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LOGAN COUNTY, COLORADO inch of snow on the ground, but the number of days va- ries greatly from year to year The average relative humidity in midafternoon in spring is less than 45 percent; during the rest of the year it is about 55 percent. Humidity is higher at night in all seasons, and the average at dawn is about 80 percent. The prevailing direction of the wind is from the northwest. Average windspeed is 10 miles per hour. The highest, 13 miles per hour, is in April Some years, blizzards with high winds and drifting snow occur in the county, and snow remains on the ground for a few weeks. High winds cause soil blowing in dry periods on both dry and irrigated farms, causing dust storms. Tornadoes occur occasionally in the county. Summer hailstorms are common and can cause severe local damage to crops in the county. Drought is always a risk in nonnTigated cropland areas. Most nonirrigated crops are grown in a sequence with summer f allow. This sequence tends to reduce the hazard of crop failure. The growing season is long enough for all crops commonly grown in the county to mature, whether nonnrigated or irrigated. Occasionally, early frost will damage corn that has been planted late. Favorable yields are generally produced by those farmers who conserve moisture, use good irrigation practices, and use other good crop and soil management practices. Natural Resources Soil, surface and underground water, oil, natural gas, sand and gravel, and native vegetation are the major natural resources of Logan County. Soil, the most widely used of the county's resources, can be expected to yield benefits without depletion if managed and used properly. The purpose of this survey is to aid in maintaining and improving the value of the soil resource. The South Platte River is the principal source of sur- face water. The number of irrigation wells have increased greatly in the past 10 years. With careful management and efficient use of water, the water resources of the county can be conserved. Oil and natural gas production expanded in the late 1940's to about 1955, but has been reduced during the past years. An abundance of sand and gravel is available for build- ing roads and other structures. The sources of these materials are the sandy and gravelly alluvial lands and the exposed sandy and gravelly old alluvium underlying plateaus of the area The native vegetation has been plowed out to make way for nonirrigated and irrigated cropland in about half of the county. The remainder is being grazed by livestock. The grass resource can be maintained by careful grazing management. Maintaining the native vegetation can also be helpful in maintaining the soil resource by minimizing soil loss through erosion. Water Supply 3 The South Platte River is the principal source of sur- face water in Logan County. Other streams contributing are Lewis Creek, Pawnee Creek and Cedar Creek The water source of the South Platte River is snowmelt and runoff waters from the foothills and mountains and ground water discharge. The creek water sources are snowmelt and runoff water from the adjacent dissected terraces and ground water discharge. Streamflows are not constant. During the late spring and summer months streamflows are low. The North Sterling (fig. 1), Prewitt and Jumbo Reser- voirs were formed by damming basinlike valleys These three lakes provide about 7,680 acres of open water. Water for these reservoirs is drawn from the South Platte River during peak flow periods of the winter months. Water from North Sterling and Prewitt Reser- voirs is used for irrigation in Logan County. Jumbo Reservoir water is used for irrigation in Sedgwick Coun- ty. The ground water recharge in Logan County is chiefly from precipitation, but there are additions from the ir- rigation water delivery system in the Platte River Valley. The amount and frequency of the recharge vary con- siderably from place to place and year to year, depending on precipitation and the amount and distribution of the water diverted for irrigation. Springs commonly occur along the contact of the rela- tively impermeable Brule Formation and the overlying more permeable gravel of the Ogallala Formation. Water derived from springs is used primarily for livestock. Irrigation water in Logan County comes from three main sources: (1) surface water diverted from the South Platte River, (2) ground water pumped from wells, and (3) water stored in the North Sterling and Prewitt Reser- voirs. Many farms with irrigation wells also obtain water from surface ditches. During years when surface water is limited, water from wells is used advantageously. A delay of a day or two in irrigating crops that are in crucial need of moisture may be the difference between success and failure. Most of the wells in the county were drilled into the sand and gravel formation of Pleistocene geologic age. This formation underlies the bottomlands and low ter- races along the South Platte River. A few wells were dried into fissures of the Brule Siltstone Formation. Other wells in this upland area were located in the gravel members of the Ogallala Formation. Irrigation wells along the South Platte Terrace system range from 30 to 100 feet in depth. Wells located on the uplands outside the valley range from 200 to 300 feet deep. Agriculture The first settlers in Logan County were mainly cattle and sheep ranchers. Dryland farming began on a small scale when land was opened to homesteading.