specific irrigation water 

 quality considerations 

 for arid 

 and semiarid regions 



must supply one-half to all or most of the soil mois- 

 ture required for crops for annual periods ranging 

 from 3 to 12 months. 



Annual precipitation varies in the Western 

 United States from practically zero in the south- 

 western deserts to in excess of 100 inches in the 

 upper western slope of the Pacific Northwest. The 

 distribution of precipitation throughout the year 

 also varies, with no rainfall during extended pe- 

 riods in many locales. Often the rainfall occurs 

 during nongrowing seasons. 



The amount of precipitation and its distribution 

 is one of the principal variables in determining the 

 diversion requirement, or "demand," for irrigation' 

 water. 



Environmental factors 



Climate 



Climatic variability exists in arid and semiarid 

 regions. In the Far West, the Pacific Ocean pro- 

 vides considerable moderation, preventing ex- 

 tremely high summer temperatures and extremely 

 low winter temperatures; this influence de- 

 creases with distance from the coast and with the 

 presence or absence of intervening mountains. 

 There are differences due to altitude, the highest 

 elevations having the shortest frost-free growing 

 season, and the lowest elevations having the long- 

 est. The latitude affects the length of the growing 

 seasons, permitting subtropical fruits and winter 

 vegetables, etc., to be grown in the low-elevation 

 southern portions. Deciduous fruits with a winter 

 chilling requirement are examples of crops favor- 

 ing the northern latitudes. There can be heavy 

 winter precipitation, generally increasing from 

 south to north, and increasing with elevation. 

 Summer showers are common, increasing north 

 and east from California. The only thing common 

 through this Western part of the country is the 

 inadequacy of precipitation during the growing 

 season. In most areas of the West, intensive agri- 

 culture is not possible without irrigation. Irrigation 



Land 



Soils of the arid and semiarid regions were de- 

 veloped under a drier regime than the soils of the 

 more humid areas. They have more weatherable 

 minerals and consequently are generally better sup- 

 plied with the nutrient elements except for nitro- 

 gen. These soils generally have relatively high ex- 

 changeable cation status, base status, and a low 

 degree of acidity. Also, if they have developed pro- 

 files, the topsoils are not as deep as in the more 

 humid areas. Because of less frequent passage of 

 rainfall through the soil profiles, they are shallower 

 and are more apt to be saline. 



For irrigation purposes, soils are sometimes 

 grouped in accord with their topographic position. 

 Upland soils are those formed in place by the 

 weathering of the underlying parent material or to 

 some extent, from materials moved laterally by 

 coUuvial forces. They are also the soils where the 

 greatest erosion is usually taking place. Most of the 

 material eroded and transported downstream as 

 sediments, largely during floods, is deposited on 

 the flood plain and deltas. The deep, medium-tex- 

 tured soils of the flood plains are recognized as the 

 prime agricultural soils. Farther down the river 

 system, soils of the basin are normally found. 

 These basins are regions of flat gradient where 

 drainage is impeded. They may be swampy, at least 

 during time of flood, but can be a filled lake or 

 estuary. The distinctive features are a high water 

 table and fine-textured materials. 



One other soil position should be mentioned — 

 the terrace. The terrace is created as a flood-plain 

 alluvium. Because of stream lowering, there is no 

 longer deposition, but its gradient is flat enough 

 to limit erosion hazard. Over thousands of years, 

 the soil can develop a profile. Percolating water 

 causes the leaching of colloidal material and solu- 

 ble constituents from the top soil to be deposited 



167 



