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Reliable parameterizations of water uptake under osmotic stresses depend on the appropriate description of the interacting effects of root distribution and activity over the root zone. These effects should be evaluated under transient salinity conditions. To show that, a crop was irrigated with three salinity inputs using a randomized block design. Root-zone water storage and pressure heads measured at the bottom of the root zone allowed to calculate the evapotranspiration under the different salinity levels imposed. Root densities were measured by direct sampling twice during the growth season. Transpiration was distributed along the root zone according to a Feddes-type sink term and a reduction function accounting for osmotic stress. These data allowed evaluating the effects on the reduction function of (1) the actual salinity and root distributions under different osmotic stress and (2) discontinuous osmotic stress conditions occurring locally in the root zone because of salt leaching. When considering the space–time salinity distribution and the specific root distribution, a unique reduction curve was found, independently of the salinity level of the irrigation water. The effect of discontinuous stress was modelled by introducing (1) a hysteretic behaviour in the salinity reduction function, to be applied only to root compartments actually experiencing discontinuous stress and (2) a scaling factor acting on the whole root zone, based on the ratio of the actual (induced by salinity) to the potential root density. Results suggested that the reduction of transpiration under salinity may be partly caused by irreversible physiological adaption of the roots to saline stress and partly by reduced root density. Copyright © 2014 John Wiley & Sons, Ltd.