Use of SWAT to Model Impact of Climate Change on Sediment Yield and Agricultural Productivity in Western Oregon, USA

Climate change predictions for the Pacific Northwest region of the United States of America include increasing temperatures, intensification of winter precipitation, and a shift from mixed snow/rain to rain-dominant events, all of which may increase the risk of soil erosion and threaten agricultural and ecological productivity. Here we used the agricultural/environmental model SWAT with climate predictions from the Coupled Model Intercomparison Project 5 (CMIP5) “high CO2 emissions” scenario (RCP8.5) to study the impact of altered temperature and precipitation patterns on soil erosion and crop productivity in the Willamette River Basin of western Oregon. An ensemble of 10 climate models representing the full range in temperature and precipitation predictions of CIMP5 produced substantial increases in sediment yield, with differences between yearly averages for the final (2090-2099) and first (2010-2019) decades ranging from 3.9 to 15.2 MT·ha-1 among models. Sediment yield in the worst case model (CanESM2) corresponded to loss of 1.5 - 2.7 mm·soil·y-1, equivalent to potentially stripping productive topsoil from the landscape in under two centuries. Most climate models predicted only small increases in precipitation (an average of 5.8% by the end of the 21st century) combined with large increases in temperature (an average of 0.05°C·y-1). We found a strong correlation between predicted temperature increases and sediment yield, with a regression model combining both temperature and precipitation effects describing 79% of the total variation in annual sediment yield. A critical component of response to increased temperature was reduced snowfall during high precipitation events in the wintertime. SWAT characterized years with less than basin-wide averages of 20 mm of precipitation falling as snow as likely to experience severe sediment loss for multiple crops/land uses. Mid-elevation sub-basins that are projected to shift from rain-snow transition to rain-dominant appear particularly vulnerable to sediment loss. Analyses of predicted crop yields indicated declining productivity for many commonly grown grass seed and cereal crops, along with increasing productivity for certain other crops. Adaptation by agriculture and forestry to warmer, more erosive conditions may include changes in selection of crop kinds and in production management practices.