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.