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 (_desague_). 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 Rafael, 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. [36] There are at present in the Mendoza province 275,000 hectares with a definitive right, and 303,000 with an eventual right. The concessions fed by the Diamante and the Atuel at San Rafael, which amount to 120,000 hectares with a definitive right and 150,000 with an eventual right, are not yet entirely developed.