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• W2798797621 abstract "The AC.U Valley is the lower part of the Rio Piranhas valley in the northwestern part of the State of Rio Grande do Norte, Brazil. It begins where the Rio Piranhas leaves the crystalline Precambrian rocks to flow across the outcrop of sedimentary rocks. The area considered in this report extends northward for about 45 kilometers ; it is terminated arbitrarily where encroachment by sea water has contaminated the aquifer and imparted a disagreeable saline taste to the water in it. The boundary was not determined in the field, however, for lack of specif 1 equipment. Part of the extensive uplands on either side of the valley are includ°d. This makes the total area approximately 2,500 square kilometers. The largest town, AQU, had a population of about 8,000 in 1960. The area is considered to be part of the Drought Polygon of northeast Brazil because the precipitation, although averaging 448 millimeters annually at Agu, varies widely from year to year and often is deficient for many months. The precipitation has been supplemented by use of irrigation wells, but irrigated agriculture is not yet far advanced, and the quantities of water used in irrigation are small. Geologically, the area consists of basement crystalline rocks (Precambrian), a wedge of sedimentary rocks thickening northward (Cretaceous), and alluvial sediments constituting a narrow band in the bottom of the valley (Alluvium and terrace deposits)'. The crystalline rocks contain water mainly in fractures and, in general, are impermeable. The sedimentary rocks of Cretaceous age comprise two units: a thick but fine-grained sandstone grading upward into siltstone and shale (Agu Sandstone), and limestone and dolomite with an included shale zone (Jandaira Limestone). The sandstone especially and the limestone to a lesser degree are ground-water reservoirs of large capacity. The limestone has been tapped at several places, but the sandstone and its contained water are practically untested and, hence, imperfectly understood. The alluvium of the first terrace is the aquifer supplying most of the ground water being used in the area. Wells in the alluvium yield as much as 80,000 liters per hour. Larger yields probably could be obtained from wells designed to take full advantage of the aquifer. There are in the valley about 300 dug wells which are used for irrigation. Half of these are equipped with pumps and engines. The rest, together with about 500 01 C2 HYDROLOGY OF LATIN AMERICA AND THE ANTILLES drive-point wells, are equipped with manual or windmill-driven pumps. In addition to irrigation, the water is used in homes and for cattle. The quantities of water currently used in irrigation are relatively small, both per hectare and in the area as a whole, but this will probably increrse substantially when intensive irrigation becomes a reality. The annual punpage from the alluvium, nearly constant since 1959, was about 2.25 million c'lbic meters in 1964, which is only about 90 cubic meters from each hectare-meter of saturated alluvium. This amount would lower the water table about 1 meter in 11 years, if there were no recharge. Actually, no such decline is likely to occur, because the recharge from precipitation alone is estimated to be more than enough to replace the water currently being pumped. Chemical analyses of eight samples show that the ground water in the aluviuin is acceptable for most uses. The water in the AQU Sandstone and Jandazra Limestone is more mineralized than that in the alluvium and at some places, at least, is not acceptable for human consumption. The available chemical data on this water, however, are not adequate to judge fully the quality of the wa ter in these formations. It is estimated that about 22 million cubic meters of water would be needed annually if irrigation were extended to all the bottom land, which totals about 25,000 hectares. This amount is only one-fourth to one-half the estimated recharge from precipitation alone. The present rate of application of water is very low but probably would be increased under intensive irrigation. An estimate of 350 million cubic meters per year by the U.S. Bureau of Reclamation exceeds the probable annual recharge, but this amount is not likely to be realized for a long time to come. Although much information not presently available will be needed before the limitations of the ground-water supply can be estimated accurately, it is evident that considerable more water can be developed for irrigation than is currently being used. INTRODUCTION The AC.U Valley is a developing farming area that has a rare combination of fertile bottom-land soils, favorable climate, and abundant ground water. It is one of the more promising areas for irrigated agriculture in Rio Grancle do Norte. Water is an important, factor in determining the degree and rate of development of the area, although it is not at present a limiting factor. Surface water developed and ready for use in irrigation is not at present adequate for all the irrigable area, nor can the future availability of increased surface-water supplies be predicted. The ground-water resources serve as a base for development of the area and should be fully integrated ir plans for use of the water resources." @default.
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• W2798797621 date "2011-01-01" @default.
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• W2798797621 title "Ground-Water Resources of the Agu Valley Rio Grande do Norte Brazil" @default.
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