Chapter Four Comparison of Water Supply with Required Needs Available Information In the previous chapter, we discussed the irrigation water requirements for agriculture in Egypt and Sudan. We are now investigating the available supply to meet these needs and to account for years of drought.

The longest series of Nile observations is from the Roda gauge (in Cairo). We have maximum readings from these observations up to the present time (1911 AD). More specifically, from 1840 to 1911 AD, and minimum readings from two periods: one from 1870 to 1890 AD, and another from 1891 to 1911 AD. There was a single break in the observation series in the recent period, and the readings in these two periods were not taken by a single gauge.

However, this does not pose a difficulty in the matter. It is likely that the data from the first period was taken from "Takhbet Al-Fikr fi Tadbeer Neel Misr" (The Pinnacle of Thought in Managing the Nile of Egypt), authored by Ali Mubarak Pasha, which was translated by Mr. G. J. Keig from the Hijri calendar to the Gregorian calendar. The data from the modern period was taken from a publication by Shiloh Bey.

We have reviewed the maximum observations, but in this chapter, we will create a table showing the average of deviations from a certain mean. We will deduce from the data of the larger period, which spans eight hundred years, that the average varied slightly from one century to another, and that it generally showed a continuous upward trend. The reason for this is undoubtedly the gradual silting up of the riverbed.

If these maximum observations are plotted graphically, they do not indicate any regular cyclical movement. However, it is evident that high floods and low floods tend to occur in groups. For example, two groups of years with low floods occurred: one from 1813 to 1815 AD, and another from 1870 to 1879 AD.

We have daily data from the Aswan and Roda gauges. The river's discharge was measured during this period, although not consistently. Some discharges were measured by floats at Aswan, and since 1870 AD, measurements have been taken regularly. However, this was not the case in all locations. Before 1870 AD, discharges were measured regularly during the low-water season through the sluices at Aswan. During the flood season, however, discharges were not taken regularly for any long period, although research continues to measure flood discharge through the Aswan sluices, which requires maintaining its records.

If we use the discharge-gauge relationship curve and apply it to the gauge readings, we can obtain approximate discharge values at Aswan. However, this method is not entirely reliable for determining discharges during periods of low water due to changes in the riverbed. But when used to measure discharges during the flood season, its results are more accurate and better. Since the volume of water during floods and the period immediately following it (July to December) exceeds three-quarters of the total annual discharge, the discharge-gauge relationship curve is worthy of providing annual discharge estimates with sufficient accuracy for comparing drought years.

It should be noted that the calculation of the average for drought years depends entirely on gauge readings. The discharge measurements in recent years enable us to compare the supply of drought years with agricultural needs.

The maximum observations of the Roda gauge have been compiled into the following table, which shows the average of deviations from the mean, considering a deviation of half a meter. All known observations have been used for this purpose, and the averages for the two periods under consideration have been presented separately.