Groundwater recharge quantity and timing across land covers in a managed agricultural landscape in NE Germany: insights using stable water isotope approaches and water storage measurements

Abstract

Improving groundwater recharge flux (GWR) estimation is key for effective or sustainable groundwater resource management. Yet, GWR assessment is challenging as direct measurements are usually limited to the point scale and specific depths of the vadose zone . In agricultural settings, the spatial variability introduced by land use management may further complicate the assessment. Tracer studies in the vadose zone, combining stable isotopes (δ2H and δ18O) in soil water and water storage measurements can aid such assessment in agriculturally managed landscapes. The stable water isotope signal can provide insights into the timing of the soil water transport while traditional water storage measurements (i.e. soil moisture, groundwater levels) can provide complementary information allowing for comparison or alignment. In this study, we aim to provide an integrative estimate of GWR under various agricultural land covers at the field scale. For that, we combine dedicated measurements of soil water stable isotope and continuous water storage observations spanning the soil profile from topsoil to groundwater table. The study was conducted in a highly instrumented research site near Potsdam, Brandenburg, situated on a gentle hillslope and covered by a variety of agricultural plots. During two sampling campaigns in spring (May) 2023 and winter (January) 2024, we collected bulk soil water from various soil profiles (0-150 cm) along with monthly groundwater samples and analysed them for stable water isotopes (δ2H and δ18O). By integrating isotope data with soil moisture observations, we trace GWR using the peak shift method. Complementary GWR estimates are derived from timeseries of tensiometers and groundwater level fluctuations. We present an overview of the experimental set up and preliminary GWR estimates. Our aim is to offer a complementary perspective on the key processes governing vertical water fluxes within the vadose zone across different depths, land covers, and hillslope positions, advancing our understanding of GWR dynamics at this managed agricultural site.