Characterizing hydrologic similarity of precipitation and catchment wetness using repeating patterns in runoff

Understanding the reasons why a certain combination of precipitation event and catchment wetness condition causes a particular runoff response serves as the basis for a sustainable water resources management. Runoff responses at the catchment scale are highly variable in space and time due to numerous influencing factors, e.g., topography, land use, geology, and climatic conditions. Yet, despite previous studies having investigated these interdependencies, it remains difficult to differentiate between the various impacts on the runoff response and to determine which driving factors dominate under which conditions. Assuming that similar precipitation and catchment wetness (e.g. in terms of soil water content or groundwater level) lead to similar runoff responses, reoccurring patterns of these hydrological flux and state variables may be useful to assess the causes for similar or different runoff responses. Therefore, we compared precipitation and catchment wetness conditions and classified them as hydrologically similar if they lead to a similar runoff response. In this study, the similarity of runoff responses was assessed for the 38.5 ha, partly forested Wüstebach catchment in Western Germany using the goodness-of-fit (GOF) criteria Nash-Sutcliffe-Efficiency and Volumetric Efficiency. If the GOF exceeded a pre-defined threshold, the runoff responses were classified as similar and grouped together. Subsequently, for similar runoff responses, the corresponding rainfall and wetness conditions were compared calculating the Spearman’s rank correlation coefficient and descriptive statistics. A total of 22 similar out of 73 runoff events were identified for the Wüstebach catchment over a period of nearly 12 years and classified into seven groups with the largest group including in total eight events. Results show that for similar runoff responses in this representative group, soil water content as well as groundwater levels in the riparian zone are well correlated with r = 0.815 and r = 0.840, respectively, indicating a possible dominant control on runoff responses. However, rainfall patterns show overall weak correlations (r = 0.406), implying that the precipitation temporal pattern control on the runoff response might be limited for these types of runoff events in the Wüstebach catchment. In a next step, the here defined hydrologically similar precipitation and wetness conditions will be searched in the data of the Wüstebach catchment to compare the corresponding runoff responses, and hydro-meteorological variables will be used to explain similar or different runoff responses.