Estimating the Water Cost of Winter Cover Crops.

Winter cover crops can provide important environmental benefits in row crop systems, protecting the soil surface against sediment and phosphorus losses and immobilizing excess nutrients.  However, farmers are reluctant to use them, in part due to fears that their transpirational water use will increase the likelihood that the following crop will be affected by water stress.  We developed a model to compare the ET of winter cover crops to that of bare fields exposed to the same atmospheric forcing.  Model performance was evaluated against eddy covariance measurements of latent heat flux in two adjacent fields, one bare and one planted with winter rye, over two growing seasons.  Both model and measurements show that frequency of precipitation, timing of cover crop removal, and soil albedo are the key determinants of the relative water use of cover-cropped versus bare fields.