Abstract:
Rainfall and streamflow variability and trends have a significant impact on the economy of the
country. In the study area, the variability of rainfall and streamflow has a critical problem for
potential water resource, irrigation development and agricultural productivity. From this
point of view, this study is aimed to evaluate annual and seasonal rainfall and streamflow
variability and trends on temporal and spatial scale for selected stations. The variability of
rainfall on both annual and seasonal scales was evaluated using the coefficient of variation
(CV), precipitation concentration index (PCI) and standardized rainfall anomaly (SAI). The
Mann–Kendall test and Sen's slope estimator were used to trend analysis of rainfall and
streamflow at a 5% significance level. In addition to this, the water potential of the basin has
been estimated using the SWAT (soil and water assessment tool) model. The average annual
rainfall was 904.9 mm, with a coefficient of variation (CV) of 32% and standard deviation
(SD) of 283.8 mm, while the mean seasonal rainfall was 297.3 mm, with a coefficient of
variation (CV) of 48.4 %. The average annual streamflow was 2.815 m3
/s, with a coefficient of
variation (CV) of 39.1% and standard deviation (SD) of 1.202 m3
/s, whereas the mean seasonal
streamflow was 2.94 m
3
/s, with a coefficient of variation (CV) of 49.3%.The maximum Sen’s
slope estimator increase in the annual rainfall was observed with a magnitude of 29.3 mm/year
at Asasa station. However, a negative trend of annual rainfall was observed at Kofele station
(-17.45mm/year). The maximum Sen’s slope estimator increase in the annual streamflow was
observed with a magnitude of 0.099 m3
/year at Asasa station, whereas the minimum decreasing
annual streamflow was observed with a magnitude of -0.045 m
3
/ year at Wabi station. There
is generally a decreasing trend in streamflow and rainfall. The decline in annual rainfall and
streamflow affected on the water potential and productivity. Overall, the results indicate
rainfall and streamflow varied considerably across the stations and catchments. The identified
significant trends and variability of rainfall and streamflow can help to support planning
decisions for water management. It is recommended that this study could be expanded to
include more stations to generalize the variability and trend of rainfall and streamflow t
