Linking Regional Aquifers Heterogeneity with Nonlinear Storage-Baseflow Relationships for European Catchments

Groundwater is the main source of fresh water on Earth. It provides drinking water, supports agriculture, and sustains ecosystems. During droughts, this subsurface reservoir also maintains river flow through baseflow. Groundwater systems can buffer temporal fluctuations in recharge and moderate the severity of both short-term dry spells and prolonged droughts.

In this study, we investigate the recession behavior of baseflow of European catchments during periods of droughts assuming a non-linear storage-baseflow relationship. We focus on two key parameters: the recession constant (k) and the nonlinearity exponent (n). Using a large set of recession segments, we show that the duration of the recession period strongly influences the parameter estimation. In particular, the degree of nonlinearity strongly depends on the length of the recession periods. Short recession periods (<1 month) show a stronger degree of nonlinearity than longer ones. For very long recession periods of about a year the nonlinearity exponent approaches 1 indicating a linear relationship between storage and baseflow.

In order to explain these findings, we develop a theory based on the assumption that regional groundwater systems contributing to baseflow are spatially heterogeneous. To that end, different parts of the groundwater system contribute with a different recession behavior to the total baseflow. Based on stochastic theory, we derive an effective nonlinear-storage-baseflow relationship which links statistical properties of a groundwater systems with the recession constant (k) and the nonlinearity exponent (n). Using our theory, we can explain why the estimated degree of non-linearity depends on the length of the recession period.

Our results emphasize the importance of the length of selected recession periods in the characterization of non-linear storage-baseflow relationships.  We will discuss how to make use of this finding and propose to use a spectrum of different recession periods to get more information on the spatial heterogeneity of groundwater systems contributing to baseflow.

Moreover, we will discuss the relevance of spatial heterogeneity of regional groundwater systems on buffering short and long term hydrological droughts.