Modeling the transpiration of grapevines on sloped vineyards

Abstract

Recent studies have shown that models, which can partition solar radiation between grapevines and soil are valuable tools to correspondingly separate potential evapotranspiration and to derive an expression for the potential transpiration of grapevines. By means of measuring sap flow, this allows to calculate relative transpiration rates (the ratio of measured actual to calculated potential grapevine transpiration), which can be analyzed regarding their feedback to drought stress or high evaporative demand or to other water transport related questions. We first calculated radiation interception fractions for direct and diffuse solar radiation intercepted by the grapevines or the soil of row oriented canopy structures with a numerical simulation based on the Monte Carlo method. Further, we considered the influence of slopes on potential evapotranspiration by calculating the amount of solar radiation which is received by tilted surfaces and by calculating horizontal equivalents of evapotranspiration. This allowed to calculate potential vine transpiration based on vineyard geometrical features and made it possible to directly compare the relative transpiration of different vineyards. We measured sap flow and soil water content on three different sites grown with Vitis Vinifera cv. ‘Riesling’ for two years. In the absence of water deficit, actual grapevine transpiration was 56% (±32%) of potential vine transpiration. A linear decrease of relative transpiration as a function of the fraction of transpirable soil water (FTSW) was observed, if soil water content dropped below a threshold of FTSW of 0.4. Responses of relative transpiration in dependence of potential evapotranspiration or vapor pressure deficit differed among the vineyards, which might be an obstacle for the development of a universally valid grapevine transpiration model.