Quantifying pluriannual hydrological memory with Catchment Forgetting Curves

Abstract. This article presents a new approach to quantifying pluriannual hydrological memory, using exclusively streamflow and climate data. The rainfall--runoff relationship is analyzed through the concept of elasticity, focusing on the relation between the annual anomalies of runoff yield and humidity index. We identify Catchment Forgetting Curves (CFC) to quantify pluriannual catchment memory, considering not only the current year's humidity anomaly but also the anomalies of the preceding years. CFCs are parameterized using a Gamma distribution. The variability of CFCs is investigated on a set of 158 Swedish and 527 French catchments. As expected, French catchments overlying powerful aquifers exhibit a long memory. In Sweden, the expected effect of the lakes is less clear. Overall, aridity appears to be one of the main drivers of catchment memory in both countries. Our work underlines the need to account for catchment memory in order to produce meaningful and geographically coherent elasticity indices.