Plausible Projections of Climate Induced Hydrologic Response: Case Study of Jhelum and Chenab Rivers

Abstract

The damage due to floods and droughts has been contributed by the inter-annual and spatial variability of climate. Climate change will have a significant influence on Pakistan's water-related disasters and environmental challenges. The country is one of the regions where excessive monsoon-related rainfall will grow, exposing a higher proportion of the population to floods. Climate change can potentially affect the frequency and extent of flood inundation in river basins, impacting lives, infrastructure, agriculture, fisheries, and ecosystems. As a result, projecting the extent of inundation caused by future flooding events is critical for river basin management. The study is required to estimate the anticipated changes in flood magnitudes under a future climate in an area where snowmelt and rainfall contribute considerably to extreme flows. The relative contributions of these sources are projected to vary due to climate change. Further, there is a need for a comprehensive framework to address the climate change impact projections and modeling, integrating all the essential aspects of the modeling chain, which can be replicated for any region to develop reliable medium- and long-range climate and flood projections and warning systems and build the capacity. This study presents a framework for synthesizing the plausible climate data product based on the outputs of General Circulation Models (GCMs) to assess climate change and its impacts. The framework has been demonstrated using the case study of Jhelum and Chenab River basins. Chain of modeling techniques has been applied to comprehensively assess the climate change impacts on the extreme hydrological response in the study area. The modeling chain includes (1) Evaluation of Gridded Reanalysis Data (2) Development of climate zones/regionalization using the selected reanalysis data (3) Synthesis of Climate data for the various climate forcing scenarios, incorporating different GCM selection methods, and bias correction (4) Climate change trend analysis (5) Assessment of Hydrological response with the help of a physically-based hydrological model, forced by the synthesized climate data (6) Projected flows trend Analysis (7) Flood Frequency analysis (8) Construction of Synthetic design Hydrograph for unsteady flow boundary conditions (9) Reservoir operation optimization (10) Projected Flood Inundation analysis. Climatic data archives, including observed climate data (1971-2004), grid-based reanalysis data (1971-2004), general circulation models (GCMs) data of baseline period (1971-2004), and projected period (2005-2099), for the climate forcing Scenarios of RCP 4.5 and 8.5, were used to identify future climate change trends and extreme hydrologic response to climate change. Different modules in python computer language have been written for the (1) development of climate zones, (2) selection of General Circulation Models (GCMs) in a climate zone using past performance and envelop-based methods and synthesizing meteorological inputs for the hydrological model (3) Spatio-temporal trend detection, and (4) generate the unsteady flow data as upstream boundary conditions for the inundation analysis. The study resulted in the divisional boundaries of the climatic zones in the study area, projected climate data product for climate forcing scenarios of RCP 4.5 and 8.5, projected Spatio-temporal climate change trends, projected hydrological responses, projected flood frequencies, and flood inundation maps in Jhelum and Chenab River basins. The significant results in the trends of precipitation and flows were obtained when the analysis was performed for the projected period. While, the maximum and minimum temperature trends were significant, even when the projected period was divided into 3-decadal periods. A negative precipitation trend was observed throughout the study area, in which 40% of the area had a significant negative trend for the warm-wet season for RCP 4.5. For the RCP 8.5, the warm-dry season again exhibited a significant positive precipitation trend, whereas insignificant positive trends were obtained for the warm-wet, cold-dry, and cold-wet seasons. Overall, the decreasing precipitation trends for the warm wet and cold dry season and increasing precipitation trends for the warm dry season were significant. All the scenarios presented a significant increase in temperature trends in approximately all the climate zones in a region. All the scenarios presented consistent results for the projected maximum and minimum temperature with a high degree of significance and evidence of increasing trends. With the help of maps that present the spatial distribution of the significant increasing and decreasing trends, the comprehensive information of the total change in temperature in the specific area can be assessed. The results presented consistently significant increasing flow trends for June, July, and December (only Jhelum), for all the flow statistics and scenarios (RCP 4.5 and 8.5), in Jhelum and Chenab Rivers. Most of the scenarios projected significant decreasing trends for September and October in Jhelum and Chenab Rivers. The flood frequency would undergo a substantial reduction under most scenarios in the twenty-first century. A possible explanation may be the remarkable increases in temperature and evaporation throughout the year, and precipitation decreases in the monsoon season. Negative changes in floods corresponding to all return periods are significant under most scenarios in the twenty-first century. The optimized projected flood outflows from Mangla station, and Marala Station, for different return periods, were used as the boundary conditions to force the calibrated Rainfall-runoff Inundation model (RRI) for Jhelum and Chenab river respectively. The flood inundation maps were generated using the outputs of the RRI model, and inundated areas were also estimated for the projected floods of 10, 25, 50, 100, 200, 500, and 1000 years return levels. The trends projected using the scenarios provide the details of the range of trend variability of climate change in the region, with the knowledge of maximum increasing and decreasing trend quantification seasonally. Thus, despite the limitations in the adopted approach, the study's findings provide a reasonable overview of potential effects on Jhelum and Chenab River basin discharge and inundation due to climate change prescribed by the plausible developed scenarios