crops is transpiration. Within the range of suitable temperatures, crop growth and production are approximately proportional to transpiration and for practical purposes are frequently assumed to be proportional to crop water use, or evapotranspiration (ET). Water from irrigation or rain is not uniformly available to the crop as some portions of the field receive more water than others. Within the same field, soils may not be uniform and water holding capacities and infiltration rates may vary considerably. Consequently, when water becomes fully adequate for all portions of the field there will be considerable excess or waste from some portions. As maximum yields are approached, efficiencies decline, producing an approximately exponential relationship between yields and water availability. Hargreaves (1975) obtained crop yield data from various crops and research trials and compared yields to the water available (initial soil water stored in the soil plus growing season rainfall plus irrigation water applied). Relative crop yields (Y) were used in order to compare different crops and yield units. A value of unity for Y was used for the maximum yield of each crop, and X is unity for the water required to produce maximum yield. The data used covered the range of values of X from 0.30 to 1.20. It was found that most of the yield data evaluated could be represented by the equation: Y is relative crop yield (fraction); and X is the depth of applied water. The equation is normalized such that for X = 1.0, Y = 1.0. Soil Aeration and Drainage Where natural drainage is inadequate and artificial drainage cannot be economically provided, thelands usually cannot be permanently irrigated. Rice is an exception although rice grows well in standing water, drainage is required for − most tillage and harvesting operations. Irrigation of lands where the original water table is less than thirty meters below the surface has usually resulted in eventual waterlogging or salinization or both. Control of salinity and methods of drainage
crops is transpiration. Within the range of suitable temperatures, crop growth and production are approximately proportional to transpiration and for practical purposes are frequently assumed to be proportional to crop water use, or evapotranspiration (ET). Water from irrigation or rain is not uniformly available to the crop as some portions of the field receive more water than others. Within the same field, soils may not be uniform and water holding capacities and infiltration rates may vary considerably. Consequently, when water becomes fully adequate for all portions of the field there will be considerable excess or waste from some portions. As maximum yields are approached, efficiencies decline, producing an approximately exponential relationship between yields and water availability. Hargreaves (1975) obtained crop yield data from various crops and research trials and compared yields to the water available (initial soil water stored in the soil plus growing season rainfall plus irrigation water applied). Relative crop yields (Y) were used in order to compare different crops and yield units. A value of unity for Y was used for the maximum yield of each crop, and X is unity for the water required to produce maximum yield. The data used covered the range of values of X from 0.30 to 1.20. It was found that most of the yield data evaluated could be represented by the equation: Y is relative crop yield (fraction); and X is the depth of applied water. The equation is normalized such that for X = 1.0, Y = 1.0. Soil Aeration and Drainage Where natural drainage is inadequate and artificial drainage cannot be economically provided, thelands usually cannot be permanently irrigated. Rice is an exception − although rice grows well in standing water, drainage is required for most tillage and harvesting operations. Irrigation of lands where the original water table is less than thirty meters below the surface has usually resulted in eventual waterlogging or salinization or both. Control of salinity and methods of drainage