COMPARISON BETWEEN MECHANISTIC AND FUNCTIONAL MODELS FOR ESTIMATING SOIL-WATER BALANCE - DETERMINISTIC AND STOCHASTIC APPROACHES

In many models used to simulate soil-water relationships, representati ons of the transport mechanisms in the soil-plant-atmosphere continuum , range from mechanistic to functional. The objective of this paper is to compare two functional models, FAO (Doorenbos and Pruitt, 1977) an d Ritchie (1985) models, with a mechanistic model (Maraux and Lafolie, 1998) to simulate the soil water balance of maize and sorghum grown i n sequence in Nicaragua. In the FAO model, the soil is described as a single reservoir which is characterized by its amount of water varying on a daily time scale, depending on the rain, drainage, and actual ev apotranspiration. In the Ritchie model, the soil is regarded as a mult ilayered soil profile. The maximum evapotranspiration is divided betwe en soil evaporation and plant transpiration, and drainage occurs if th e amount of water arriving in the last layer corresponds to a water co ntent greater than the field capacity. The mechanistic model is based on the Richards' equation. Comparison of the three models was first ma de according to a deterministic approach with parameters coming from t he same database. We then considered a stochastic approach for which 8 00 hydraulic characteristics of the soil were generated, according to the spatial variability observed at the field scale and to the scaling theory applied to similar porous media. A distribution of the stochas tic parameters used in the three models was thus derived. Results show ed that the order of magnitude of the evapotranspiration was similar f or the three models (902, 874, 842 mm cumulative evapotranspiration fo r a 203 day period for the MM, Ritchie, and FAO models, respectively). Adding a capillary rise mechanism in the functional models improved m oderately the soil-water balance. Evapotranspiration and drainage show ed moderate sensitivity to spatial variability in soil hydraulic prope rties (coefficients of variation than 1.6%), whereas final water stora ge (after 203 days) showed a greater sensitivity (coefficients of vari ation from 7.9-15.7%, depending on the model). (C) 1998 Elsevier Scien ce B.V. All rights reserved.