WATERSHED MANAGEMENT IN CHANGING CLIMATE - A CASE STUDY OF NEPAL

Abstract

In the present study, watershed management of Rangoon river basin of Dadeldhura District (Nepal) was carried out. The satellite image having resolution 10m×10m grid cells is taken for study. The GIS software ArcMap 10.3, ERDAS IMAGINE 2014 software used for image processing by adding spatial input data and the Universal Soil Loss Equation (USLE) was used to estimate the potential soil loss from the watershed. Mann-Kendall and Sen’s slope estimator were used to investigate and assess the significance of the potential trend of three variables viz. rainfall, temperature and runoff over the Rangoon watershed. The trend analysis has been carried out on monthly, seasonal and annual basis using the data period between 1979 to 2010 for rainfall and temperature and 1967 to 1996 for runoff. The most important findings from trend analysis are, i) There is warming trends over the Rangoon watershed as Mann-Kendall statistic (Z-value) for most of the maximum temperature values are positive, ii) Rainfall and runoff affected by fluctuations every year though the annual rainfall showing a rising trend whereas runoff showing a falling trend. The rainfall seasonal trend analysis indicates that monsoon and post-monsoon period showed a positive rainfall trend with z statistics of +1.93, and +1.12 respectively, whereas pre-monsoon and winter seasons showed a negative trend with z statistics of -1.02, and -0.54 respectively. However, the annual rainfall in the Rangoon watershed showed a positive trend with a z value of +1.70. The rainfall erosivity factor (R) for Rangoon watershed was estimated about 530 MJ mm/ha/h/year. The minimum soil erodibility factor (K) for low slope region is 0.01 t ha h/ha/ MJ/mm and maximum of 0.3 t ha h/ha/MJ/mm is adopted. The slope length factor (LS) is given by the model based on input factors and DEM raster image is 0 to 70 for the watershed. The crop management factor (C) is adopted based on the land use type is from 0.000 to 0.20. Similarly, the value of conservation support practice factor (P) is taken 0.7 for main river Rangoon where some protection works had been done and value of 1 for all other region. The potential average annual soil loss obtained from the USLE model in the Rangoon watershed is 8 t/ha/year having maximum erosion of more than 50 T/ha/yr in some areas of exceptional case. The result shows that about 84.20% of land of total watershed has very low erosion due to dense forest and flat grassland. The remaining erosion potential area is divided based on the severity of low, high, very high, severe and very severe potential zones having percentage area 11.23%, 1.46%, 0.83%, 0.42% and 1.87% respectively. The mean annual flow out from the outlet, average monthly flow depth in the channel, sediment concentration in the flow out from the watershed is also computed by the SWAT model run. But calibration and validation is not applied because of absent of field data. Therefore, these areas need immediate attention from soil and water conservation point of view. In order to take action against this crucial soil erosion due to impact of climate change, the various structural and non-structural measures (vegetation) were recommended to control erosion within acceptable limit.