ANALYSIS AND FIELD-EVALUATION OF THE CERES MODELS WATER-BALANCE COMPONENT

The soil water status partly determines the N losses from soil-crop sy stems, With the ultimate objective of estimating N losses, the capacit y-based water balance module of the Ceres models was tested against he ld data collected from various pedoclimatic regimes in France, A proce ss-oriented analysis of initial simulation results for a loamy soil pr ompted introduction of Darcy's law in the drainage and capillary rise parts of the model, As a result, a more accurate prediction of the soi l water storage and surface water content was achieved. This was confi rmed by comparing model output against independent data from bare or m aize (Zea mays L.)-cropped conditions and for silt loam or sandy loam soils, For a 1-yr period, the mean square error between modeled and me asured water storages was in the range 1.9 to 3 cm(2) water for the mo dified model, in contrast with 4 to 12 cm(2) using the original model (which performed best on well-drained soils). A unidimensional sensiti vity analysis was conducted with regard to the three new parameters in troduced in the revised model: the saturated hydraulic conductivity an d two texture-dependent constants used in simple analytical representa tions of the moisture retention and hydraulic conductivity curves. The sensitivity analysis proved that this more physical approach in capac ity-based models required less rigorous parameterization than mechanis tic models. Moreover, the accuracy of the simulations performed with t he modified model fell within the experimental error in the measuremen ts.