Assessment of Non-Point Source Pollution in Lake Awassa Watershed Using the Annualized Agricultural Non-Point Source (AnnaGnps) Model
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Date
2008-07
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Addis Ababauniversity
Abstract
Lake Awassa watershed is a Closed-watershed located in the Central Main Ethiopia Rift
valley (MER). Despite its ecological, social, and economical functions and values, the
lake and its watershed have been and being inflicted by non-point sources of pollution
associated with deleterious anthropogenic activities. Better management of the
environment to halt the existing problem of pollution and possibly to restore the
ecosystem, requires informed decision on the root causes of the problem as well as on
alternative management measures.
In this study, the Annualized Agricultural Non-Point Source (AnnAGNPS) model has
been used for relative assessment of non-point source pollution, prioritization of the
sources of pollution and formulation and evaluation of alternative management
measures.
The relative comparison of the AnnAGNPS model average annual output, with respect to
land use, for the year 2004 showed that cultivated land with Maize and Coffee, monocropped
Maize land, and Bare land are leading sources in terms of runoff yield. In
addition, Bare land, Pasture poor condition and mono-cropped Maize land are top
yielding in terms of sediment. Based on nutrient yield mono-cropped Maize land,
cultivated land with Maize and Coffee, and cultivated land with Maize and ‘Chat’ are
principal sources in terms of Nitrogen. Bare land, Shrub land with poor condition, and
Pasture poor condition are leading in terms of Organic Carbon. Mono-cropped Maize
land and cultivated land with Maize and Coffee are chief sources in terms of
Phosphorous. Accordingly, the study identified mono-cropped Maize land, Bare land and
cultivated land with Maize and Coffee as priority areas that need alternative
management option to control the non-point source pollution.
The study also showed that a change from mono-cropped Maize land to mixed cultivation
of Maize and ‘Chat’ would result a decrease up to 78% for water and Organic Carbon,
91% for sediment and more than 95% for Phosphorus yields, while Nitrogen yield remain
unchanged. A change from Bare land to mixed cultivation of Maize and ‘Chat’ would
result a decrease up to 99%, 94% and 78% for sediment, Organic Carbon and water
yield, respectively, while an increase up to 100% for Nitrogen. A switch from the mixed
cultivation of Maize and Coffee to mixed cultivation of Maize and ‘Chat’ would result a
decrease up to 81%, 75%, 60% and 35% for water, Nitrogen, sediment and Organic
carbon yield, respectively, as well as a decrease of more than 85% for Phosphorus yield.
Comparison of 1965 and 2004 scenarios showed that a decrease of watershed runoff by
about 28%, while there is a decrease of sediment yield by about 73%. Similarly, there
were a decrease of around 32%, 50% and 35% for Nitrogen, Organic Carbon and
Phosphorus yields, respectively, for the 1965 scenario from that of 2004.
The result of the study showed that a change in land use system can bring about
significant reduction in non-point source pollution and the spatial distribution of areas in
the watershed that need immediate attention
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Keywords
Annualized Agricultural