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Role of changing land use and land cover on the 2018 megafloods over Kerala, India

Ankur Dixit*, Sandeep Sahany, Balaji Rajagopalan, Sweta Choubey

*Corresponding author:

ABSTRACT: The state of Kerala in southern India was hit with massive floods in August 2018. The episodes of extreme rainfall were unprecedented and left the region with huge loss of livelihoods and infrastructure. There are several studies that reported the role of improper operations of dams in this region and climate change attribution. However, the changing Land Use / Land Cover (LULC) is an important driver of flood modulation, therefore we studied of LULC change (over the four decades) and its impact on flooding in this region during this event. We investigated the impact of changes in the LULC on the mega-floods using the Weather Research Forecasting (WRF)-Hydro model. In this, we downscaled the FNL (Final operational global analysis data) analysis data using WRF to provide initial and boundary conditions to the gridded WRF-Hydro model. These downscaled boundary conditions, along with observed discharge (at stations distributed over Kerala), were used to calibrate WRF-Hydro. The calibrated WRF-Hydro was run with the August 2018 flood's meteorological forcing using LULC corresponding to 1985, 1995, 2005, and 2018. Analysis of LULC changes suggests a significant loss of evergreen forest and reductions in shrubland during 1995-2005 and considerable loss of mixed forest during 2005-2018. The model results indicate that the most extensive changes in flooding attributes (discharge, flood inundation area, and flood surface height) were found for LULC changes from 1995 to 2005. Specifically, comparing 2005 to 1995, we found that high flows (estimated by Q10) increased by more than 10% at many stations, and for few locations, the increase was as high as ~50%. The increase in surface water head was ~40% over some parts with an increased inundation. During 2005–2018 afforestation measures reduced the steep land cover decline observed during 1995–2005. Thus, the LULC change contribution to the August 2018 floods is mainly due to the deforestation related land cover changes during the 1995 - 2005 period. This substantial role of land cover in enhancing flood hazards exacerbates the need to manage land cover in the watersheds better.