Experiments were conducted on sandy loam soils at Cambridge University Farm over the period 1989–99 to examine the effects of irrigation regime and variety on water uptake (WU) in potatoes. Unirrigated crops extracted water from considerable distances ahead of the rooting front but frequently watered crops took up water from depths shallower than the current depth of rooting. There was an increase in the extraction of soil water at depth if crops were irrigated less frequently at moderate (i.e. 40 mm) soil moisture deficits (SMD). The SMD measured at different positions across the ridge always differed and the relationship changed during the season. This is of concern since most reports on water use in potatoes are based on a single measurement position for the neutron probe in the centre of the ridge and this location over-estimates crop water use. Crops grown on the flat had a more uniform extraction of soil water across the row width than crops grown in ridges but there was no evidence that having one part of the rooting system drier than another affected overall crop water use. Once rooting systems were established to considerable depth, WU continued from deeper roots even though upper horizons were periodically re-wetted by irrigation. For this reason, it proved impossible to relate WU to rooting density in specific horizons over the course of the season. Only early in the season did the proportion of total WU correspond reasonably closely with the proportion of total root length in each horizon. It appeared that the pattern and extent of soil drying created by a crop changes the horizons where water is extracted at different growth stages and the relative rooting density in a particular horizon is not a good indicator of the potential to take up water from that depth. Although rooting density decreased rapidly with increasing depth, roots deeper in the profile contributed a considerable component of total crop water requirement irrespective of the water status of horizons closer to the soil surface.

A series of close relationships were established between the ratio of actual : potential evapotranspiration and SMD for different daily evaporative rates. These showed that there was a limiting deficit at which the ratio of actual : potential evapotranspiration decreased rapidly with increasing SMD and this limiting deficit was inversely related to daily evapotranspiration rate. However, even at small SMDs, as daily evapotranspiration rate increased there was a significant, slow decrease in actual : potential evapotranspiration ratio. In order to maintain potential evapotranspiration rates in conditions of extreme demand in the UK (e.g. 5–7 mm/day), crops need to be maintained at <25 mm deficit but allowable deficits can be increased as demand moderates.