Modelling the impact of conservation structures and climate change of water yield in a watershed

Abstract

Hydrological models have been increasingly used for the impact assessment of climate change and management practices on hydrological processes. In the Thuthapuzha watershed, where extreme events due to climate change and resulting changes in patterns of river flow predominate, proper management of water resources through soil and water conservation needs to be adapted in the future. In this research, SWAT model was used to simulate hydrological processes on a daily time-step in Thuthapuzha watershed, subbasin of Bharathapuzha located in Kerala, India. SWAT performs satisfactorily with Nash-Sutcliffe efficiency value (NSE) of 0.88, coefficient of determination (R2) of 0.88 and Percent bias (PBIAS) of -1.4 for the calibration period (1989-2009) and R2, NSE and PBIAS values of 0.8, 0.8 and 5.4 respectively for the validation period (2010-2017). The study concluded that the developed SWAT model can be used to predict streamflow from the watershed. So the developed model was then used for studying the impact of climate change and conservation structures on the hydrology of the watershed. Quantification of changes in the water balance and soil erosion over a long period of time is necessary for watershed management. The developed SWAT model was used to understand the impact of conservation practices on hydrological processes. Major conservation practices in the study area were modelled as ponds and Kanjirapuzha reservoir within the study area was modelled as dam. The results obtained were analysed to study the impact of conservation structures on streamflow and found that monthly streamflow increased during summer season (9-17%) when the river has a very lean flow with the effect of conservation practices which helps in maintaining a better environmental flow regime. Conservation structures impact on sediment yield was also analysed by comparing the outputs with and without the addition of structures. In addition to the structural details, sediment yield analysis requires equilibrium sediment concentration value which is very difficult to estimate. Thus, a calibration process was again done for calibrating equilibrium sediment concentration using sediment yield output at the Pulamanthole gauging station (Jalowska and Yuan, 2018). For the study, it was assumed that the sediment yield output obtained from the calibrated model as the sediment yield with the addition ofstructures. Monthly sediment yield showed a slight increase (0.001-0.04%) during the summer months whereas sediment yield decreased (0.2-1.3%) during peak flows with the addition of conservation structures. Climate data are collected from CMIP5 and CORDEX-SA datasets of GFDL-CM3 climate model for RCP4.5, RCP6 and RCP8.5 scenarios and the bias corrected weather data were used as input in SWAT model. Comparison of streamflow and drought intensity based on predicted climate change scenarios is evaluated. The results of the future simulations of streamflow in SWAT reveal that, river flow increased under all RCP scenarios with predominant increase in RCP6 scenario (37-60%) followed by RCP4.5 (13-16%) and RCP8.5 (9-16%) from 2021-2070. Significant increase in streamflow was found during the end periods of simulation for all the scenarios taken for the study purpose. Results show the importance of climate change effect on water resources, where it does not have only an effect on precipitation and temperature, but the streamflow is also directly influenced by climate change. Thus, necessary steps should be taken to mitigate the extreme events due to streamflow increase during future periods. In order to study the climatic condition in the Thuthapuzha watershed, drought intensity was calculated. Drought intensity was predicted using the SPI and RDI index for the period 1989-2017 and found that severely dry events have occurred once during 2015-16 when using SPI index. Comparison and regression analysis between both the indices showed that both were well correlated and similar trend with little variation in the drought period was observed. Thus, SPI index was selected for studying the impact of climate change on drought intensity and found that the wet years are more than drought years for all the RCP scenarios with RCP 8.5 shows more drought period followed by RCP4.5 and RCP6. For the projected period from 2021-70, extreme drought condition will occur only once and severe drought condition will occur six times for RCP8.5 whereas no extreme and severe drought conditions were observed for RCP4.5 and RCP6.SWAT successfully achieved the aim of this research; to assess the impact of climate change and conservation practices in the Thuthapuzha watershed. Nevertheless, uncertainty cannot be avoided in this study since climate model datasets were used for making the future prediction. The results of the entire research work will give an insight to hydrologists in solving climate change related issues as well as provides water resources managers with an effective tool for the integrated catchment management.