Correction and Informed Regionalization of Precipitation Data in a High Mountainous Region (Upper Indus Basin) and Its Effect on SWAT-Modelled Discharge

Abstract

The current study applied a new approach for the interpolation and regionalization of observed precipitation series to a smaller spatial scale (0.125° by 0.125° grid) across the Upper Indus Basin (UIB), with appropriate adjustments for the orographic effect and changes in glacier storage. The approach is evaluated and validated through reverse hydrology, guided by observed flows and available knowledge base. More specifically, the generated corrected precipitation data is validated by means of SWAT-modelled responses of the observed flows to the different input precipitation series (original and corrected ones). The results show that the SWAT-simulated flows using the corrected, regionalized precipitation series as input are much more in line with the observed flows than those using the uncorrected observed precipitation input for which significant underestimations are obtained. stations, the coverage is still very thin and the data less representative, especially, for different elevation zones. The available data also needs a lot of preprocessing, as it represents uncorrected raw precipitation readings, and, therefore, needs checking for quality issues and correction for losses or gaps. Similarly, while most of the weather stations have become operational after the mid-nineties, long-term data is a rear commodity and only available at a limited number of locations.Owing to the complex orography of the UIB region and to the co-action of different hydro-climatic regimes (which affect the amounts, spatial patterns and the seasonality of precipitation), neither the sparse observed station data (or the gridded data products based on them) nor the sensors-based data, fully represents the precipitation regime of the region [11][12][13][14]. Several studies have pointed out that precipitation in the HKH (Hindu Kush Himalayan) -region exhibits large changes over short distances and has a considerable vertical gradient [15][16][17][18][19][20]. This also explains the fact that the average precipitation amounts over the UIB (based on the sparse and low-altitude climatic station network), are unrealistically low to be able to sustain the observed discharge at the basin outlet.These factors have led many researchers to find ways to assess methods for precipitation correction that may lead to a more realistic water balance [21] in many basins and have also compelled a number of hydrological studies in the UIB region to use, in addition to the observed station data, a variety of other reference climate data from different sources, either directly or with prior modifications and adjustments (e.g. TRMM Data [22]; modified APHRODIT [23], or modified WFDEI data [13] etc).This study proposes a simple method to regionalize and correct precipitation data in the UIB through accounting for the orographic effect at a scale finer than the one covered by the streamgauging network, by applying a new method based on a step-wise correction and informed regionalization. This method consists of three steps: 1) Correction for systematic errors;...