Abstract:
Flood hazard and water erosion are key environmental problems in degraded
watersheds. This study was conducted in Iresha watershed in Guraghe zone where there
is high incidences of floods and upland degradation. The objectives of this study are to
examine LULC change and dynamics in the last three decades, to assess the retrospective
and prospective state of soil erosion in the watershed, to analyze flood discharge and
future flood risk potentials, to examine local land management practices and to suggest
appropriate management interventions for the watershed. LULC change and dynamics
analysis was conducted with RS and GIS. Soil loss was analyzed using RUSLE model
with GIS and RS and also GIS and RS based analysis of runoff was determined with
cook’s method and management practices of the watershed is assessed using interviews of
local farmers, NGOs and government workers. The results show that Iresha watershed is
experiencing high LULC change and dynamics on cultivated lands, Enset, Erica bush
land and natural vegetation with Eucalyptus and Cupressus. By now the watershed is
dominantly covered by cultivated land (26%) followed by natural vegetation with dense
grass (23%). In the watershed the most changed land cover is Erica bush land with
sparse trees and it is changing 26 ha yr-1. The soil loss from the watershed is increasing
over years and cultivated lands generate more soil (17.35 tha-1yr-1) that was beyond the
tolerable limit for the area with the same agro-ecological setup and also for Africa and
for global average. Sub-watersheds on the upper part of the study watershed (32.4%) are
generating more soil with high future potentials. Runoff discharge was increasing
annually and sub-watersheds in the upper slopes generates more runoff while subwatersheds
in the lower slopes are generating less runoff which was calibrated by
recorded discharge values at the out let of the river by MoWR that ranges from 95m3s-1 to
305m3s-1. Maximum soil and runoff is generated from cultivated lands and the maximum
runoff and soil generating capacity is shifting upwards and more soil and runoff is
generated from the upper slopes due to the encroachment of cultivated lands to the
natural forests and Erica bush lands, free grazing in the natural forests and Erica bush
lands and lack of proper focus on natural resources management. Finally the cause for
flood hazard in Iresha watershed is identified and to protect those causes of flood the
possible corrective measures are planned.
Key words:- Runoff discharge, Flood risk assessment & mapping, soil loss, soil erosion risk
mapping, LULC Change detection, LULC dynamics analysis, GIS & RS, flood plain, flood
hazard zone, RUSLE, Cook’s method, Catchment Characteristics, etc