Influence of soil type on dry down patterns of the North Fork of the American River Basin

This study examines how soil type influences the way the North Fork (NF) of the American River Basin located west of Sacramento, CA drains following snowmelt or significant rainfall. Understanding the hydrometeorological response of the American River Basin to precipitation is one of the key goals of the National Oceanic and Atmospheric Administration's Hydrometeorology Testbed (NOAA-HMT). In this study, the NF of the American River Basin was roughly divided based on soil characteristics from soil surveys. The Lower Basin is characterized by soils with high clay content, while the Upper Basin is comprised mostly of alluvium and soils with volcanic origins. Soil monitoring sites, operated by the NOAA Earth Systems Laboratory (ESRL), measured soil water content at 10.0 and 15.0 cm in the Basin for water seasons 2008, 2009, and 2010. Soil moisture changes as a function of time were examined from the peak soil wetness fraction at six locations to the point in time where the soil moisture reached its seasonal minimum. This period is commonly referred to as the dry down period. Precipitation amounts during the water season prior to the dry down were examined at each site to ensure that seasonal variations in precipitation did not affect the dry down pattern from year to year. The Upper Basin sites dried down more rapidly compared to the Lower Basin sites. In general, more precipitation fell over the Upper Basin than the Lower Basin, with the maximum precipitation falling over the region where the basin soil type changes from having high clay content to soils that are primarily alluvial and volcanic in origin. These results imply that while the Lower Basin collects less precipitation in the season prior to the dry down, its soils retain higher amounts of water and for longer periods of time compared to soils in the Upper Basin. Climate change scenarios suggest the height where the maximum precipitation falls as rain could move to higher elevations in the Basin. Because the Upper Basin soils can hold less water, more of the precipitation could flow into the NF of the American River on a faster time scale in the future, increasing the flood risk to the Sacramento area.