EFFECTS OF CHANGES IN LAND USE LAND COVER ON THE HYDROLOGICAL RESPONSE IN UPPER INDUS BASIN

Abstract

Empirically based lumped hydrologic models have an extensive track record of use for various watershed managements and flood related studies. This study focuses on the impacts of land use land cover change for a 10 year period on the stream discharge in a sub-watershed of Indus River using lumped model HEC-HMS. The Indus above Tarbela region acts as a source of main flood events in the middle and lower portions of Indus basin because of the amount of rainfall and geomorphological setting of the region. The discharge pattern of the region is influenced by the LULC associated with it. In this study the Landsat TM images were used to conduct LULC analysis of the sub-watershed. Satellite daily precipitation estimation of Tropical Rainfall Measuring Mission (TRMM) data was used as input rainfall. The input variables for model building in HEC-HMS were calculated based on the Digital Elevation Model, Land use Land cover, soil and precipitation data which was collected and pre-processed in HEC-GeoHMS. SCS-CN was used as transform model, SCS unit hydrograph method was used as loss model and Muskingum method was used for flow routing. For discharge simulation years 2000 and 2010 were selected. HEC-HMS was calibrated for the year 2000 and then validated for 2010. The model calibration statistics for n = 90 (monsoon season during July, August and September) are correlation coefficient = 0.96, relative Bias = 2.5% and Nash-Sutcliffe coefficient of efficiency (NSCE) = 0.89. The model validation statistics are CC = 0.95, Bias = -3.7% and NSCE = 0.94. The model results confirm the results of the LULC change analysis using Landsat TM images that in 10 years the impact of LULC change on discharge has been negligible in the study area overall. This is possibly due to the fact that the proportion of the built-up area in the sub-watershed, which is the main causative factor of change in discharge, is less than 1% of the total xii area. This conclusion was further supported by the LULC change analysis conducted on a local scale metropolitan district of Mansehra, where the impact of development was found significant in the built up area and corresponding discharge. The analysis was done on Mansehra city sub-watershed with an area of about 16 km2. Built up area in this sub-watershed increased from 8.2% in 2000 to 13.76% in 2010, whereas the corresponding river discharge increased by 33.61% from year 2000 to 2010, proving strong positive correlation on a local scale between the LULC and the river discharge. The rainy days of more than 15 mm rain were considered to generate river discharge and the change in percentage was averaged. The results showed that with an increase of 40% built-up area in the city from 2000 to 2010 the discharge values increased about 33 percent, indicating the impact of LULC change on discharge value.