Assessing the Relationship between the Standardized Precipitation Evapotranspiration Index, Watershed Characteristics, and Hydrological Drought Conditions in the Pacific Northwest and California

Drought indices are often based on atmospheric observations, rather than on other hydrologic conditions that more directly affect economic activity and ecosystem function, such as streamflow. We investigate associations between popular meteorological drought indices (e.g. standardized precipitation evapotranspiration index or SPEI) and streamflow in the Pacific Northwest and California hydrologic regions of the United States. We examine the differences in linear correlation patterns between meteorological drought indices (SPEI) and dry-season streamflow with particular focus on differences between watersheds characterized by different types and magnitudes of human influences, such as agricultural versus urban land uses and dammed versus undammed rivers. In addition, we model flow as a function of SPEI, watershed attributes, including vegetation (using the NDVI), to better understand how the effect of meteorological conditions on local streamflow is mediated by watershed characteristics. This study gives an advanced understanding of how well meteorologically-based indices can identify local drought conditions when used for drought monitoring and early warning efforts and how the impacts of meteorological drought can vary across watershed conditions.