Modelling the impact of prescribed global warming on runoff from headwater catchments of the Irrawaddy River and their implications for the water level regime of Loktak Lake, northeast India

摘要

Climate change is likely to have major implications for wetland ecosystems,which will include altered water level regimes due to modifications in localand catchment hydrology. However, substantial uncertainty exists in theprecise impacts of climate change on wetlands due in part to uncertainty inGCM projections. This paper explores the impacts of climate change uponriver discharge within three sub-catchments of Loktak Lake, aninternationally important wetland in northeast India. This is achieved byrunning pattern-scaled GCM output through distributed hydrological models(developed using MIKE SHE) of each sub-catchment. The impacts of climatechange upon water levels within Loktak Lake are subsequently investigatedusing a water balance model. Two groups of climate change scenarios areinvestigated. Group 1 uses results from seven different GCMs for an increasein global mean temperature of 2 °C, the purported threshold of''dangerous'' climate change, whilst Group 2 is based on results from theHadCM3 GCM for increases in global mean temperature between 1 °C and 6 °C. Results from the Group 1 scenarios show varying responses betweenthe three sub-catchments. The majority of scenario-sub-catchmentcombinations (13 out of 21) indicate increases in discharge which vary from<1% to 42% although, in some cases, discharge decreases by as muchas 20%. Six of the GCMs suggest overall increases in river flow to LoktakLake (2–27%) whilst the other results in a modest (6%) decline. Incontrast, the Group 2 scenarios lead to an almost linear increase in totalriver flow to Loktak Lake with increasing temperature (up to 27% for 6 °C), although two sub-catchments experience reductions in meandischarge for the smallest temperature increases. In all but one Group 1scenario, and all the Group 2 scenarios, Loktak Lake water levels arehigher, regularly reaching the top of a downstream hydropower barrage thatimpounds the lake and necessitating the release of water for barragestructural stability. Although elevated water levels may permit enhancedabstraction for irrigation and domestic uses, future increases in hydropowergeneration are limited by existing infrastructure. The higher water levelsare likely to exacerbate existing ecological deterioration within the lakeas well as enhancing problems of flooding of lakeside communities.