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Estimating Long-Term Drainage at a Regional Scale Using a Deterministic Model

Unité de Science du sol — SESCPF, INRA Orléans, Avenue de la Pomme de Pin, 45160 Ardon, France

L. Bruckler and P. Bertuzzi

Unité de Science du sol — INRA Avignon, Site Agroparc, 84914 Avignon, France

B. Guillet

Laboratoire de Géologie de la Matière Organique, UMR 6531 du CNRS et de l'Université d'Orléans, Bâtiment Géosciences, B.P. 6759, 45072 Orléans, France

*Corresponding author (ary.bruand{at}orleans.inra.fr).

ABSTRACT

This study discusses the use of a deterministic model, at a regional scale, when the soils have small heterogeneity. Long-term soil drainage was simulated on 3500 ha in Beauce (France), taking into account variation in soil thickness, agricultural practices, and climate characteristics for a 32-yr period. The model was based on a numerical solution of Richards' equation, using a finite element method. Hydraulic properties were determined at four sites that are 4 km apart. The results show a small variability of hydraulic parameters, with coefficient of variation (CV) ranges from 1 to 16% for the parameter, the empirical constant n, the saturated water content _s, and the residual water content _r, and from 60 to 75% for the saturated hydraulic conductivity K_s. Hence, the spatial and temporal variability of drainage are mainly related to soil thickness, agricultural practices, and climate characteristics. Assigning small spatial variability to hydraulic properties, we analyzed the role of soil thickness and initial water content (IWC), induced by agricultural practices and climate characteristics, on drainage prediction. Five IWC were defined: 100, 90, 80, 70, and 55% of field capacity (FC). Four soil thicknesses were studied: 30, 60, 100, and 150 cm. The IWC appeared as a significant factor in drainage prediction for 60% of the 32 yr, regardless of soil thickness. Soil thickness became the most significant factor (20% of the 32 yr) for low IWC. The model was spatially extrapolated by combining the calculated drainage (which varied with soil thickness and IWC) and the spatial distribution of crops and soil thickness classes. Simulated drainage was closely related (r = 0.90) to groundwater recharge during the 32 yr, indicating that the model could be applied to other areas with appropriate parameters.

Received for publication February 29, 1996.