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a,;'/. 81 u 1 t " 12. . for artesian pres sures. It is believed that these deep waters are interconnected throughout the valley. Above the last confining strata are more recent deposits of coarser sands and gravels containing a water table influenced only to a minor extent by escape of water under pressure. Most of the water pumped is being taken from this zone. ., The Rio Grande coming in from the west has built up a fan in such manner as to isolate the northern half of the valley into a closed basin. There is a ground-water divide a few miles no rth of the river and any waters entering the valley north of that divide remain in the valley. Water in excess of agricultural needs moves to the lowest point called the sump which lies well to the eastern side of the valley. Here it is lost by evaporation from the San Luis Lakes and by transpiration from useless vegetation. Irrigation began in the central part of the valley near Hooper and Mosca about 1887. The farmers practiced sub-irrigation and the area was extremely productive for about 20 years. The end came when the land became water-logged and the soil saline. People moved westward and as of now the main body of farmed land is many miles from Mosca. Sub-irrigation is still practiced but by means of controlled drains, the danger of water logging is avoided. Ordinarily the water table must be within 18 inches from the surface during the growing season in this type of irrigation. When the crops are maturing, the drains are opened and the water passes overland towards the sump. In periods of short water supply, insufficient to bring up the water table to the desired elevation, surface methods must be employed. To meet these emergencies, farmers began to employ wells in about 1929. Many frowned upon these activities as being imcompatible with sub-irrigation. However in a few years hundreds of wells were drilled into the water table. They have been of immense value during drought periods. Sub-irrigation remains the dominant met hod of applying water and each year, every effort is made to bring the water table up to the desired elevation. After a year of very low water supply a very large reservoir has to be filled to restore conditions. Thus an above-normal supply must be available. This sinking of what some might call surplus water could lead to contractual difficulties with New Mexico. " I would place all the area north of the ground-water divide in Classification II, and that between the divide and the river in Classifi- cation I at least in Rio Grande County. - 12 -