What becomes of the rainfall?

The water that falls on the land is disposed of in three ways:  First, under ordinary conditions, a large portion runs off without entering the soil; secondly, a portion enters the soil, but remains near the surface, and is rapidly evaporated back into the air; and, thirdly, a portion enters the lower soil layers, from which it is removed at later periods by several distinct processes.  The run-off is usually large and is a serious loss, especially in dry-farming regions, where the absence of luxuriant vegetation, the somewhat hard, sun-baked soils, and the numerous drainage channels, formed by successive torrents, combine to furnish the rains with an easy escape into the torrential rivers.  Persons familiar with arid conditions know how quickly the narrow box canyons, which often drain thousands of square miles, are filled with roaring water after a comparatively light rainfall.

The run-off

The proper cultivation of the soil diminishes very greatly the loss due to run-off, but even on such soils the proportion may often be very great.  Farrel observed at one of the Utah stations that during a torrential rain—­2.6 inches in 4 hours—­the surface of the summer fallowed plats was packed so solid that only one fourth inch, or less than one tenth of the whole amount, soaked into the soil, while on a neighboring stubble field, which offered greater hindrance to the run-off, 1-1/2 inches or about 60 per cent were absorbed.

It is not possible under any condition to prevent the run-off altogether, although it can usually be reduced exceedingly.  It is a common dry-farm custom to plow along the slopes of the farm instead of plowing up and down them.  When this is done, the water which runs down the slopes is caught by the succession of furrows and in that way the runoff is diminished.  During the fallow season the disk and smoothing harrows are run along the hillsides for the same purpose and with results that are nearly always advantageous to the dry-farmer.  Of necessity, each man must study his own farm in order to devise methods that will prevent the run-off.

The structure of soils

Before examining more closely the possibility of storing water in soils a brief review of the structure of soils is desirable.  As previously explained, soil is essentially a mixture of disintegrated rock and the decomposing remains of plants.  The rock particles which constitute the major portion of soils vary greatly in size.  The largest ones are often 500 times the sizes of the smallest.  It would take 50 of the coarsest sand particles, and 25,000 of the finest silt particles, to form one lineal inch.  The clay particles are often smaller and of such a nature that they cannot be accurately measured.  The total number of soil particles in even a small quantity of cultivated soil is far beyond the ordinary limits of thought, ranging from 125,000 particles of coarse sand to 15,625,000,000,000 particles of