Shyft and Rasputin: a toolbox for hydrologic simulations on triangular irregular networks

Abstract

Terrain topography controls insolation variations at catchment scale. This effects are known to be important in cold and mountainous regions due to high diurnal and seasonal variability in incoming radiation. However, meteorological data in such areas lacks accuracy due to sparse station network and coarse re-analysis grids. Simulation tools that model hydrologic processes at local scales require ways to overcome the lack of accuracy in the observational data, particular at high elevations, so downscaling to cell level is done carefully. With an introduction of irregular triangular networks into distributed hydrologic modelling framework Shyft, steps are taken to account for hillslope-scale terrain structures within scope of operational hydrology. This new functionality allows translation of radiation measurements or re-analysis data onto inclined surfaces improving the predictive power of the model. Based on the Shyft and Rasputin toolbox we show importance of topographic details such as slope and aspect on predicting snowmelt and rainfall-runoff simulations in snow-covered mountainous region of Himalaya. We conduct the series of experi- ments for catchments in Narayani area of central Nepal in two steps. First, we demonstrate sensitivity of streamflow simulation to mesh size and shape. The results show that there is an upper limit after which further mesh refinement is not useful for simulations. Second, we incorporate "on the fly" correction of incoming solar radiation depending on surface inclination. The experiments with both coarse and fine tin meshes demonstrate that snow-water equivalent and potential evapotranspiration are directly affected by variations in insolation, with less snow and more evaporation on south-facing slopes. Finally, we perform 10-years of hydrological simulation of Budhi-Gandaki catchment with several model configuration incorporating both pro- posed features (tins and radiation correction), where we reveal better correspondence of simulated and observed discharge forgrid-based solution, which is contradictory to our previous study at Marsyangdi-2 catchment.This is an unfinished study: the Budhi-Gandaki catchment hydrology has to be analyzed carefully. One of the possible reasons of the unexpected results is a complex precipitation pattern in the catchment, which was not caught by the modelproperly.