Photograph by the Author.

Plate VII.

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Another problem, which the smaller oases of the north-west hardly know—the problem of drainage—is of paramount importance at San Juan and Mendoza, as far as a large part of the irrigated surface is concerned. The water infiltrating into the soil forms a subterranean sheet which approaches more or less to the surface according to the topography. It comes to the surface at the foot of the cone, where the slope diminishes and the cone gradually passes into the plain. Hence the cone has, at its base, a belt of marshes (ciénagas), and sometimes a line of good springs (barbollon). At San Juan, if you move far enough away to get a comprehensive view of the whole of the estates, you see that they occupy the middle belt, half-way down the cone, the top of which is composed of coarse shingle, while the bottom is too wet. The advance of the plots upward and the steadily increasing use of the available water tends to raise the level of the underground sheet and enlarge the area of marsh.

There is a fine black soil, very fertile when it is drained, and no irrigation is needed; as it is possible, according to the depth of the drainage-trenches, to regulate the level of the underground water so as to make it reach and feed the roots. The draining of the marshes, again, opens up a field for the further expansion of the estates, especially at San Juan, where it has scarcely begun. Moreover, the water that is obtained by draining the marshes enables them to create new irrigated estates further on. At Mendoza there is already a considerable area irrigated by drainage-canals (desagüe).

The level of the water in the marshes sinks in the summer and rises in winter, at the time when the irrigation of the upper districts is suspended or greatly reduced, and when the surplus of the acequias, which the fields no longer take, flows or infiltrates downward in any way that it can. Thus, contrary to the torrent itself, it is in winter that the drainage-canals are at their fullest. At Barriales (Mendoza), and on the lower course of the Zanjon canal, thousands of acres, watered by the drainage-canals and exposed to drought in the summer, have the right to take water from the river or the canal during the three summer months, from November to January. During the remainder of the year they are restricted to the use of the drainage-canals. This sort of concession seems to provide a means of using the surplus of the river during the summer.

With this exception there are no temporary rights limited to the high-water season and enabling them to raise quick crops, that ripen in a few months, round the area of perennials. At least, the expansion of the estates and the wish to use the full water-supply have led to the creation of eventual rights, besides the definitive rights. They do not come into play, theoretically, until the definitive rights have had their full supply, and then only in a fixed order. They are subordinated to the ordinary rights, and the market value of land with eventual water-rights is much lower than that of land with definitive rights.[36] At San Rafaël, where colonization preceded the systematic inventory of the natural resources, the concession of eventual water-rights was a means of facilitating the development of estates; though they were very badly informed as to the surplus of the Atuel and the Diamante and the area that the new land might cover.

In practice, the co-existence of eventual and definitive rights presents many difficulties, and more than one pretext for fraud. Somtimes Sometimes the owners of eventual rights have access to the river higher up than the older intakes, which ought to be served first. A whole group of canals feeding land with eventual rights is in this way grafted upon the Tunuyan above La Paz, the rights of which are definitive and ancient.