Irrigation Water Requirements
ü Irrigated agriculture is facing new challenges that require refined management and innovative design.
ü Formerly, emphasis centered on project design; however,current issues involve limited water supplies with several competing users, the threat of water quality degradation through excess irrigation, and narrow economic margins.
ü Meeting these challenges requires improved prediction of irrigation water requirements.
ü Irrigation water requirements can be defined as the quantity, or depth, of irrigation water in addition to precipitation required to produce the desired crop yield and quality and to maintain an acceptable salt balance in the root zone.
ü This quantity of water must be determined for such uses as irrigation scheduling for a specific field and seasonal water needs for planning, management, and development of irrigation projects.
ü The amount and timing of precipitation strongly influence irrigation water requirements. In arid areas, annual precipitation is generally less than 10 inches and irrigation is necessary to successfully grow farm crops.
ü In semiarid areas (those typically receiving between 15 to 20 inches of annual precipitation), crops can be grown without irrigation, but are subject to droughts that reduce crop yields and can result in crop failure in extreme drought conditions.
ü Subhumid areas, which receive from 20 to 30 inches of annual precipitation, are typically characterized by short, dry periods.
ü Depending on the available water storage capacity of soils and the crop rooting depth, irrigation may be needed for short periods during the growing season in these areas.
ü In humid areas, those receiving more than 30 inches of annual precipitation, the amount of precipitation normally exceeds evapotranspiration throughout most of the year.
ü However, drought periods sometimes occur, which reduce yield and impair quality, especiallyfor crops grown on shallow, sandy soils or that have a shallow root system.
ü Irrigation is not needed to produce a crop in most years, but may be needed to protect against an occasional crop failure and to maintain product quality.
ü The primary objective of irrigation is to provide plants with sufficient water to obtain optimum yields and a high quality harvested product.
ü The required timing and amount of applied water is determined by the prevailing climatic conditions, the crop and its stage of growth, soil properties (such as water holding capacity), and the extent of root development.
Water within the crop root zone is the source of water for crop evapotranspiration.
ü Thus, it is important to consider the field water balance to determine the irrigation water requirements.
ü Plant roots require moisture and oxygen to live.
ü Where either is out of balance, root functions are slowed and crop growth reduced.
ü All crops have critical growth periods when even small moisture stress can significantly impact crop yields and quality.
ü Critical water needs periods vary crop by crop.
ü Soil moisture during the critical water periods should be maintained at sufficient levels to ensure the plant does not stress from lack of water.
The calculation of irrigation water requirements
ü Delineation of major irrigation cropping pattern zones.
ü These zones are considered homogeneous in terms of types of irrigated crops grown, crop calendar, cropping intensity and gross irrigation efficiency.
ü Represented on the map of Africa, they should be viewed as regions where some homogeneity can be found in terms of irrigated crops.
ü The cropping pattern proposed for the zone should be viewed as representative of an 'average' rather than a 'typical' irrigation scheme.
ü Definition of the area of influence of the climate stations (in GIS) and quality check on the climate data.
ü Combination of the irrigation cropping pattern zones with the climate stations' zones (in GIS) to obtain basic mapping units.
ü Calculation of net and gross irrigation water requirements for different scenarios.
ü Comparison with existing data and final adjustment.