to address runoff on California farms

The Mediterranean climate and productive soils of California make the state one of the most productive agricultural regions in the world. California produces over 250 different crops and leads the nation in production of 75 commodities. Californian agriculture is characterized by its high production capacity through very intensive management of irrigated cropping systems. Intensive management is characterized by a combination of chemical inputs and a combined use of extensive tillage and surface irrigation practices. Population growth, climate, and competing land uses are raising water quality concerns for Delta inflows, a source of drinking water for two-thirds of Californians.  Agricultural activities are potential non-point sources of pollution of California’s surface water. New regulations begun in January 2005 are holding California growers accountable for known pollutants draining off of their land.  Reduced tillage, winter cover cropping, and post-sediment traps are sustainable practices for reducing runoff and minimizing nutrient and sediment losses.

Professor William Horwath quantified the effectiveness of these practices in reducing runoff and non-point pollution export. A network of automated water samplers was used on various farms in the Sacramento Valley (Yolo County) to monitor storm season and irrigation tail water discharge. The alternative practices of winter cover cropping and reduced tillage were compared to conventional practices to determine their effect on agricultural runoff.  Potential nonsource pollutants examined included total suspended sediment (TSS), turbidity, inorganic phosphate (PO4-P) and nitrogen (NO3-N, NH4-N), total dissolved nitrogen and phosphorous (DON, DOP), dissolved organic carbon (DOC), and pesticides.
The use of winter cover crops dramatically affected the hydrology of the agricultural fields compared to conventionally managed fields where up to 20% of winter precipitation ran off. The use of winter cover crops practically eliminated runoff and exports of nonsource pollutants. The use of winter cover crops should be considered in areas prone to flooding and runoff. The effect of reduced tillage on runoff was inconclusive with some fields seeing reduced runoff and others increased runoff.  Increased runoff was likely due to soil compaction and soil surface crusting that can occur when soils are disturbed less under reduced tillage.

In conclusion, the use of cover cropping was very effective on fields prone to winter runoff. However, planting cover crops before the onset of fall rains is generally a constraint facing farmers.  In addition, farmers who cover crop may experience significant delays in spring field entry due to cover crop management putting them at a competitive disadvantage compared to growers who do not cover crop. Besides these issues, the planting of winter cover crops greatly enhances the soil by improving structure retaining nitrogen and sequestering soil carbon.  These extra benefits are crucial towards attaining sustainable cropping practices.