The mean annual areal rainfall over the UBNRB, within
the studied period, is 1260 mm as shown in Fig. 2. The rainfall
distribution is highly variable both spatially – decreasing from
the southwest to the east and northeast – and temporally, i.e.
over the yearly seasons. Moreover, as the rainfall over the
UBRNB is highly seasonal, the Blue Nile river also possesses
a strongly-varying seasonal flood regime, whereby over 80%
of the annual discharge occurs during the four months from
July to October. The average annual flow of the Blue Nile at
the Sudan–Ethiopian border is about 48,660 million m3 which
represents more than 40% of Ethiopia’s total surface water
resources [24]. Hence, the UBNRB represents a substantial
water resource for Ethiopia and as well for the downstream
countries Sudan and Egypt.