The Effect of Topography on Rainy season Rainfall Variability over Blue Nile River Basin
Creators: name: Habte, Shambel
Publisher: name: National Academic Digital Repository of Ethiopia
Year: 2013

description: The effect of topography on rainfall variability in the East Africa highlands is less studied rainfall processes. 37 rain gauges and 5 complete weather sensors along four transects in the complex terrain region of the Blue Nile River Basin were deployed. The transects span along elevation ranges from about 677 m in lowland areas around the border between Sudan and Ethiopia to 4000 m in the central Ethiopia mountains. High quality summer monsoon rainfall data from the newly deployed rain gauges along each transects was used for this study. Based on the high temporal scale data obtained from the sensors, we presented the effect of topography on the spatial and temporal variability of rainfall across transects, and rain event properties using statistical techniques. Over the Blue Nile basin, the maximum % of hourly rainfall occurrence with a range of 18% to 24% at south west of choke mountain and western edge of the basin namely, Mendi, Nejo, Gimbi and the minimum % of hourly rainfall occurrences is 10.78% at south east of choke mountain .There is high spatial variability of total rainfall depth during the study period over the Blue Nile River Basin which is varied from 471.17mm south east of the choke mountain to 1569.97mm north west of choke mountain near Tana sub bain water divide. There was observed two rain reach zones over the basin, one is at south west of the basin and the other on south west of Choke Mountain. The terrain elevation greater than 2500 m along transects have the capacity to block the moist air movement over the basin. There is high variability of rainfall at relatively high elevation stations compared to low elevations at various temporal scales (15 min, 30 min, 1 hr, 3 hr and daily). Generally low total rainfall depth and frequency is observed from late morning to early after noon (10:00 to 14:00 LST) on each LST and high total rainfall depth and frequency from 4:00 to 7:00 after midnight and 15:00 LST to 19:00 LST during the afternoon while stations at Zanbit, chagni, Anjeni, Kuchit the low total rainfall depth and frequency for each hour during the day is from mid night to late morning and high total rainfall depth and frequency is only during afternoon which are located at the foot and mid elevation of mountain Unlike other station over the basin. The rain event properties, rain event depth, rain event duration, mean event intensity increase as elevation increase while Inter event time decrease as elevation increase. These results on the rainfall variability, effect of topography on rainfall rate and space time variability have significant importance for the understanding of rainfall processes, and its distribution, for evaluation of accuracy of satellite based rainfall estimates, for designing of ways of merging satellite rainfall estimates and ground based observations given sparsely distributed rain gauges. Keywords: Topography, Transect, rainfall variability, rain event properties, rainy season, Blue Nile River Basin