Trade-offs between winter cover crop production and soil water depletion in the San Joaquin Valley, California

Cover crops are currently not widely used in annual crop production systems in California's semiarid Central Valley due to concerns about lost opportunity costs and uncertainties about water use. From 1999 through 2014, we quantified cover crop biomass production for a variety of mixtures under winter precipitation and limited supplemental irrigation. In a separate study, we also determined changes in soil water storage under three cover crop mixtures compared to fallowed plots during two (2013 and 2014) winter periods to investigate tradeoffs associated with water use by cover crops in this region. Over the 15 years of the project that were characterized by recurring drought, a total of 22.8 Mg ha(-1) (20,360 lb ac(-1)) of aboveground cover crop biomass was produced with a total precipitation of 209 and 20 cm (82 and 8 in) of supplemental irrigation applied in 1999, 2012, and 2014. Cover crop biomass varied from 0.39 Mg ha(-1) (348 lb ac(-1)) in the low precipitation period (winter of 2006 to 2007) to 9.34 Mg ha(-1) (8,340 lb ac(-1); winter of 2000 to 2001). Soil water storage in the sampled depth (0 to 90 cm [0 to 35 in]) for the fallow and each of the cover crop mixtures was compared each year from January to March, the primary growing period for cover crops in this region. Net soil water storage increased during this period by 4.8 and 4.3 cm (1.9 and 1.7 in) in 2013 and 2014, respectively, for the fallow system, but in the cover crop mixture plots, there was no additional water storage. Instead, water use by the cover crop mixes resulted in a negative water balance over the cover crop growth period on an average of 0.47 and 0.26 cm (0.19 and 0.10 in) in 2013 and 2014, respectively. Thus, compared to the fallow system, cover crops depleted 5.3 and 0.67 cm (2.1 and 0.26 in) and more water from the 0 to 90 cm (0 to 35 in) profile in 2013 and 2014, respectively. From this long-term systems research, we conclude that while vigorous growth of winter cover crops in the Central Valley of California may not be possible in all years due to low and erratic precipitation patterns, there may be benefits in terms of providing ground cover, residue, and photosynthetic energy capture in many years. However, cover crop biomass production may come at a cost of soil water depletion in this semiarid, drought-prone region.