OCR Text |
Show CONSUMPTIVE USE OF WATER IN IRRIGATION 1355 and, similarly, there is no definite consumptive use. In Equation (2), Df depends on: (a) The quantity of irrigation water applied in each irrigation; (&) The uniformity of distribution; (c) The frequency of irrigation; (d) The length of land irrigated in a single run and the size of stream used; (e) The texture of the soil and subsoil; (/) The depth to the water-table; (g) The dryness of soil between the water-table and the zone of root action, provided the water-table is very deep; (h) The kind of crop and depth of root zone; and, (t) The moisture conductivity of the soil and other less important factors. In many localities, Df was large during the early years of irrigation, but it is now relatively small because: (1) The dry desert soil to great depths has been fully moistened through the years of irrigation; (2) the land is better prepared for applying water; and (3) general improvement in irrigation methods has been made. The quantity, Dp, of Equation (4) is likely to be larger than Df of Equation (2) since it includes deep percolation from canals, ditches, streets, yards, ponds, borrow-pits, and sloughs. It may be high on one project and negligible on a neighboring lower project. To measure the evaporation and transpiration losses from non-cropped areas by direct means the engineer can measure the areas for which such losses occur and make an approximation. Eelative to the (U + D)-quantities, such quantities are usually very small. It seems advantageous, however, to endeavor to approximate the consumptive use on the farm, project, and valley areas by indirect means as indicated in Equations (3), (5), and (7). More detailed consideration of the difficulties of measurement and. sources of error in the elements of moisture •supply and run-off are, therefore, desirable. Provided the farm, project, or valley considered has a deep uniform soil and deep water-table, the elements of supply may be measured with fair precision, but on gravelly or greatly variable soils the draft on soil moisture cannot be effectively measured. Moreover, moisture loss cannot be precisely measured on large projects or in large valleys without prohibitive expense; it can only be estimated. The rainfall may vary greatly from point to point; and it is, therefore, essential carefully to estimate percentage inaccuracy in the elements of supply which may result from rainfall variation, provided the rainfall factor is comparatively large. Possibly a still greater source of error is inflowing ground-water from neighboring high land, measurements of which are sometimes impractical. In some of the larger valleys of the West, of which many in California, Colorado, and Arizona are typical, the inflowing ground-water is of great importance. Water from melting snows and rains in the mountain ranges may sink into the coarse textured soil of the valley rim immediately after leaving the canyons. These waters gradually move underground toward the middle or lower areas of the valley and may here form high water-tables, |