Agricultural Best Management Practices (BMPs) have been employed for years as erosion reduction measures on cultivated lands. The ability to model the effects of BMPs at the watershed scale using a standardized process-based method provides watershed managers with an important decision making tool for addressing large-scale water quality concerns. This study uses a process-based BMP simulation method with the Soil and Water Assessment Tool (SWAT) to evaluate five BMPs including grassed waterways, channel stabilization structures, strip crops, cover crops and vegetative filter strips on almond orchards in northern California, USA. The sediment reduction rate of the five BMPs was compared to a base case where no BMPs were applied to the watershed and analyzed for several precipitation scenarios to compare BMP effectiveness. In-channel BMPs which included grassed waterways and channel stabilization structures, reduced sediment load at the watershed outlet by 8% to 14%, respectively, depending on the annual precipitation scenario. Grassed waterways consistently outperformed channel stabilization structures, reducing sediment load by an additional 2% to 5% compared to channel stabilization structures. Upland BMPs including strip crops, cover crops and vegetative filter strips reduced sediment yield by 15 to 100% for the various precipitation scenarios. For years with median and above median precipitation, strip crops were most effective, reducing sediment yield by 63% in both cases, whereas cover crops only reduced sediment yield by 54% and 15%, respectively. For the below median precipitation year, the cover crop reduced sediment yield from fields completely (100%), whereas strip crops and vegetative filter strips only reduced sediment load by 64% and 59%, respectively. For all BMPs, a positive correlation between sediment load/yield and increasing precipitation amount and intensity was observed. The methods presented in this study are easily applicable to watershed scale studies of other basins and for other water quality concerns, such as the fate and transport of agricultural pesticides and nutrients.

Global climate change has the potential to dramatically alter hydrologic conditions in California by changing the spatial and temporal patterns of snow accumulation and snow melt. The water management infrastructure in California has been designed and is operated in accordance with historic hydrologic patterns. Understanding if and how this infrastructure can be managed in the face of global climate change in order to meet the array of vital water management objectives for the system is a critical research question addressed in part by this study. Here an application of the Water Evaluation and Planning (WEAP) system, developed by the Stockholm Environment Institute, is presented for California's Sacramento River Basin. WEAP is an integrated hydrology and water resources systems model that allows for assessment of climate change impact and adaptation in the water sector based solely on future climate time series. The model is used to evaluate the impact of four future climate scenarios on agricultural water management in the region, and to investigate whether water management adaptation could reduce potential impacts.