Rain event properties at the source of the Blue Nile River

Abstract

Abstract. In the present study, spatial and temporal patterns of rain event properties are analysed. These event properties are rain event depth, event duration, mean event rain rate, peak rain rate and the time span between two consecutive rain events which is referred to as inter-event time (IET). In addition, we assessed how rain event properties change when the period over which rainfall data is aggregated changes from 1 to 6 min and when the minimum inter-event time (MIT) changes from 30 min to 8 h. Rainfall data is obtained from a field campaign in two wet seasons of June-August (JJA) of 2007 and 2008 in Gilgel Abbay watershed that is situated at the source basin of the Upper Blue Nile River in Ethiopia. The rainfall data was automatically recorded at eight stations. The results revealed that rain event depth is more related to peak rain rate than to event duration. At the start and towards the end of the wet season, the rain events have larger depth with longer duration and longer IET than those in midseason. Event rain rate and IET are strongly related to terrain elevation. Sekela which is on a mountain area has the shortest IET while Bahir Dar which is at the south shore of Lake Tana has the longest IET. The period over which rainfall data is aggregated significantly affected the values of rain event properties that are estimated using relatively small value (30 min) of MIT but its effect diminished when the MIT is increased to 8 h. It is shown that increasing the value of MIT has the largest effect on rain event properties of mountain stations that are characterised by high rainfall intermittency.

Changes in land cover, rainfall and stream flow in Upper Gilgel Abbay catchment, Blue Nile basin – Ethiopia

“…Simple tests in this work on the daily time series of rainfall as well as stream flow indicated that data only is weakly correlated. We note that the weak auto-correlation is supported by research findings in Haile et al (2009Haile et al ( , 2011 on rainfall diurnal cycle assessments and rainfall variability issues.…”

“…We also observed that deforestation is a day-to-day activity of the people that use the wood for heating and cooking. The catchment is characterized by a rainfall regime with a pronounced dry and wet season with very high rainfall intensities (>20 mm h −1 ) and high variability in space and time dimension (see Haile et al, 2009Haile et al, , 2010Haile et al, , 2011. During our field visits we observed that high rainfall intensity and Hortonian overland flow affected the sheet and rill erosion processes while saturation excess overland flow was observed at overland flow source areas at the down side of hill slopes and in erosion gullies.…”

Rainfall timing and runoff: The influence of the criterion for rain event separation

“…It allows rain to be recorded with rain gauges; additionally, it allows addressing a diversity of objectives, at different temporal scales, by choosing the best MIT for those goals. As shown here and in several studies (Dunkerley, 2008b;Haile et al, 2011), the MIT chosen to define rainfall events has important consequences on the properties of the events. This indicates that, in research based on rain events, the choice of criterion to separate them can be crucial.…”

“…To describe the events generated in each case we calculated the following parameters: total rain depth, duration, mean intensity, maximum intensities at 60 and 5 minutes (I 60max and I 5max ), number of single-tip events, total volume collected in single-tip events and mean duration of dry periods between rainfalls, known as interevent time (IET) (Haile et al, 2011).…”

“…Understanding the characteristics of rain is essential for both hydrological and climatic studies (Haile et al, 2011). Rain depth, duration and intensity influence the amount of water that plants intercept, as well as evaporation, infiltration, ponding and horizontal flow (Hawke et al, 2006;Struthers et al, 2007).…”