| 28 Oct 2025
Larger hydrological simulation uncertainties where runoff generation capacity is high: insights from 63 catchments in southeastern China
Abstract. Traditional parameter calibration strategies that focus on a single optimal parameter set may lead to large uncertainties and biases in simulating internal hydrological processes because of parameter equifinality. This study used the semi-distributed Tsinghua Hydrological Model based on Representative Elementary Watershed (THREW) to investigate the influence of parameter equifinality on uncertainties in surface–subsurface runoff partitioning. The model was implemented in 63 catchments in southeastern China with high-quality rainfall and streamflow data. Behavioral parameter sets were selected based on KGE thresholds to quantify uncertainty in estimates of the contribution of subsurface runoff (Csub). Correlation analyses were conducted to investigate factors influencing these uncertainties. Results showed that: (1) the THREW model performed well across the 63 catchments, with an average optimal KGE (KGEopt) of 0.846. Csub varied widely among catchments, ranging from 1.0 % to 74.1 % (mean = 31.7 %), and was below 50 % in 84 % of the catchments, indicating that surface runoff was the dominant runoff generation mechanism in the study area. (2) Substantial uncertainty in Csub can arise from small differences in KGE, with notable variability among catchments. The uncertainty in Csub was modest in most catchments, with mean Bias (difference between the Csub estimated using the optimal set and the average across all behavioral parameter sets) and Range (max–min across behavioral sets) of 2.7 % and 15.8 %, respectively. However, the uncertainty can be large in some catchments, where reliance on a single optimal parameter set is likely inappropriate. (3) Runoff ratio was identified as an important catchment attribute significantly correlated with Csub and its uncertainty. In catchments with stronger runoff-generation capacity, the model tended to be less sensitive and the simulation of internal runoff-component partitioning tended to exhibit larger uncertainties. Such evidence can provide empirical a priori guidance on the likely magnitude of uncertainties and help inform calibration-strategy selection.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Hydrology and Earth System Sciences.
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