A new precipitation-based method of baseflow separation and event identification for small watersheds (<50 km2)

Baseflow separation methods are often impractical, require expensive materials and time-consuming methods, and/or are not designed for individual events in small watersheds. To provide a simple baseflow separation method for small watersheds, we describe a new precipitation-based technique known as the Sliding Average with Rain Record (SARR). The SARR uses rainfall data to justify each separation of the hydrograph. SARR has several advantages such as: it shows better consistency with the precipitation and discharge records, it is easier and more practical to implement, and it includes a method of event identification based on precipitation and quickflow response. SARR was derived from the United Kingdom Institute of Hydrology (UKIH) method with several key modifications to adapt it for small watersheds (<50 km2). We tested SARR on watersheds in the Choptank Basin on the Delmarva Peninsula (US Mid-Atlantic region) and compared the results with the UKIH method at the annual scale and the hydrochemical method at the individual event scale. Annually, SARR calculated a baseflow index that was ∼10% higher than the UKIH method due to the finer time step of SARR (1 d) compared to UKIH (5 d). At the watershed scale, hydric soils were an important driver of the annual baseflow index likely due to increased groundwater retention in hydric areas. At the event scale, SARR calculated less baseflow than the hydrochemical method, again because of the differences in time step (hourly for hydrochemical) and different definitions of baseflow. Both SARR and hydrochemical baseflow increased with event size, suggesting that baseflow contributions are more important during larger storms. To make SARR easy to implement, we have written a MatLab program to automate the calculations which requires only daily rainfall and daily flow data as inputs.