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At least 1,153 records · Page 64Linked to original sources
USGS1964

Ground water for irrigation in the Snake River Basin in Idaho

The Snake River basin, in southern Idaho, upstream from the mouth of the Powder River in Oregon, includes more than 50 percent of the land area and 65 percent of the total population of the State. More than 2.5 million acres of land is irrigated ; irrigation agriculture and industry allied with agriculture are the basis of the economy of the basin. Most of the easily developed sources of surface water are fully utilized, and few storage sites remain where water could be made available to irrigate lands under present economic conditions. Because surface-water supplies have be come more difficult to obtain, use of ground water has increased greatly. At the present time (1959), about 600,000 acres of land is irrigated with ground water. Ground-water development has been concentrated in areas where large amounts of water are available beneath or adjacent to tracts of arable land and where the depth to water is not excessive under the current economy. Under these criteria, many of the most favorable areas already have been developed; however, tremendous volumes of water are still available for development. In some places, water occurs at depths considered near or beyond the limit for economic recovery, whereas in some other places, water is reasonably close to the surface but no arable land is available in the vicinity. In other parts of the basin large tracts of arable land are without available water supply. Thus the chief tasks in development of the ground-water resources include not only locating and evaluating ground-water supplies but also the planning necessary to bring the water to the land. Irrigation began in the 1860's ; at the present time more than 10 million acre feet of surface water, some of which is recirculated water, is diverted annually for irrigation of more than 2.5 million acres. Diversion of this large quantity of water has had a marked effect on the ground-water regimen. In some areas, the water table has risen more than 100 feet and the discharge of some springs has more than doubled. Large-scale development of ground water began after World War II, and it is estimated that in 1959 about 1,500,000 acre-feet of ground water was pumped for irrigation of the 600,000 acres irrigated wholly with ground water in addition to a substantial amount of ground water pumped to supplement surface-water supplies. Ground water is also the principal source of supply for municipal, industrial, and domestic use. The water regimen in the Snake River basin is greatly influenced by the geology. The rocks forming the mountains are largely consolidated rocks of low permeability; however, a fairly deep and porous subsoil has formed on them by decay and disintegration of the parent rock. Broad intermontane valleys and basins are partly filled with alluvial sand and gravel. The subsoil and alluvial materials are utilized very little as a source of water supply but are important as seasonal ground-water reservoirs because they store water during periods of high rainfall and snowmelt. Discharge from these reservoirs maintains stream flow during periods of surface runoff. Because these aquifers are fairly thin, they drain rapidly and are considerably depleted at the end of each dry cycle. The plain and plateau areas and tributary valleys, on the other hand, are underlain chiefly by rocks of high permeability and porosity. These rocks, mostly basaltic lava flows and alluvial materials, constitute a reservoir which fluctuates only slightly from season to season. Large amounts' of water are withdrawn from them for irrigation and other uses, and discharge from the Snake Plain aquifer is an important part of the total flow of the Snake River downstream from Hagerman Valley. The ultimate source of ground water in the basin is precipitation on the basin. In the mountainous areas, aquifers mostly are recharged directly by precipitation.

Idaho↗
USGS1964

Hydrogeology of northwestern Nassau and northeastern Queens Counties, Long Island, New York

A detailed investigation of the geology and ground-water occurrence in northwestern Nassau and northeastern Queens Counties, N.Y., has been completed by the U.S. Geological Survey, in cooperation with the Nassau County Department of Public Works and the New York State Water Resources Commission. The area, about 63 square miles, includes the peninsulas of Great Neck and Manhasset Neck and the western part of Nassau County as far south as Garden City. Unconsolidated sediments of Pleistocene and Cretaceous aged 200 to 800 feet thick, constitute the ground-water reservoir of the area. The reservoir includes three discrete aquifers, not all found in the same locality, which transect geologic boundaries. The shallow unconfined aquifer consists of those permeable Pleistocene and Cretaceous deposits that lie within the upper part of the zone of saturation, which extends from slightly below sea level to more than 110 feet above sea level. It is an abundant source of water in much of the area, particularly in the southern part, where the aquifer consists of coarse sand and gravel in a glacial-outwash plain. The intermediate or principal aquifer consists largely of that part of the Magothy(?) formation that occurs from about 50 feet below sea level downward to the top of the clay member of the Raritan formation. Locally, however, the aquifer includes Pleistocene deposits which blanket the Magothy(?) formation or lie in channels cut into it. Although the principal aquifer has limited extent to the north, it is the chief source of water in most of the area, except the peninsulas. The lower or deep confined aquifer is found beneath the entire project area, and consists of the Lloyd sand member of the Raritan formation and the hydraulically contiguous Jameco gravel. The lower limit of the aquifer is the bedrock surface; its upper limit is defined by the overlying clay member of the Raritan formation and the Gardiners clay, the latter abutting against the clay member on the north or, in some valleys and embankments, lying directly upon the clay member. Thus, hydraulic continuity exists between the Lloyd sand member of the Raritan formation (Cretaceous) and the Jameco gravel (Pleistocene) in the northern part of Manhasset and Great Necks. The two contiguous clay bodies overlying the aquifer, the clay member of the Raritan formation and the Gardiners clay, form an effective confining bed which probably extends beyond the shore lines of the project area. The deep confined aquifer is particularly important on Manhasset and Great Necks where locally it is the only source of water available for large public-supply or industrial needs. In general, ground-water supplies in sufficient quantity and of excellent quality can be obtained from the three aquifers underlying northwestern Nassau and northeastern Queens Counties, N.Y. Ground-water withdrawals for public supply have increased with population growth and expanded use from an average of about 10 millions of gallons per day in 1940 to 30 mgd in 1957. In addition, about 10 mgd are pumped for various industrial, institutional and private uses. Much of the water pumped by industry is returned to the ground by diffusion wells and recharge basins. However, an increasing amount of water is lost from the ground-water reservoir due to the expanding network of server systems discharging directly to the sea. In the Manhasset Neck and Great Neck areas, ground-water resources are approaching full development in terms of the total available supply, if such development has not been reached already (1959). The chief limiting factor to further expansion lies in the geometry of the aquifers and the reduced thickness of permeable saturated sediments. In the northern part of both peninsulas, the ground-water reservoir is only about 200 to 400 feet thick, and, locally, as much as 75 percent of the strata in the reservoir consists of rather impermeable silt and clay of the Raritan formation or the Gardiners clay. I

Water Supply Paper↗
USGS1964

Ground-water resources of the South Platte River Basin in western Adams and southwestern Weld Counties, Colorado

The area described in this report consists of about 970 square miles in western Adams and southwestern Weld Counties in northeastern Colorado. It includes that part of the South Platte River valley between Denver and Kuner, Colo., all of Beebe Draw, and the lower part of the valley of Box Elder Creek. The stream-valley lowlands are separated by rolling uplands. The climate is semiarid, the normal annual precipitation being about 13 inches; thus, irrigation is essential for stable agricultural development. The area contains about 220,000 acres of irrigated land in the stream valleys. Most of the remaining 400,000 acres of land is used for dry farming or grazing because it lacks irrigation water. Most of the lowlands were brought under irrigation with surface water during the early 1900's, and now nearly all the surface water in the area is appropriated for irrigation within and downstream from the area. Because the natural flow of the streams is sometimes less than the demand for water, ground water is used to supplement the surface-water supply. Wells, drilled chiefly since 1930, supply the supplemental water and in some places are the sole supply for irrigation use. Rocks exposed in the area are of sedimentary origin and range in age from Lato Cretaceous to Recent. Those that are consolidated, called 'bedrock' in this report, consist of the Fox Hills sandstone and the Laramie and Arapahoe formations, all of Late Cretaceous age, and the Denver formation and Dawson arkose of Late Cretaceous and Tertiary age. The surface of the bedrock was shaped by ancestral streams, the valleys of which are reflected by the present surface topography. Dune sand, slope wash, and thin upland deposits of Quaternary age mantle the bedrock in the divide areas, and stream deposits ranging in thickness from 0 to about 125 feet partly fill the ancestral valleys. The valley-fill deposits consist of beds and lenses of clay, silt, sand, gravel, cobbles, and boulders. Abundant supplies of ground water for irrigation, municipal, and industrial use are obtained in the principal stream valleys from wells tapping valley-fill deposits beneath the flood plain and bordering terraces. Many domestic and stock wells obtain water from the unconsolidated deposits both on the uplands and in the valleys. The ground water in the valley-fill deposits generally is unconfined but in a few places is under slight artesian pressure. The bedrock formations yield small to moderate supplies of water to municipal, industrial, domestic, and stock wells, but the yields are not sufficient for irrigation. Ground water in the South Platte River valley moves downstream and toward the river and is discharged into the river. The direction of ground-water movement in Beebe Draw and Box Elder Creek valley is nearly parallel to the streams. Beebe Seep, the stream in Beebe Draw, gains water from the groundwater reservoir in some reaches and loses water in others, but Box Elder Creek loses water to the ground-water reservoir throughout its course especially during floods. The shape and slope of the water table are affected chiefly by the permeability of the valley-fill deposits, the location and altitude of the areas of recharge and discharge, and the configuration of the underlying bedrock floor. The depth to water in the South Platte River valley ranges from less than 1 foot beneath the flood plain to as much as 80 feet beneath the terraces. In Beebe Draw the depth to water ranges from less than 1 foot to about 60 feet and in Box Elder Creek valley from about 5 feet to about 40 feet. During the period of record the annual fluctuation of water levels in wells in the area has ranged from 2 to 13 feet. Precipitation within the area and infiltrating water from irrigated tracts, reservoirs, canals, and streams are the principal sources of recharge to the ground-water reservoir; some recharge results from underflow from outside the area. Ground water is discharged by evapotranspiratio

Water Supply Paper↗
USGS1964

Summary of floods in the United States during 1958

This report describes the most outstanding floods that occurred in the United States during 1958. A series of storms from January 23 to February 16 brought large amounts of precipitation to northern California and produced damaging floods, particularly in the Lower Sacramento Valley where losses totaled about \$12 million. Major floods, notable because of the large area affected, occurred on many small streams in central and south Texas, following heavy general rains in late February. Extensive flooding occurred along the Gulf Coastal plain on the lower reaches of the major streams from the Brazos River to the Nueces River. Two lives were lost, and property damage exceeded \$1 million. Damaging floods of April 1-7 followed one of the wettest winters in California history. Swollen streams overflowed their banks throughout the central part of the State, and discharge peaks on many streams exceeded those .of the floods of December 1955. Most severely flooded was the San Francisco Bay area. Total flood damage was estimated at \$23 million. The storms and floods of April-May in Louisiana and adjacent States outranked all other floods in the United States during 1958 with respect to intensity of rain over a large area, number of streams having maximum discharge of record, rare occurrence of peaks, and great amount (\$21 million) of resultant damage. Heavy rains on June 8-15 caused one of the greatest summer floods of record in central Indiana. Peak discharges were high and of rare occurrences. Failure of numerous levees along the Wabash River caused great damage. Crop damage alone was estimated at \$48 million. Intense rains of July 1-2 caused record-breaking floods in southwestern Iowa. Rapid rises and the great magnitude of the floods on small streams resulted in 18 deaths and many injuries. Six towns and cities along the East Nishnabotna River and its tributaries were particularly hard hit; rural damage was also high. Total damage was estimated at \$15 million. Heavy rains (as much as 40 inches during the last 2 weeks in September) from the middle of September to the middle of October caused destructive floods along the Rio Grande in Texas and Mexico. Many communities were isolated by the flood waters, and damage to crops was great. In addition to the 7 floods mentioned above, 21 others of lesser magnitude are reported in this annual summary.

Water Supply Paper↗
USGS1963

Specific Yield--Column drainage and centrifuge moisture content

The specific yield of a rock or soil, with respect to water, is the ratio of (1) the volume of water which, after being saturated, it will yield by gravity to (2) its own volume. Specific retention represents the water retained against gravity drainage. The specific yield and retention when added together are equal to the total interconnected porosity of the rock or soil. Because specific retention is more easily determined than specific yield, most methods for obtaining yield first require the determination of specific retention. Recognizing the great need for developing improved methods of determining the specific yield of water-bearing materials, the U.S. Geological Survey and the California Department of Water Resources initiated a cooperative investigation of this subject. The major objectives of this research are (1) to review pertinent literature on specific yield and related subjects, (2) to increase basic knowledge of specific yield and rate of drainage and to determine the most practical methods of obtaining them, (3) to compare and to attempt to correlate the principal laboratory and field methods now commonly used to obtain specific yield, and (4) to obtain improved estimates of specific yield of water-bearing deposits in California. An open-file report, 'Specific yield of porous media, an annotated bibliography,' by A. I. Johnson, D. A. Morris, and R. C. Prill, was released in 1960 in partial fulfillment of the first objective. This report describes the second phase of the specific-yield study by the U.S. Geological Survey Hydrologic Laboratory at Denver, Colo. Laboratory research on column drainage and centrifuge moisture equivalent, two methods for estimating specific retention of porous media, is summarized. In the column-drainage study, a wide variety of materials was packed into plastic columns of 1- to 8-inch diameter, wetted with Denver tap water, and drained under controlled conditions of temperature and humidity. The effects of cleaning the porous media; of different column diameters; of dye and time on drainage; and of different methods of drainage, wetting, and packing were all determined. To insure repeatability of porosity in duplicate columns, a mechanical technique of packing was developed. In the centrifuge moisture-content study, the centrifuge moisture-equivalent (the moisture content retained by a soil that has been first saturated and then subjected to a force equal to 1,000 times the force of gravity for 1 hour) test was first reviewed and evaluated. It was determined that for reproducible moisture-retention results the temperature and humidity should be controlled by use of a controlled-temperature centrifuge. In addition to refining this standard test, the study determined the effect of length of period of centrifuging and of applied tension on the drainage results. The plans for future work require the continuation of the laboratory standardization study qith emphasis on investigation of soil-moisture tension and unsaturated-permeability techniques. A detailed study in the field then will be followed by correlation and evaluation of laboratory and field methods.

Water Supply Paper↗
USGS1965

Specific yield - laboratory experiments showing the effect of time on column drainage

The increasing use of ground water from many major aquifers in the United States has required a more thorough understanding of gravity drainage, or specific yield. This report describes one phase of specific yield research by the U.S. Geological Survey's Hydrologic Laboratory in cooperation with the California Department of Water Resources. An earlier phase of the research concentrated on the final distribution of moisture retained after drainage of saturated columns of porous media. This report presents the phase that concentrated on the distribution of moisture retained in similar columns after drainage for various periods of time. Five columns, about 4 cm in diameter by 170 cm long, were packed with homogenous sand of very fine, medium, and coarse sizes, and one column was packed with alternating layers of coarse and medium sand. The very fine materials were more uniform in size range than were the medium materials. As the saturated columns drained, tensiometers installed throughout the length recorded changes in moisture tension. The relation of tension to moisture content, determined for each of the materials, was then used to convert the tension readings to moisture content. Data were then available on the distribution of retained moisture for different periods of drainage from 1 to 148 hours. Data also are presented on the final distribution of moisture content by weight and volume and on the degree of saturation. The final zone of capillary saturation was approximately 12 cm for coarse sand, 13 cm for medium sand, and 52 cm for very fine sand. The data showed these zones were 92 to 100 percent saturated. Most of the outflow from the columns occurred in the earlier hours of drainage--90 percent in 1 hour for the coarse materials, 50 percent for the medium, and 60 percent for the very fine. Although the largest percentage of the specific yield was reached during the early hours of drainage, this study amply demonstrates that a very long time would be required to reach drainage equilibrium. In the layered columns the middle (medium sand) layer functioned as a hanging water column accelerating the drainage of the overlying coarse-sand layer. After the middle layer started to drain, the moisture distribution as retained in all three layers showed trends similar to that obtained when the same materials were tested in homogenous columns.

Water Supply Paper↗
USGS1967

Specific yield: compilation of specific yields for various materials

Specific yield is defined as the ratio of (1) the volume of water that a saturated rock or soil will yield by gravity to (2) the total volume of the rock or soft. Specific yield is usually expressed as a percentage. The value is not definitive, because the quantity of water that will drain by gravity depends on variables such as duration of drainage, temperature, mineral composition of the water, and various physical characteristics of the rock or soil under consideration. Values of specific yields nevertheless offer a convenient means by which hydrologists can estimate the water-yielding capacities of earth materials and, as such, are very useful in hydrologic studies. The present report consists mostly of direct or modified quotations from many selected reports that present and evaluate methods for determining specific yield, limitations of those methods, and results of the determinations made on a wide variety of rock and soil materials. Although no particular values are recommended in this report, a table summarizes values of specific yield, and their averages, determined for 10 rock textures. The following is an abstract of the table. [Table]

Water Supply Paper↗
USGS1962

Ground-water provinces of Brazil

As part of a study of the status of investigations and development of ground water in Brazil, made under the auspices of the United States International Cooperation Administration and with the cooperation of the Government of Brazil, the country was divided into seven ground-water provinces. The identification and delineation of the provinces were based on the regional distribution of the dominant geologic units which are known or inferred to have distinctive water-bearing characteristics. Three of the provinces, covering most of the country, are underlain by Precambrian crystalline rocks. Three others coincide in part with four extensive sedimentary basins--the Parnaiba or Maranhfio basin and the contiguous Sao Francisco basin in the northeast and east, the Amazon basin in the north and northwest, and the Paranfi basin in the south and southwest. In addition, the narrow, discontinuous coastal plain is considered as a province. the occurrence of ground water is discussed briefly, and pertinent data are given on the more important aquifers, together with information on some existing wells. Because of the widespread distribution of crystalline rocks of low permeability, it is difficult in many areas to develop large or even adequate ground-water supplies. In general, satisfactory supplies of water are available in most of the rest of the country. Some problems include the relative deficiency of rainfall in the northeast together with the occurrence, in parts of this region, of mineralized water in the crystalline rocks. Also, there is a potential problem of excessive lowering of water levels and interference among wells in the intensively developed area of the city of Sao Paulo.

Water Supply Paper↗
USGS1968

Ground-water resources of the Acu Valley, Rio Grande Norte, Brazil

The Acu 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 special equipment. Part of the extensive uplands on either side of the valley are included. This makes the total area approximately 2,500 square kilometers. The largest town, Acu, 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 Acu, 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 (Acu 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 S0,000 liters per hour. Larger yields probably could be obtained from wells designated 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 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 increase substantially when intensive irrigation becomes a reality. The annual pumpage from the alluvium, nearly constant since 1959, was about 2.5 million cubic 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 alluvium is acceptable for most uses. The water in the Acu 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 water 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

Water Supply Paper↗
USGS1963

Ground water for public supply in St. Croix, Virgin Islands

The ground-water resources of St. Croix, V.I., if properly developed should be adequate to supply the present and near-future demand for water for public supply. Ground water is obtained from weathered volcanic and intrusive rocks (bedrocks), from limestone, and from alluvium. The water obtained from weathered bedrocks and from alluvium in the valleys of the North Side Range is generally of better quality than that obtained elsewhere on the island. The greatest yields are obtained from the limestone and alluvium of the south-central coastal plain, but the water there is generally of poor to only fair quality. In the East End area no water, or salty water, is obtained from wells.

Water Supply Paper↗
USGS1966

Hydrology of the Upper Capibaribe Basin, Pernambuco, Brazil - A reconnaissance in an Area of Crystalline Rocks

The upper Capibaribe basin is the western three-fourths, approximately, of the valley of the river that empties into the Atlantic Ocean at Recife, the capital of the State of Pernambuco, Brazil. It is the part of the drainage basin that is within the Drought Polygon of northeast Brazil, and it totals about 5,400 square kilometers. It receives relatively abundant precipitation in terms of the annual average, yet is regarded as hot subhumid to semiarid because the precipitation is uneven from year to year and place to place. The dependable water supply, therefore, is small. The basin has water, which could be put to better use than at present, but the opportunities for augmenting the usable supply are not great. The streams are intermittent and therefore cannot be expected to fill surface reservoirs and to keep them filled. The ground-water reservoirs have small capacity--quickly filled and quickly drained. A rough estimate based on the records for 1964 suggests that, of 4,700 million cubic meters of precipitation in the upper Capibaribe basin, 2,700 million cubic meters (57 percent) left the basin as runoff and 2,000 million cubic meters {43 percent) went into underground storage or was evaporated or transpired. The bedrock of the upper Capibaribe basin is composed of granite, gneiss, schist, and other varieties of crystalline rocks, which have only insignificant primary permeability. They are permeable mainly where fractured. The principal fracture zones, fortunately, are in the valleys, where water accumulates and can feed into them, but the volume of fractured rock is small in relation to the basin as a whole. A well in a large water-filled fracture zone may yield up to 20,000 liters per hour, but the average well yields less than one-fourth this amount, and some wells yield none. The saprolite, or weathered rock, is many meters thick at some places especially in the eastern half of the upper Capibaribe basin. It contains water locally, but ordinarily will yield only small quantities to wells. The alluvium probably is the most productive aquifer in the basin, but is limited to narrow bands along the rivers that generally are no more than a few hundred meters wide and 5 meters thick. The alluvium contains variable amounts of silty sand capable of yielding small to moderate quantities of water to wells. Wells driven or dug into the alluvium could solve many small water problems. The chemical quality of the water in the upper Capibaribe basin ranges from good to bad and generally presents a major problem that cannot be solved solely by applying geological criteria. Mineralized water is widespread in the area, both in streams and underground, and .the choice of aquifers is small. All known aquifers contain, at one place or another, water that is mineralized, leaving no alternative for a natural supply of good-quality water. Although much of the available water is unsatisfactory for human consumption, it is generally acceptable for animals and therefore meets one of the principal water needs. Some of the ground water could be made potable by diluting it with rainwater, which could be collected during rainy seasons and temporarily stored in cisterns or reservoirs.

Water Supply Paper↗
USGS1969

Ground-water resources of the Lambayeque Valley, Department of Lambayeque, northern Peru

Ground water in the Lambayeque Valley has been developed mainly for irrigation of sugarcane and rice. The locality is on the coastal plain of northern Peru, about 650 km (kilometers) northwest of Lima, the national capital. The area considered in this study is about 1,670 sq km (square kilometers) and is mainly on the alluvial fan of Rio Chancay and entirely in the Department of Lambayeque. Chiclayo, the departmental capital and largest city, has a population, of about 46,000. The climate is hot and virtually rainless. Agriculture is dependent on irrigation. The available water, whether in stream s or underground, is introduced from the Andean highlands by Rio Chancay. Rocks in the area range in age from Cretaceous, or possibly Jurassic, to Quaternary and in lithology from dense and hard igneous, sedimentary, and metamorphic rocks to unconsolidated sediments. The bedrock contains and yields water only in small quantities, if at all. The principal water-bearing strata are in the alluvium comprising the fan of Rio Chancay. Where ground water in the alluvium has been most intensively developed, the productive zone is within 20 m (meters) of the land surface and is composed approximately as follows: (1) relatively impermeable soil, clay, and clayey sand, 5 to 10 m thick, (2) permeable sand and gravel, 6 to 10 m thick, at places including one or more layers of clay, so that several water-bearing beds are distinguishable, and (3) relatively impermeable mixtures of clay, sand, and gravel extending below the bottom of wells. Unit 3 in the deepest test continued to 102 m. Unit 2 is the principal source of water tapped by irrigation wells. In the northern part of the area wells locally yield water rather freely from strata as deep as 73 m, but elsewhere in the area the strata deeper than 20 m are not very productive. Wells at and near Chiclayo yield only small amounts, and the deepest well disclosed, in 100 m of material, only 5.5 m of material that can be considered as possibly water bearing. Water in the alluvium of the eastern part of the area occurs under water-table conditions at depths from 1 to 8 m below the land surface. The water table declines during pumping for irrigation and rises when pumping is stopped. Recharge comes mainly from infiltration on irrigated fields and from irrigation ditches and probably varies greatly from year to year at any given place. The ground-water reservoir is replenished when pumps are idle; therefore, it is concluded that the recharge is sufficient to offset withdrawal at a rate comparable to that of 1957, which was about 81 million cum (cubic meters). A study of the effect of protracted pumping on yields of wells suggests that the rate of recharge locally, and for a short period, was more than 76,000 cu m per day. This recharge presumably declined rapidly to zero when irrigation was suspended in the locality. A pumping test showed the transmissivity to be about 950 cu m per day per m and the storage coefficient to be about 0.07. Based on these coefficients, the drawdown caused by one well discharging 10 lps (liters per second) for 6 months would be only 0.066 m at points 4,000 m distant, but 50 wells at the same rate and distance would create 3.3 m of drawdown. As actual distances between wells range from 100 to 300 m where the wells are most numerous and as the average discharge rate is nearer to 20 than to 10 lps, the cumulative effect of the actual pumping is certain to be considerable. If it were not for the recharge resulting from infiltration of irrigation water, the pumping of so many wells probably could not be long sustained. The waters from wells of the Lambayeque Valley compare favorably, in most respects, with the standards established by the U.S. Public Health Service for water for human consumption. Chemical analyses of 10 samples of ground water show that the dissolved solids, silica, bicarbonate, sulfate, and sodium increase in the downstream direction, where

Water Supply Paper↗