Impact of Climate and Land Use/Cover Changes on the hydrology of Lake Hawassa Watershed (Central Ethiopian Rift):
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Date
2020-06
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Addis Ababa University
Abstract
It is generally concurred that climate of the world is changing or at least its variability
is increasing and that is bringing significant hydrologic influences on water resources.
Consequently, the variation has transformed the way hydrologists view prediction of
future hydrologic parameters. The impact to water resources is not only limited to
climate variations however. Land use changes have large impacts on these resources
as well. Therefore, impact assessment on water resources is a compound effect of
natural phenomena and manmade alterations to the environment. Although these
modifications have a profound impact on many aspects of the environment, wetlands,
especially lakes, are among the most significantly affected.
Lake Hawassa is one of the extensively impacted Ethiopia Rift Valley Lakes. Perhaps
the most notable recent changes in the Hawassa watershed were the Lake Hawassa
level rise and the flooding of Hawassa town in 1998. Tikur Wuha River discharge
increase and the decline thereby disappearance of Lake Cheleleka was attributed to
the increasing siltation caused by alarming deforestation of the eastern catchment. The
impacts were reported to be of both natural and anthropogenic origin. However, little
was done to scientifically address these effects. The investigation entail understanding
the hydrologic regime shift caused by the climate variability in the recorded
meteorological parameters and anthropogenic factor as in the land use land cover
change in the watershed. Therefore, this thesis focuses in testing and detecting the
presence of significant trend in hydro-meteorological variables, identifying the
amount, distribution and time of land use land cover changes, assess the
morphometric change in Lake Hawassa and evaluate the dominant hydrological
processes using coupled surface and groundwater modeling framework.
Mass and double mass curves analyses of rainfall in four stations (namely Hawassa,
Haisawita, Yirba and Wendo Genet) within and nearby the watershed showed that
there is no trend in the rainfall of the area while Tikur Wuha River flow at Tikur Wuha
Bridge and Dato village depicted jumps. Trend and homogeneity test of Lake
Hawassa level, rainfall, temperature and flow at Tikur Wuha by Mann-Kendall and
Pettitt’s test revealed that all had a trend (at 5% significant level) and nonhomogeneity characteristics except rainfall. Change point analysis illustrated the year
1987-1988 was the year where most of the hydrometeorological parameters showed
changes. As a result, Landsat images taken in the years 1973, 1987 2003 and 2019 were
selected for image classification and the period from 1973 to 1987 and 1988-2003 were
taken as statistically stationary periods for model development (calibration and
validation) and study the impacts. The results of hydrometeorological analyses were
the bases for land use land cover change analysis and hydrological modeling
activities.
Land use and land cover maps were derived from ground truthing and satellite
imagery by supervised image classification technique. The analysis identified six
major land use land cover forms (Built up, Cultivated, grassland, grassed wetland,
shrub and forested land and water body). The change investigation displayed
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cultivated land was the dominant land use form in Hawassa watershed. Built up area
increased by 188% while shrub and forested lands diminished by almost 23% during
1973-2003. Recently, urban areas increased by over 800% proving to be the fastest
growing land use type. The land use changes in the only perennial Tikur Wuha River
catchment also followed similar patterns with the entire Hawassa watershed.
In this study, a bathymetric map was prepared using advances in global positioning
systems, portable sonar sounder technology, geostatistics, remote sensing and
geographic information system (GIS) software analysis tools with the aim of detecting
morphometric changes against the first extensive hydrographic map of Lake Hawassa
in 1999. Results showed that the surface area of Lake Hawassa increased by 7.5% in
1999 and 3.2% in 2011 from that of 1985. Between 1999 and 2011, while water volume
decreased by 17%, silt accumulated over more than 50% of the bed surface has caused
a 4% loss of the Lake’s storage capacity. The sedimentation patterns identified may
have been strongly impacted by anthropogenic activities including urbanization and
farming practices located on the northern, eastern and western sides of the lake
watershed. The investigation also demonstrated geostatistical modeling approach to
be a rapid and cost-effective method for bathymetric mapping.
Finally, a coupled surface and groundwater modeling system, MIKE SHE, suitable to
model lake watersheds effectively, was used to diagnose the responses of Tikur Wuha
catchment in Hawassa watershed to LULC changes and climate variability. Two
models based on the Bridge and Dato village stations flow data were developed to
tackle the huge difference between the two data sets. The models were calibrated and
validated, and were able to capture the dominant runoff processes and streamflow
dynamics of the catchment. Streamflow simulations and water balance assessment
indicated that, evapotranspiration accounted 85%, while the other components
represented 22% of the total rainfall the area received. This showed that the watershed
had given off its reserves to satisfy the water balance of the hydrologic components.
The models demonstrated that climate variability was found to have impacted
unsaturated zone storage but have smaller impact on the rest of the water balance
components in the watershed during the study period. Tikur Wuha River flow and
the components of the catchment water balance were adversely modified by land use
land cover changes; especially evapotranspiration, overland flow, unsaturated zones
storage, base flow to river and the saturated drain to river components. These
indicated that impacts in the watershed are reversible with the proper catchment
management supported by sound land use policies. Annual water balance was
moderately affected by the changes while streamflow was most susceptible to land
use change for both models of the catchment. Simulated streamflow indicated that the
Bridge model deteriorated with time while the Dato model simulated well but failed
to distinguish land use impacts. The study highlighted the importance of soil and
water conservation interventions in the various LULC classes particularly in the
agricultural land use systems. The study showed the gaps in streamflow data accuracy
and emphasized on the reassessment of the stations condition.
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Keywords
Change Detection, Bathymetry, MIKE SHE, Land Use Land Cover (LULC), Lake Hawassa