Hydrologic extreme event-based feedback dynamics over a tropical monsoonal river basin

With the increase in the number of extreme hydrologic events under the changing climate, the catchment and regional scale water balance gets altered. This affects the incoming-outgoing radiation-based energy balance in the lower stratosphere zone causing short-term feedbacks in the land-atmosphere interaction process. This study assesses the short-term feedback dynamics of the evapotranspiration (ET) and surface runoff (SRF) fluxes during the Phailin cyclone, occurred in September 2013, over the tropical monsoonal Brahmani River basin in eastern India. Soil and Water Assessment Tool (SWAT) is used as the land-surface model to generate the water balance components of ET and SRF for which the observed streamflow and remote-sensing (MOD16A2) based ET datasets are used as the benchmark. The forcing datasets used in setting up SWAT include the grid-based observed precipitation and maximum / minimum- temperature datasets are from the India Meteorological Department and the processed dynamically downscaled NCEP Final analysis products, derived using the nested Weather Research and Forecasting (WRF) model for finer resolution analysis. The results reveal that, during the post-Phailin cyclone, the ET flux was decreased by 5-8% from the pre-cyclone phase; whereas the corresponding SRF flux was increased by 11-15%. This has resulted in an increase in the atmospheric water content and soil moisture amount by 2-4% and 8-10%., respectively. As a feedback mechanism to the atmosphere, the atmospheric water has reduced the corresponding short and long radiative forcing by 3-5% decreasing the atmospheric temperature by 0.02%. Conclusively, during the hydrologic extreme events such as cyclones, the land-surface fluxes of ET and SRF provide positive and negative feedbacks to the atmospheric temperature and radiative forcing with affecting the local weather, respectively.