Scale Effects of Distributed Hydrological Simulation: Forcing, Structure and Mechanism

The spatial discretization of hydrological sub units (HSU) is an inevitable and effective way to achieve refined distributed simulation. It can not only strengthen the distributed characteristics of the model, but also help simulate the runoff process on a small-scale watershed. However, the spatial variability of the runoff characteristics of HSU divided on a smaller spatial scale increases, and the hydrological process mechanism shows different characteristics under the refined spatio-temporal scale, which then forms the spatio-temporal scale effect. Therefore, it is of great significance to study the theory of spatial discretization of basin hydrological simulation at different spatio-temporal scales, construct a division method for runoff simulation HSUs, quantitatively describe and calculate the corresponding runoff characteristics, develop corresponding runoff simulation methods, and construct a refined distributed basin hydrological model.In this paper, We studies the influence of the spatial scale of HSU on the runoff simulation of hydrological models, and proposes an optimization method based on the division of HSU scales, the matching of input data time scales, the quantitative calculation of model parameters, and the evaluation of basin applicability. With the refinement of the HSU scales, the spatio-temporal resolution of precipitation and evaporation input data should be improved accordingly to ensure the precise matching of rainfall evaporation process with hydrological response. Specifically, the calculation units were first divided into different scales, gradually refined from large scale to small scale, and the time scale changes of precipitation and evaporation data were simulated, using time steps of 3 hours, 2 hours and 1 hour respectively to improve the resolution of the hydrological response of the basin. Secondly, the input parameters of the Xin'anjiang (XAJ) model were optimized based on the quantitative calculation of the basin's underlying surface characteristics (such as area, morphological factors, river network density, slope, etc.). By combining quantitative calculations and empirical derivations at different scales, the effects of confluence parameters at different scales were analyzed. Finally, this paper verifies the rationality of the basin division, especially by evaluating the closure of the calculation unit based on DEM and bedrock depth data to ensure that each calculation unit has the hydrological mechanism characteristics required for hydrological model. In order to optimize the spatial scale of the calculation unit, a multi-objective optimization algorithm (Pareto frontier optimization) was used, and the rationality of the basin selection was verified through empirical research, thus ensuring the the model’s reliability. The results show that with the refinement of the calculation unit scale, the temporal and spatial scales of the precipitation evaporation input data and the hydrological response are better matched, but if the unit scale is too small, it may not meet the requirements of basin closure and hydrological mechanism. Therefore, the selection of the calculation unit scale should comprehensively consider the basin characteristics, the temporal and spatial scales of the data and the model mechanism. The reasonable calculation unit scale should usually not be less than 100 km² to ensure the accuracy of the model and the reliability of the mechanism.