Based on 106 years of measured streamflow of the Nile River above Aswan, Egypt, and on readings of the annual high river flow at Cairo from the year 1700, the question of possible short-period climate changes in the source region for the water is examined. The problem of random time series is discussed., the runoff record of the Colorado River is considered in comparison.
A 115-year cycle from 1725 to 1840 is compared with the still continuing subsequent regime of high flow followed by low flow. The difference between high- and low-flow periods is near 25% of average in both cases, representing a large change in available water supply. Mean annual discharge was measured as 91 km3 year−1 from 1870 to 1976 and estimated as slightly less, 87 km3 year−1, for 1725–1840, due to lower computed annual discharge in about half of the years with high flow.
Variability in Nile discharge is thought to be mainly related to changes in precipitation over the Ethiopian mountains. Preliminary analysis of satellite radiation temperatures for June–September 1974 shows a marked diurnal regime with low radiation temperatures concentrated in the late part of the day. However, there are periods from 1 to 8 days with suppressed diurnal variation. Five such periods account for half of the days when radiation temperature of −30°C or lower, indicative of substantial cumulonimbus presence, prevailed both day and night, mainly clustered around the high mountains. Suggestions for future research evolve from these observations.