Comparative Assessment of Runoff and Its Components in Two Catchments of Upper Indus Basin by Using a Semi Distributed Glacio-Hydrological Model

Abstract

ABSTRACT:The hydrology of Upper Indus basin is not recognized well due to the intricacies in the climate and geography, and the scarcity of data above 5000 m a.s.l where most of the precipitation falls in the form of snow. The main objective of this study is to measure the contributions of different components of runoff in Upper Indus basin. To achieve this goal, the Modified positive degree day model (MPDDM) was used to simulate the runoff and investigate its components in two catchments of Upper Indus basin, Hunza and Gilgit River basins. These two catchments were selected because of their different glacier coverage, contrasting area distribution at high altitudes and significant impact on the Upper Indus River flow. The components of runoff like snow-ice melt and rainfall-base flow were identified by the model. The simulation results show that the MPDDM shows a good agreement between observed and modeled runoff of these two catchments and the effects of snow and ice are mainly reliant on the catchment characteristics and the glaciated area. For Gilgit River basin, the largest contributor to runoff is rain-base flow, whereas large contribution of snow-ice melt observed in Hunza River basin due to its large fraction of glaciated area. This research will not only contribute to the better understanding of the impacts of climate change on the hydrological response in the Upper Indus, but will also provide guidance for the development of hydropower potential and water resources assessment in these catchments.

Differentiating Snow and Glacier Melt Contribution to Runoff in the Gilgit River Basin via Degree-Day Modelling Approach

“…Previous hydrological modelling studies within UIB [1,24,25], assessing the impact of climate variability on projected future flows, found significant biases in runoff contribution which they attributed to the selection of large catchment areas, poor quality/sparse precipitation data, and choice of Degree Day Factor (DDF) and/or temperature lapse rate (TLR) [23]. More recent studies have sought to address these shortcomings by employing more specific DDF and TLR applied at sub-basin scales [3,[26][27][28][29].…”

“…A number of studies [25,27,28] have employed the Snowmelt Runoff Model (SRM), which was designed for runoff simulation in snow dominant basins. However, when this model is used in a partly glacier-fed basin resulting in the neglection of the glacier-melt contribution, model accuracy can be compromised.…”

“…Importantly, it may also be necessary to consider that glacier cover melt-rates vary depending on whether a glacier consideration is debris-covered or clean-ice [31][32][33]. Similarly, the most recent studies based on snow cover assessment and hydrological modelling were carried out in the Gilgit River Basin (GRB) and neighboring river basins [27][28][29]34]. These studies focused on the climatic and snow cover trends for both the present and future, without considering glacier coverage (clean ice/debris-covered) [35] for the selected basins.…”

“…These studies focused on the climatic and snow cover trends for both the present and future, without considering glacier coverage (clean ice/debris-covered) [35] for the selected basins. Alternatively, [27] classified clean and debris-covered ice and employed the Modified Positive Degree Day Modelling (MPDDM) approach to address snow and ice contributions in GRB, rather than including the precise glacier melt contribution to flow. While a number of studies addressed the contribution of glaciers in terms of melting from snow on land (SOL), snow on ice (SOI), and exposed glaciers ice (EGI) [3], results were based on the melting rate generated by clean-ice glaciers rather than including the precise debris-covered glacier fraction.…”

“…Over the period 2001 to 2012, the GRB had between 15-80% snow-covered areas, with 8% covered by glacier ice [13]. According to recent estimates [27], around 1139 km 2 of the total 12,655 km 2 area is glacier-covered (around 9%). Despite representing a relatively small percentage of the total area, the glacier coverage contribution to snowmelt runoff should be taken into account in order to calculate more reliable runoff estimates, particularly in the context of any future changes in climate in the region.…”