A Physically Based Model of a Two‐Pass Digital Filter for Separating Groundwater Runoff From Streamflow Time Series

Abstract Groundwater runoff is an important component of river runoff and is sensitive to climate change. However, it is difficult to differentiate groundwater runoff from total river runoff by current base flow separation methods. Here, we propose a physically based model with a two‐pass digital filter for separating groundwater runoff from streamflow time series. The proposed method is based on the fundamental theory of surface–groundwater interactions under a variable groundwater recharge process that is highly associated with river stages. Our results show that the groundwater flow recession constant and the base flow recession constant can be determined separately by performing variogram analysis on long‐term streamflow time series data. For watersheds with areas of 10 3 –10 4 km 2 , the groundwater flow recession constant differs from the typical values of the base flow recession constant by an order of magnitude or more. Therefore, the developed method can be used to quantify groundwater runoff dynamics on a temporal scale spanning tens of years. Unlike other approaches to base flow separation, our proposed method can reveal the effects of climate change on groundwater runoff.