Modelling climate change impact on surface runoff and sediment yield in a watershed of Shivalik region

Abstract

The climate change refers to the seasonal changes over a long duration in relation to the increasing amount of greenhouse gasses in the atmosphere. Global warming leads to a more vigorous hydrological cycle, including higher amount rainfall and more frequent high-intensity rainfall events. The Himalayan region is suffering from a serious problem of soil erosion and rivers flowing through this region transport a massive load of sediment. Climate change has a significant contribution to soil erosion. It leads to loss of nutrient-rich top soil which in turn can affect the nation’s food security. The present study depicts modelling climate change impact on surface runoff and sediment yield in a watershed of Shivalik region of Himachal Pradesh using a process-based Agricultural Policy/ Environmental eXtender(APEX) model. Terrain characteristics were analysed with the aid of Cartosat DEM. Land use/land cover characteristics were extracted from Resourcesat-2 LISS-IV and ground observations. Soil samples were collected from the field were analysed to identify soil physical and chemical properties. Surface runoff and sediment yield data required for model calibration and validation were collected from the gauging station constructed in the field. The future climate scenarios (temperature and rainfall) namely A2 and B2 of the study area were downscaled using statistical downscaling model (SDSM). APEX model parameterization was done as per local conditions. The APEX model was calibrated on a daily basis for 2017 and 2018. For calibration and validation of the model used low to medium rainfall days. The model calibrated quite well for surface runoff (r2 - 0.92) and sediment yield (r2 - 0.88) with RMSE of 4.98 mm and 0.20 t/ ha for surface runoff and sediment yield, respectively. The model was validated well for surface runoff (r2 - 0.81) and sediment yield (r2 - 0.81) with RMSE of 2.6 mm and 0.11 t/ha for surface runoff and sediment yield respectively. The model performance was identified based on Nash- Sutcliffe efficiency (NSE). The model performed quite well for surface runoff and sediment 224 yield of NSE 0.71 and 0.70 respectively. The change in soil loss under A2 and B2 scenarios with respect to baseline period were predicted for the study area to recognize the effect of climate change on soil loss. The general trend in future climate shows there is an increase in rainfall under both A2 and B2 scenario. Under the A2 scenario, rainfall increases marginally higher than B2 scenario. A total of 41.35 per cent increase in rainfall during 2080, 20.14 per cent during 2050, and 27.27 per cent during 2020 were observed. But in B2 scenario due to lower emission, change in rainfall is relatively lower than A2 scenario. It was observed that 24.71 per cent, 29.13 per cent and 35.16 per cent increase during 2020, 2050 and 2080 respectively. Maximum temperature increases 3.7 oC during 2080 under A2, while under B2 scenario the increase is 2.6 oC. Similarly, minimum temperature also rising at 3.6 oC during 2080 under A2 scenario and 2.7 oC under B2 scenario. The increase in temperature under both scenarios is almost similar and a marginal difference was observed. Highest soil loss was estimated from scrub land (38.42 t/ha/yr) followed by agriculture (26.97 t/ha/yr) then open forest (21.69 t/ha/yr) and lowest in the dense forest cover (14.70 t/ha/yr) under baseline period. The average annual soil loss from the watershed is 25.45 t/ha/yr. It was observed that 64.61 per cent of the study area was under moderate (10-20 t/ha/yr) erosion risk class. 24.15 per cent area with severe (20-40 ton ha-1 yr-1) erosion and 11.23 per cent area contribute very severe (>40 ton ha-1 yr-1) erosion. Under A2 scenario the average soil loss during 2020s, 2050s and 2080s may increase 27.71, 21.84 and 46.94 per cent respectively. Similarly under B2 scenario average soil loss may increase 23.24, 30.71 and 38.80 per cent, respectively. The climate change impact on soil erosion under both scenarios suggests that there is an increasing soil erosion due to the increase in rainfall in Shivalik region of Himachal Pradesh. Due to the high intensity of rainfall and steep slopes of the study area the mechanical conservation measures are preferred. The agronomic, mechanical and biological measures can be also used to conserve the soil and water.