By means of numerical modeling we investigate the impact of within-field variability in the soil hydraulic properties on actual transpiration and dry matter yield for three different climate scenarios. We first show that the sensitivity of the simulated actual transpiration and dry matter yield to soil hydraulic parameters increases with the dryness of the climate. The numerical simulations with soil independent stress factors further demonstrate that the impact of the within-field variability in soil hydraulic properties on the simulated transpiration and dry matter yield can be very large. The generated spatial variability in transpiration and dry matter yield increases systematically with the dryness of the climate, with coefficients of variation increasing from 7 to 14.6% for actual transpiration and from 6.7 to 16% for dry matter yield. In a subsequent analysis, all agrohydrological simulations are rerun considering that the water stress parameters are spatially variable and soil dependent. While the results obtained with the adjusted water stress parameters are quite different, the spatial variability in simulated transpiration and dry matter yield still increases for drier conditions. The different results obtained, although not validated experimentally, illustrate that the use of an agrohydrological simulation model in a stochastic mode requires accurate estimates of the water stress parameters, which should be soil dependent. Finally, we show that simultaneous estimation of water stress and soil hydraulic parameters cannot be robustly performed using measurements of transpiration or dry matter yield alone. Consequently, the use of an agro-hydrological model in a stochastic mode for a vegetated surface requires alternative strategies for specifying reliable water stress parameters. Adjustment of water stress parameters from reference unsaturated hydraulic conductivity values seems attractive, but needs more research.