Modelling the impact of land use land cover changes on the runoff processes of Chalakudy basin using HEC-HMS model

Abstract

Fast development of urbanization alongside other expanding anthropogenic factors have been distinguished as significant reasons for land use changes and land transformations. This eventually causes several devastations like floods, droughts, water contamination and soil debasement. There is a need for target evaluation and investigation on the land utilization patterns and the mode of operation of water conserving structures in order to take up any preventive and additional healing measures. The state of Kerala in particular is notable for significant level of development as far as socio–monetary components, education, human services and so forth are considered. The broad financial changes have prompted expanded pace of framework, building development and several land use changes in the most recent decade. Evaluating the spatial and temporal changes in land use and land cover (LULC) of a basin is one of the analytic strategies to comprehend the issues continuing in a basin and gives significant understanding of its effect on runoff processes. The Chalakudy river basin in Kerala was one of the worst affected basins during the floods of 2018 and has experienced unaccountable damages to human life, ranches, gardens, domesticated animals, buildings, roads etc. The present study compares the LULC changes over two different decades 1997- 2007 and 2007-2017 by analysing the LULC maps and the effect of these changes on the runoff processes in Chalakudy river basin. From the LULC maps, the area under each class, the percentage area coverage and decadal percentage change for each class were calculated. The Hydrologic Modelling System HEC-HMS, developed by the US Army Corps of Engineers Hydrologic Engineering Centre (HEC) was used to model the flood flows of the basin. Calibration and validation of the model was done by employing the SCS CN as the loss method. Calibration of the model was done for five years (2003- 2007) to discover the best parameters of HEC-HMS model while validation of the model was done for three years (2015- 2017). The final analysis of the model showed CN to be the most sensitive parameter for simulating the runoff in the basin. The Nash-Sutcliffe model efficiency (E) for the calibration period was found to increase from 0.726 to 0.766 and 0.816 for the validation period. The correlation coefficient (R2) value was observed to increase from 126 0.80 to 0.83 before and after the calibration and a value of 0.85 was obtained for the validation period respectively indicating good performance of the model. Simulation runs of the model were done separately for another three years i.e., 1997, 2007 and 2017 in order to analyze the changes in runoff with respect to land use changes. It was observed that the vegetation area decreased consequently from 886.21 km2 to 803.09 km2 while the urban area was found to increase from 31.74 km2 to 41.93 km2 (1997-2017). Aside from that the annual rate change for each class was calculated and results showed an increment in the class of paddy, palm, barren land and urban area while a decrease in annual rate change of vegetation class was also observed. LULC transition matrix was also prepared for 1997-2007 and 2007-2017. From the net loss and gain calculation it was observed that the highest loss from 1997-2007 was found to be for vegetation (-52.52 km2) and the highest gain was of Paddy (54.39 km2). In between 2007-2017 the highest loss was noticed to be for vegetation (-30.59 km2) while the highest gain was for barren land (54.39 km2). The study highlights a disturbing observation in the last two decades and how this change has prompted the occurrence of floods and runoff. After analyzing the decadal land use changes and the simulated runoff values, it was understood how, loss of vegetation cover and increase in urbanization being the most significant reasons for LULC changes have altered the overall basin ecology.