Urban Hydrological Connectivity and Response Patterns Across Timescales: An Integrated Time-Frequency Domain Analysis

Abstract

Understanding the interconnections between rainfall, groundwater and stream flow in urban and peri-urban catchments, along with the factors affecting response times between causally related hydrological variables, is essential for predicting and managing future urban water resources. In this study, we applied autocorrelation, cross-correlation and time-frequency analyses over 20 years of daily hydrometeorological data to extract relevant indicators of hydrological connectivity in the Wuhle catchment, one of the main urban streams in Berlin, the capital of Germany. Despite the strong influence of urban storm drainage, our analysis showed a high degree of persistence of the groundwater signals, reflecting the important role of storage in regulating fluxes from the regional aquifer. For wet years, spectral analysis of the precipitation and discharge signals highlighted regions of high common power and coherence between precipitation and discharge at the seasonal scales (6–12 months). These were attenuated during periods of consecutive wet years when the influence of individual rainfall events was less dominant. For dry years, including through the persistent 2018–2020 west-central European drought, the analysis revealed a high degree of statistically significant coherence between groundwater levels and discharge at scales of 1 year and longer. Phase angles were used to estimate response times between the signals. A virtually instant response between groundwater and streamflow throughout the analysed period confirmed the importance of groundwater in sustaining streamflow, and the severe consequences of multi-year droughts that can deplete groundwater storage and, in extreme cases, lead to a cessation of stream flow. The study thus demonstrates the benefits of integrated time series analysis in understanding the important role of groundwater at time scales of ~3–6 years, green spaces and non-urban areas in the integrated management of complex urbanised catchments, even when dominated by urban storm runoff from impervious areas at sub-daily time scales.