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Increasing salinity is a significant factor affecting the future agricultural productivity in semiarid irrigated regions of the world. Computer simulation models, which can be used to evaluate the consequences of differing management strategies on crop yield and salt distribution in the soil profile, would be valuable. Simulated results from models must be compared with measured results from field experiments to establish their validity. The simulated salt distribution from the ENVIROGRO model were compared with measured distribution at the end of the growing season from an experiment that had treatment variables of irrigation water electrical conductivity (EC) of [approximately equal to] 1.7, 4.0, 5.0, 8.0, and 10.2 dS [m.sup.-1] and average irrigation intervals of 3.5, 7, 14, and 21 d. Root distribution is an input variable to the model. Therefore, a second objective of the study was to test the sensitivity of the model results to the root distribution. In general, the agreement between measured and simulated salt distributions were better for the longer than for the shorter irrigation intervals. For the shorter irrigation intervals, the measured salt concentration near the soft surface was greater than was simulated. This result is explained by the fact that the model does not separate the transpiration (T) from the evaporation (E) component of evapotranspiration (ET), and assumes that all water is lost by T. The E:T ratio would be expected to increase as the irrigation frequency increases, and E would carry salts to the soil surface. Except for nonsaline conditions with frequent irrigation, the simulated yields were increased by having a deeper root distribution. The effect of a deep root system was greater for the longer irrigation interval when the water storage capacity within the root zone becomes more important.

Sustained irrigation agriculture is critical for food and fiber production to support the growing human population. Increasing salinity is a significant factor in many irrigated lands. Knowledge on the proper time and amount of water of various salinities to be applied for optimum yield of a given crop is important. Because of the numerous variables, computer simulation models would be valuable to partially replace expensive field experiments. Simulated relative yields of corn from the ENVIRO-GRO model were compared with experimentally measured yields which had irrigation water electrical conductivities (ECs) of 1.7, 4.0, 5.0, 8.0, and 10.2 dS [m.sup.-1] and average irrigation intervals of 3.5-, 7-, 14-, and 21-d treatments. The simulations were run for 5 yr although the field experiment was conducted only 1 yr. The simulated root-weighted average matric and osmotic heads prior to irrigation were evaluated during the year. Depending on treatment, yields were depressed because of either osmotic or matric effects or a combination of the two. The agreement between simulated and measured yields was good for all treatments indicating that the model appropriately accounted for both osmotic and matric effects. On the basis of these results, the ENVIRO-GRO model can be used with confidence in simulating the consequences of irrigation management options under saline conditions. The simulated yields were lower the second and subsequent years compared with the first year, emphasizing the fact that results from 1-yr experiments cannot be used reliably for long-term predictions because the results are highly dependent on the soil conditions at the beginning of the growing season.

Effective rainfall (ER) is the portion of total rainfall that plants use to help meet their consumptive water requirements, and is an important component of water resource budgeting for irrigation. The USDA's Technical Release no. 21 (TR-21) is used to predict ER and irrigation requirements for south Florida citrus, but its accuracy is in question due to high-intensity rainfall, poorly drained soils, and partial irrigation coverage in microirrigated orchards. We evaluated the calculation of ER by TR-21 under these conditions by monitoring rainfall, irrigation, water table depth, evapotranspiration (ET), and soil water content inside and outside of the microirrigation-wetted pattern in four orchards for a total of 83 site-months. We developed a soil water budget to calculate daily water table upflux, root zone water content, water used, and ER separately for the irrigated and nonirrigated root zones. Water budget ER calculated by site ranged between -3.3% and +18.2% of TR-21 ER, with a mean of + 10%. A linear correlation between water budget ER and TR-21 ER using pooled data from all four sites yielded the equation: Water Budget ER (mm) = 0.79 [TR-21 ER (mm)] + 17.7, r = 0.84. A hypothetical ER comparison using 30-yr mean rainfall and ET data showed that annual ER calculated by TR-21 amounted to 673 mm, while water budget ER totaled 744 mm, or +10.5%. We suggest that the TR-21 method has the level of accuracy needed to allocate water for microirrigated citrus on poorly drained south Florida soils.

The SWMS 2D finite-element model helps determine water and CaCl2 movement in alternate-furrow irrigation (AFI) and every-furrow irrigation (EFI) with furrow-placed fertilizer bands. Simulation studies show that water distribution is more uniform with AFI in the clay loam than in the crook loamy sand. In both soils, water contents exhibit higher uniformity in EFI than in AFI. In both types of furrow irrigation, placement of fertilizer in the ridge decreases leaching of the fertilizer and retains the fertilizer in the root zone.