WSP02862 - Laserfiche WebLink

Home Browse Search

..,...,vv'"'u . . .~ 3 with conventional irrigation management, the salt concentration of the soil water is essentially uniform near the soil surface regardless of the leaching fraction (L, the fraction of infiltrated water that actually passes through the rootzone) but increases with depth as L decreases. Likewise, average rootzone salinity increases as L decreases; crop yield is decreased when tolerable levels of average salinity are exceeded. Once the soil solution has reached the maximum salinity level compatible with the crop tolerance, at least as much soluble salt as is brought in with additional irrigations must be removed from the rootzone; a process called "maintaining salt balance". Without net leaching, such as occurs by the abandonment of irrigation and by the USDA's WRP and CRP set- aside programs, moderately saline soils may become extremely salinized, especially if the water table remains at a shallow depth (-<6 feet). In most irrigation projects, the currently used leaching fractions (and resulting drainage volumes) are excessive (L is typically about 0.4). They can be reduced appreciably without harming crops or soils; they should be minimized because the resulting excesses in leaching and drainage volumes are the major, fundamental causes of both soil and water salinization, for the reasons explained previously. Minimized leaching reduces drainage volume and the associated problems of waterlogging; it also reduces salinization. in three ways. Less salt is discharged with reduced leaching because less irrigation water, and hence less salt, is applied. Reduced leaching reduces the discharged salt-load still more because the fraction of applied salt that precipitates as minerals (such as calcite and gypsum) in the rootzone of the soil increases. A further benefit of reduced leaching is that less additional "geologic" salts are "picked-up" by the percolating water from the weathering and dissolution of soil and substrata minerals, because the through-put of drainage water is reduced and the "solvent" capacity of the more saline water resulting from low leaching is likewise reduced. Thus, as compared to high leaching, minimized leaching reduces the amount of salt added to soils and discharged from irrigated rootzones because it maximizes the precipitation of applied Ca, HC03 and S04 salts as carbonate and gypsum minerals in the soil, and it minimizes the application of salts and the "pick-up" of weathered and dissolved salts from the soil and substrata. The extent to which leaching and drainage can be minimized is limited in theory by the salt tolerances of the crops being grown, but in practice it is limited more by the application efficiency of the irrigation system and by the variability in soil infiltration rates existing within the field. To prevent waterlogging and secondary salinization, drainage must remove the precipitation and irrigation water infiltrated into the soil that is in excess of crop demand and any other excessive water (surface or subsurface) that flows into the irrigated soils; it must provide a,n outlet for the removal of salts that accumulate in the rootzone in order to avoid excessive soil salinization, and it must keep the water table sufficiently deep to permit adequate root development, to prevent the net flow of salt-laden groundwater up into the rootzone by capillary forces and to permit the movement and operations of farm implements in the fields without excessive compaction. Artificial drainage systems should be provided in the absence of adequate natural drainage. The water table depth required in order to prevent a net upward flow of groundwater and salt into the rootzone is dependent on irrigation management, as well as on soil hydraulic properties. .