© CSIRO 2003 10.1071/FP02222 1445-4408/03/070699 Functional Plant Biology , 2003, 30, 699–710 www.publish.csiro.au/journals/fpb CSIRO PUBLISHING Modelling the seasonal dynamics of the soil water balance of vineyards Eric Lebon A , Vincent Dumas B , Philippe Pieri C and Hans R. Schultz D A UMR Écophysiologie des plantes sous stress environmentaux, LEPSE, INRA–ENSAM, 2 place Viala, F-34060 Montpellier cedex 1, France. B UMR Vigne et Vin d’Alsace, INRA–Université Louis Pasteur Strasbourg, 28 rue de Herrlisheim BP 507, F-68021 Colmar cedex, France. C Unité d’Agronomie, INRA–Centre de Recherches de Bordeaux, 71 avenue Edouard Bourlaux BP 81, F-33883 Villenave d’Ornon cedex, France. D Institut für Weinbau und Rebenzüchtung, Fachgebiet Weinbau, Forschungsanstalt, von Lade Straße 1, D-65366 Geisenheim, Germany. Corresponding author; email: h.schultz@fa-gm.de Abstract. A geometrical canopy model describing radiation absorption (Riou et al. 1989, Agronomie 9, 441–450) and partitioning between grapevines (Vitis vinifera L.) and soil was coupled to a soil water balance routine describing a bilinear change in relative transpiration rate as a function of the fraction of soil transpirable water (FTSW). The model was amended to account for changes in soil evaporation after precipitation events and subsequent dry-down of the top soil layer. It was tested on two experimental vineyards in the Alsace region, France, varying in soil type, water-holding capacity and rooting depth. Simulations were run over four seasons (1992–1993, 1995–1996) and compared with measurements of FTSW conducted with a neutron probe. For three out of four years, the model simulated the dynamics in seasonal soil water balance adequately. For the 1996 season soil water content was overestimated for one vineyard and underestimated for the other. Sensitivity analyses revealed that the model responded strongly to changes in canopy parameters, and that soil evaporation was particularly sensitive to water storage of the top soil layer after rainfall. We found a close relationship between field-average soil water storage and pre-dawn water potential, a relationship which could be used to couple physiological models of growth and/or photosynthesis to the soil water dynamics. Keywords: model, soil evaporation, soil water balance, vine transpiration, Vitis vinifera L., water deficit. Introduction The importance of water supply in influencing wine quality has long been recognised, because of its major effect on the balance between vegetative and reproductive growth (Matthews et al. 1987; van Leuween and Seguin 1994). The maintenance of an equilibrium between shoot growth (vigour), and production and maturation of fruit is one of the major vineyard management challenges. Water use in vineyards is a function of available water content of the soil, rootstock and the presence of cover crops. In contrast to non-irrigated, cool and temperate climatic zones, water use in semi-arid or arid situations with moderate to severe drought periods can be controlled to some degree by irrigating at below-evapotranspiration demand using water- saving irrigation practices, such as regulated deficit irrigation (Pritchard et al. 1995) or partial root zone drying (Dry and Loveys 1999). Water management in a vineyard requires tools to evaluate water use and to assess the degree of stress, where applicable. Direct determination of soil water availability is difficult because of the heterogeneity of soils and uncertainty about the rooting depth of the vines. Other plant water-status measurements including canopy temperature (van Zyl 1986), stomatal conductance, pre-dawn or stem water potential (Matthews et al. 1987; Naor 1998; Choné et al. 2001), or whole-plant transpiration (Lascano et al. 1992; Yunusa et al. Abbreviations used: α, vineyard albedo; D, distance between rows; ES, actual soil evaporation; ES d , actual soil evaporation on a specific day; ES p , potential soil evaporation; ETP, potential evapotranspiration; ETR, evapotranspiration rate; FTSW, fraction of soil transpirable water; g max , maximum stomatal conductance; g s , stomatal conductance; H, foliage height; L, foliage width; NEU, vineyard site 1; P , precipitation; P d , rainfall on any specific day; P o , proportion of canopy gaps; Ψ PD , pre-dawn water potential; R av , solar radiation absorbed by the vineyard; R g , incident global radiation; R gv , global radiation absorbed by the canopy; THT, thermal time; TSW d , soil transpirable water on any specific day; TTSW, total amount of transpirable water; TV, daily vine transpiration rate; TV d , vine canopy transpiration on a specific day; TV p , potential vine transpiration rate; ϕ, threshold value between non-limited and limited transpiration; U, threshold value for cumulative soil evaporation; VPD, vapour pressure deficit; WIN, vineyard site 2.