An evaluation of the crop-growth simulation submodel of EPIC for wheat grown in a Mediterranean climate with variable soil-water regimes

The EPIC model (Erosion-Productivity Impact Calculator) was used to simulate leaf area index (LAI), above-ground biomass (DM(ag)), and grain yield of wheat grown in Southern Italy under variable soil-water regimes and for two seasons (1986-87 and 1987-88). The EPIC calibration parameters, obtained from two irrigation treatments in 1987-88, were utilized in simulating the six and four other irrigation treatments in 1987-88 and 1986-87, respectively. During the calibration procedure, some inconsistencies were observed in the simulation behaviour. EPIC sets the threshold temperature for winter dormancy at 5 degrees C and adopts an algorithm that resets the accumulation of the heat units (HU) on which many growth processes are based. Furthermore, leaf area decline during senescence is governed by late autumn temperature relative to the average annual temperature. The way EPIC handles winter dormancy and the heat units of crops through winter time appears to be inadequate for the Mediterranean climate and, moreover, the threshold for the former and the constraining control on the latter are not accessible to the users for adjustment. Further, when water stress developed during vegetative growth, EPIC consistently overestimated DM(ag), LAI, and yield, as occurred with the 1986-87 simulation. An important conclusion was that, although final yield may be correctly simulated, this may be purely accidental as errors in intermediate processes may compensate each other, while biomass and LAI development in time might not be related to final yield at all.