A modelling approach to discriminate contributions of soil hydrological properties and slope gradient to water stress in Mediterranean vineyards

Grapevine is a widespread crop for grape and wine production, often cultivated on hilly areas. Moderate vine water stress plays an important role in determining high-quality viticulture. However, a precise quantification of the effect of hydraulic properties and slope gradient on hilly soil water balance and consequently on vine stress has not yet been addressed in the literature. The slope-gradient effect is generally taken for granted, without providing experiments validating such a qualitative assumption. We tested the hypothesis that soil hydraulic properties play a greater role than slope gradient in driving soil water status and vine stress. In two consecutive years, we studied an "Aglianico" vineyard grown along a 90m slope, with up-slope soil having lower water holding capacity than down-slope. Up-slope vines were more stressed, as shown by lower leaf water potential, stomatal conductance, leaf CO2 assimilation and leaf area index, than down-slope vines.Water flow was simulated by using the Hydrus (2D/3D) model, calibrated and validated towards a two-year set of soil water content measurements. Significantly lower soil pressure head and higher transpiration occurred in the up- than the down-slope site.A viable procedure was developed to separate the effects of soil hydraulic properties and slope gradient by modelling soil water balance and therefore vine transpiration with and without the slope gradient. The simulations indicated that the difference in seasonal relative transpiration of 8% in 2011 and 5% in 2012 between the two sites was due to their differing soil hydraulic properties, and that the slope gradient had no contribution to this variation.