Abstract:
Land use/land cover change has been one of factors responsible for altering the hydrologic
response of watersheds leading to impacting river flows. Various water resources projects’
planning and implementation will require knowledge of the extent of these changes on catchment
hydrology. The main objective of this study was to assess the impact of the environmental
changes on the flow of the Awassa catchment which is located in the Main Ethiopian Rift valley
in Ethiopia. In particular, the study analyses the land cover change between the 1986, 2000 and
2011, and the effect these changes have had on flows in the catchment. Land use maps of 1986,
2000 and 2011 were derived from satellite images using ERDAS Imagine 9.1 software. The land
cover changes within the catchment were examined through classification of satellite images
with integrated use of image and spatial analysis. The Soil and Water Assessment Tool (SWAT)
model was used to investigate the impact of land cover change on stream flow. The model was
calibrated and validated using historical flow data. Land cover change analysis has shown an
increment in the proportion of that cultivated land from 9.5% to 17.6%, agriculture land 9.1% to
19%, and built‐up areas 5.2% to 14.4%, while a decrement in the forest area from 31.15 to 19%,
water body, from 7.2 to 7% shrub from15.9% to 8.2%, wet land from 8.8 to 4.9 and grass land
from 13.2 to 9.6% decreased between 1986 and 2011. Sensitivity analysis has shown that the
curve number is the most sensitive parameter that affects the hydrology of the watershed. The
model was calibrated for the year using flow data from 1992 to 1999 and validated from 2000 to
2002. The R
2
and Nash‐Sutcliffe Efficiency (NSE) values for the watershed were 0.90 and 0.89
for calibration, and 0.88 and 0.87 for validation, respectively. The evaluation of the SWAT
model response to the land cover has shown that the mean wet monthly flow for 2011 land cover
increased by 39% compared to the 1986 land cover. On the other hand, dry average monthly
flow decreased by 46% in 2011 compared to 1986 land cover. According to the uncertainty
analysis carried out, the significance of the LULC impacts for the catchment was analyzed. As a
result it might be possible to conclude that for the catchments the impacts were significant.
Therefore, it can be deduced that LULC impact for the study area might be the most sensitive
than the propagated uncertainty on catchment flow.
