Effects of Land Use Land Cover Dynamics on Watershed Hydrology and Reservoir Sedimentation in Chancho and Sorga Sub-Watersheds, Diga District and Nekemte City, Oromia, Ethiopia
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Date
2020-06-06
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Addis Ababa University
Abstract
Land use/cover and their change have an impact on watershed hydrology Human activities,such as
steep slope and reservoir buffer zone cultivation and urban land expansion at the expense of forest
and wetland causes for the dynamism of the watershed in the study area. The study were undertaken
to quantify water balance change, water quality change, reservoirs sedimentation, and degradation
of forest and wetland, due to difference in land use/cover,slope gradient and soil types in Chancho
and Sorga sub-watersheds. The SWAT model was used to quantify hydrological components,
sediment and nutrient loss and RUSLE model for soil loss estimation, sediment yield and subsequent
sedimentation of reservoirs. Surface water quality was assessed by taking water sample from each
surface water and analyzed in the laboratory for TN, TP, TSS, TDS, and PH. Three times historical
images of 20 years were used to assess the effects of urban land expansion on the natural
environment (water, forest, and wetland). The combined impact of land use/cover, slope gradient
and soil types on watershed hydrology were also estimated by the SWAT model. It is found that the
predictive capacity of the SWAT model for hydrological and water quality variables was evaluated
by using recorded river flow and sediment yield and showed a good agreement with simulated
values. The result indicated that urban land produced more surface runoff (642mm) than other land
types, followed by cultivated land; it is because of the impervious nature of urban land and steeper
slope and continues cultivation of farmland with no soil conservation practices. Forestland and
grassland produced (621mm and 610mm) of surface runoff and water yield,respectively, as
compared to cultivated and urban land because of their higher infiltration capacity,
evapotranspiration and percolation in the study area. Human activities such as land cultivation and
expansion of urban built-up areas are the main factors responsible for change of the watershed
hydrology in terms of change in water balance and water quality deterioration. The slope gradient,
soil type and land use/cover type also contribute for the change in water balance in the watershed.
As the slope increased from 0% to 20%, surface runoff and water yield increased in cultivated,
grass, and forest lands and then declined, whereas it increased in urban land as the slope increased
beyond 20%.. The combined effects of land use/cover and soil types had no significant impact on
hydrological components. The highest soil loss produced in the cultivated land which accounted for
76% (30.38ton/ha/yr) of the total soil loss and the least soil loss was recorded in the grass and
forestland (15.33 & 17.08ton/ha/yr), respectively, in the study sub-watersheds. Agriculture
dominated sub-watershed produced 8524.34 t/yr and forest dominated sub-watershed larger area
produces 14909t/yr of sediment and subsequently result in sedimentation of the reservoir differently.
Agriculture-dominated surface water was greater in total nitrogen, total suspended solids, and
acidity than the other two land uses. Urban-dominated surface water was greater in total dissolved
solids than others. Forestland and wetland continuously decreased from 17% and 14% in 1996 to
5.1% and 2.7% in 2016, respectively, due to an increased average in the annual urban growth rate
by 3.6% in the past 20 years. Therefore, the study areas are exprinced in an increased mean annual
local temperature, variability of rainfall and relative humidity, and surface water quality reduction.
Land use/cover and its change, slope gradient, and lesser extent of soil types are the most influential
factors in affecting the hydrology of the watersheds in terms of water balance, water quality
reduction, storage capacity loss of reservoirs and forest and wetland resource degradation.
Therefore, appropriate land use practice and watershed management strategies should be
implemented
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Keywords
Land Use/Cover, Slope Gradient, Soil, Reservoirs, Sub-Watershed, Watershed, Hydrology