Influence of calculation criteria on the values of low-flow recession constants in a non-homogenous catchment in Slovenia

In this paper we analyse the influence of three different calculation criteria on the values of low-flow recession constants, namely the influence of the calculation method, recession curve segment length (4, 5, 6, or 7 days) and of the selected period (whole period, monthly, seasonally) for the calculation of the initial discharge, which determines the threshold on the falling limb of the hydrograph for the recession analysis. The analysis is based on daily discharge data of 11 gauging stations in the hydrogeologically non-homogenous Ljubljanica River catchment. For each of the stations, we calculated 24 values of recession constants using a different combination of criteria. Based on these values we determined the influence of each criterion on the final result using statistical tests and graphical approaches. Analysis showed that the recession constants calculated using various methods are statistically significantly different for all gauging stations. The recession constants obtained using the individual recession segment method (IRS) are on average 3.1 days higher than those calculated using the master recession curve method (MRC). The influence of the other two criteria, namely the length of the segment and the period for calculating the initial discharge, is not so obvious, since it is most likely conditioned by catchments’ characteristics influencing the time dynamics of the runoff.