Figure 2 Elevation based classified map of upper Awash Basin
Result and Discussion
The climate of the Awash basin is traditionally known by four seasons.
These seasons are Arfaasaa refers to Spring (March, April, and
May), Ganna is Summer (June, July, and August), likewise,Birraa is Autumn (September, October, and November) andBona is Winter (December, January, and February). The basin is
known by wet season during Arfaasaa and Ganna including
September from Birraa season while the rest seasons are dry. That
means months from April to September are the wet and hot times. So, the
basin showed an increasing trend in different stations with various
seasons and a decreasing trend as well. Here the trend is expressed as
increasing and decreasing while the statistical significance level is
used to show the level of confidence to quantify basin’s hydroclimate
trend.
The upper awash river basin has two great streams that enter the Koka
reservoir. These streams are the Hombole and Mojo main tributaries.
These tributaries are exhibiting seasonally incremental and decremental
trend.
For the entire hydroclimate trend analysis daily data elements are
collected to evaluate seasonal fluctuation of study time series. Because
the socio-economy of the basin is influenced seasonally.
Seasonal and annual Meteorological analysis
Precipitation of eleven selected stations within the basin is evaluated
for 1991 to 2015 time series (Figure 3).
All stations’ daily data were
converted to monthly for evaluation since seasonal trend change is
considered as important. The basin during 25 years of study time series
had got 1019.41mm mean rainfall.
Furthermore, it had 1339.78 mm
maximum and 872.76mm minimum rainfall during 1993 and 1997 respectively.
The wet seasons Arfaasaa and Ganna the most basin’s
economically important seasons with annulling trend results are
considered.
Lowland stations (Bishoftu, Mojo,
and Hombole) show a change in precipitation and temperature. These
stations have annually and seasonally increasing trend with
precipitation and temperature. The Hombole station seasonal temperature
is autocorrelated except during the Arfaasaa season. So, the
analysis is subjected to a modified Mann Kendall trend analysis that
resulted in an increasing trend. This annual increasing trend is
statistically significant with a 95% significance interval.
Precipitation of Hombole station resulted in an insignificant increasing
trend. Mojo station revealed insignificantly increasing trend except for
the Birraa season. During the Birraa season, precipitation
and temperature of the station significantly increasing with 95% and
90% significance level respectively. Bishoftu station insignificantly
increased the precipitation annually and during all except Bona seasons.
Likewise, its temperature got increased annually and seasonally.