A field test of a conceptual computer simulation model for predicting water and salt distribution

Summary The unsaturated flow and unsaturated chemistry submodels of a conceptual computer simulation model, developed at the United States Bureau of Reclamation to predict water and salt behaviour in soils, were tested under field conditions. The comparison of results was done with data obtained from an irrigation trial with alfalfa over a period of four years. The model simulated the actual physical and chemical processes taking place in the soil to a fair degree of accuracy. The predicted moisture contents compared reasonably well with ob served data, especially in the soil layers between 480 and 1200 mm (Figs. 1 and 2, Tables 3 and 4). The simulation of moisture distribution was more accurate for plots receiving a high irrigation frequency than for plots given a low irrigation frequency. On a depth-weighted-mean scale, the predicted salt concentration of the soil layers shallower than 1200 mm was within 11% of the observed data. However when the salt content of the soil layers between 1200 and 1920 mm was taken into account, the percentage error increased to about 40% (Table 6). The results indicate that this simulation model can be extremely useful in predicting the long and short term effect of irrigation water on the root zone of a soil. Field data are however at all times needed to calibrate the unsaturated flow model for specific soil types.