Can winter cover cropping improve water quality of tail-water discharge?
By Aaron Ristow, Sam Prentice, Wes Wallender, and William Horwath

New regulations are holding California growers accountable for detected pollutants draining off their land. In response, there has been growing interest among farmers, researchers, governmental agencies, and environmental conservation groups in investigating the viability of alternative crop production practices that function to conserve soil and water resources. One option SAFS researchers are exploring is the use of winter cover crops to minimize discharge concentrations and/or load of targeted materials that affect water quality parameters. Preliminary analysis has demonstrated a dramatic decline in amount of surface discharge from growers’ fields while also improving the quality of discharge managed with winter cover crops (SAFS Newsletter Fall 2005, Vol.6, No.1).

These results for winter discharge are not surprising. Winter cover cropping provides protection from water erosion by improving aggregate stability and increasing soil water retention and infiltration. However, research has shown that cover cropping in the winter may conflict with water conservation in the summer. While cover crops may increase rainfall infiltration, their increased evaporative demand has been shown to deplete soil moisture from deeper layers of the soil as they mature, limiting soil water availability for the ensuing crop. In addition, enhanced infiltration from cover cropping during the winter months may extend into the growing season. Thus, in order to meet the water needs of summer crops, more surface water deliveries may be required.

To address these concerns, SAFS researchers have established a network of automated water samplers in grower fields in the surrounding Central Valley. The network of automated samplers provide year-round monitoring of surface runoff that generates data with considerable resolution to more precisely compare the effectiveness of cover crops in minimizing runoff quantity and improving overall runoff quality. Runoff volume and materials that affect water quality parameters are being determined, including suspended sediment, turbidity, phosphate, inorganic nitrogen, total dissolved nitrogen and phosphorous, dissolved organic carbon, and herbicides.

Soil water retention and infiltration

The net effect of increased water use vs. increased infiltration depends on several factors, such as cover crop dry matter production rate, degree of soil cover and soil slope, soil infiltration rate, and rainfall intensity. Preliminary analysis of our data suggests significant enhanced infiltration of rainwater during the winter months due to the winter cover cropping. Interestingly, for the summer months, approximately 50 percent of the irrigation water applications to the winter fallow (bare) field were not discharged and are assumed to have infiltrated. Surprisingly, during the same period, only 19 percent of surface water deliveries are assumed to have infiltrated from the winter cover cropped field. Perhaps cover crop root channels were developed during the winter, or changes to it and to other soil physical characteristics as a result of cover crop residues enhanced infiltration deeper into soil. Enhanced biotic activity, ranging from earthworms and microbial turnover, may increase aggregate stability and soil structure to promote infiltration in cover cropped fields. The result suggests that cover cropped fields may offset evapotranspiration through increased winter infiltration or by infiltrating water past the rooting zone in this part of California. Research in the San Joaquin Valley has suggested the opposite, that cover crop evapotranspiration may negatively affect water balance. The decreased infiltration in the cover cropped field during irrigation implies declining water use efficiency if established irrigation schedules are used in fields managed for winter runoff with winter cover crops. More research is needed to determine whether irrigation schedules or frequency of water application can be reduced to increase water use efficiency in cover cropped fields.

Concentration

Runoff from the cover cropped field had lower seasonal average concentrations of nitrate, ammonium, and total dissolved nitrogen. Concentration of phosphate and dissolved organic carbon were not statistically different. It is important to recognize that seasonal average concentrations of materials that affect water quality parameters were all below 5 mg/L (Fig. 1). This is well within drinking water quality standards for the city of Davis.

Nitrogen (N) assimilation in the cover crop throughout the winter could account for the reduced concentrations of N in the cover crop tail water. For example, it is known that a winter cover crop can effectively immobilize large quantities of nitrate, delaying the nitrogen availability until newly planted crop roots become active in the spring. In addition, the increased biomass of the residue may immobilize N during the summer when decomposition of the cover crop residues is at its highest.

Load

We have remarked on how winter cover crops during the winter storm season can significantly reduce loads of all materials that affect water quality parameters during winter months (SAFS Newsletter Fall 2005, Vol.6, No.1). Further analysis is needed on total seasonal loads in irrigation tail water between the fallow and cover cropped systems across crop rotations and soils to conclusively state the benefits of cover crops.

In conclusion, it was surprising to discover that the winter cover cropped field infiltrated only 19 percent of irrigation water deliveries in contrast to the 50 percent infiltrated deliveries from the winter fallow field. This suggests that the grower could apply less water to the cover cropped field. We stress this may be region specific with these results reflecting higher rainfall areas. Further investigation is required before recommendations can be made. In addition, N concentrations were lower in the winter cover cropped tail water compared to the winter fallow field, suggesting residual water quality enhancement from the use of winter cover crops.

• Table of Contents •