Assessment of consistency of low-flow indices of a hydrogeologically non-homogeneous catchment: A case study of the Ljubljanica river catchment, Slovenia

Abstract

This study attempts to assess the consistency of various low-flow indices evaluated at 12 water stations in the Ljubljanica river catchment, Slovenia, which is due to its high diversity of hydrogeological properties recognized as a non-homogenous catchment. Daily discharge data were used in the study for evaluating low-flow statistics (baseflow index, discharges from flow duration curves, mean annual minimums), seasonal indices, and analysis of hydrograph recession curves. The indices were compared at each station and their values were interpreted in terms of the connection between surface water and groundwater as well as the extent of contribution of delayed sources to surface water. Although all low-flow statistics point to the same stations in terms of the greatest and the lowest contribution of water from groundwater, we identified inconsistencies at some water stations. The hydrograph recession analysis and seasonal indices support the conclusions drawn from low-flow statistics analysis, with the seasonality ratio lower than 1 at all stations, meaning that summer low-flow conditions prevail across the catchment. However, at the torrential stream Gradaščica, an inconsistency was found with recession constants that could be attributed to the inappropriate choice of computational parameters. We assessed the overall consistency heat map visualisation. Although it shows consistency between low-flow indices, some variations demonstrate that in case of hydrologically and geologically heterogeneous catchments, an analysis of several low-flow indices is needed in order to obtain a holistic overview of low-flow conditions and to avoid bad decisions in water resources management due to inappropriate consideration of low-flow characteristics.