Genesis 41:34

The fifth part. That only a fifth of the harvest should be collected each year implies that even in the lean years the land was to produce something. The fertility of Egypt has always been dependent upon the yearly inundation of the land by the Nile, since rain is practically unknown.

Before the construction of the Aswan Dam and levees along the Nile in the last century to regulate the inundation, dikes were built to control a normal rise in the river. This was 25 or 26 ft. above the low level of the river in the dry season at Aswan. If the inundation reached 27 ft., dikes were washed away; if it reached 30 ft., villages were destroyed and lives lost. On the other hand, an inundation of but 23 ft. would not bring water to fields lying 2 mi. from the river, and would result in partial drought. Of the inundation of the Nile, Pliny wrote: “The ordinary height [of the inundation] is 16 cubits. When the waters are lower, they do not overflow the whole ground; when higher, it takes a long time for them to recede. In the first case the ground is not saturated; in the second, the waters are detained so long on the ground that seed-time is lost. The administration takes cognizance of both. At a height of only 12 cubits a famine is the consequence. Even at 13 cubits hunger prevails; 14 cubits produces general rejoicing; 15, perfect security; and 16, all the luxuries of life” (Natural History, v. 10).

Since Egypt produced more grain in normal years than it needed for home consumption, and was therefore able to export great quantities, the collection of 20 percent of the crop in plentiful years would work no hardship and would, at the same time, add up to an enormous amount of grain. It would not be wise to require too great an amount; otherwise the good will and cooperation of the farmers and landlords would be lost. With abundant harvests they could easily pay this increase in taxes and not feel it to be oppressive.