Conversion of a Wheat Field in Colorado to Mixed Perennials for Soil and Ecosystem Conservation: Watershed Simulation of Changes.

A 56-ha semi-arid watershed in eastern Colorado has been instrumented since about 2002. Most of the watershed area was cropped with rainfed winter wheat and alternating fallow strips. Conversion to native grasses under the USDA Conservation Reserve Program started in 2013. This producer decision provided an unplanned land-use change experiment with continued and enhanced measurements of soil hydrology, weather and surface energy variables. Here, we integrate high-resolution temporal measurements at multiple landscape positions with space-time simulations across scales within the watershed. Measurements include hourly soil water content (SWC) using capacitance sensors at multiple landscape positions and four depth intervals. SWC and snow water equivalent are also inferred at a scale of approximately 10 ha using two cosmic ray sensors installed in 2015 and 2016. The Agricultural Ecosystem Services (AgES) distributed watershed model was developed and used to simulate soil-plant-water interactions in space and time. Soil hydrology and vegetation dynamics at this site are simulated under dryland wheat-fallow rotations in alternating strips (2003-2012) followed by transitional (2013-2017) perennial vegetation (grasses, alfalfa, and remnant weeds during the conversion). Results are compared with spatially averaged point and CRS measurements of SWC at simulated scales. Conversion to perennial vegetation cover has changed the soil-water regime with generally reduced SWC, and increased the precipitation threshold for generation of overland flow, which essentially eliminated surface runoff to the outlet.