A SIMULATION-MODEL OF THE WHEAT CROP IN RESPONSE TO WATER AND NITROGEN SUPPLY .2. MODEL VALIDATION

The simulation model of soil water, mineral nitrogen supply and crop r esponse of the wheat crop described in Part I was validated against fi eld data from two semi-arid sites in northwest Victoria, Australia, ov er a period of 4 yr. The experimental data consisted of the four fallo w combinations of stubble retention and zero tillage plus a tilled sum mer fallow prior to pea and wheat crops in rotation. Total soil water was simulated well (R(2) = 0.95; RMSE = 0.02 Mg/m(3)) with a generally good characterization of the soil water profile. Some under-estimatio n of water content occurred at depth during the final crop growth stag e as a consequence of over-estimation crop growth and water use. Total soil mineral nitrogen (SMN) was stimulated with less accuracy than so il water (R(2) = 0.61; RMSE = 1.84 g/m(3)). Although the simulation of surface (0 - 10 cm) SMN was generally poorer, the model gave a good a ccount of SMN at depth that ensures good simulation of nitrogen uptake late in the growing season when nitrogen responses are important to c rop yield. The stimulation of biomass clearly followed the general pat tern observed throughout the season (R(2) = 0.77; RMSE = 1655 kg/ha) w ith final biomass closely matching the observed data in most years. Gr ain yield was simulated with less accuracy than biomass (R(2) = 0.42; RMSE = 756 kg/ha). This was largely because grain number was poorly si mulated (R(2) = 0.07; RMSE = 1425 m(-2)) and when observed grain numbe r was used in the model the accuracy of yield was increased (R(2) = 0. 59; RMSE = 615 kg/ha). Simulated grain size (R(2) = 0.61; RMSE = 5.6 m g) was more accurate than grain nitrogen concentration (R(2) = 0.29; R MSE = 0.31% N). Seasonal crop water use was simulated well (R(2) - 0.8 3; RMSE = 34 mm). There was no evidence of bias between the various ti llage methods tested in the simulations with respect to the soil water and nitrogen and crop response. Copyright (C) 1996 Elsevier Science L td