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

Abstract

The state of Kerala, India, experienced massive flooding in August 2018. Unprecedented extreme rainfall left the region with huge losses of infrastructure. Several studies reported the role of improper dam operations and climate change in this region. However, changing land use/land cover (LULC) is an important driver of flood modulation; we therefore studied regional LULC changes (over 4 decades) and their impacts on flooding during this event using the Weather Research Forecasting (WRF)-Hydro model. We downscaled the NCEP final operational global analysis data using WRF to provide meteorological inputs for the gridded WRF-Hydro model. WRF downscaled forcing, along with observed discharge, were used to calibrate WRF-Hydro, which was run with the meteorological forcing of the August 2018 floods using LULC corresponding to 1985, 1995, 2005, and 2018. Analysis of LULC change indicated a significant loss of evergreen forest and reductions in shrubland during 1995-2005 and considerable loss of mixed forest during 2005-2018. The most extensive changes in flooding attributes (discharge, 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 >10% at many stations (up to ~50% for several locations). The increase in surface water head was ~40% over some parts with increased inundation. During 2005-2018, afforestation measures reduced the steep decline in LC observed during 1995-2005. Thus, the contribution of LULC changes to the August 2018 flooding is mainly due to the deforestation-related changes during 1995-2005. This substantial role of LC in enhancing flood hazards highlights the need to better manage LC in watersheds.