Responses of potatoes (Solanum tuberosum L.) to irrigation and nitrogen in a hot, dry climate -: I.: Water use

The effects of differential irrigation and fertiliser treatments on the water use of potatoes (Solanum tuberosum L. cv. Desiree) were studied over 2 years in the hot dry climate of northeast Portugal. Total actual evapotranspiration (ETc) ranged from 150 to 320 mm in 1988, and from 190 to 550 mm in 1989 depending mainly on irrigation treatment, potential evaporation rates (ETp) and duration of the growing season. By comparison, the effects of nitrogen fertiliser on total water use were relatively small. Although nitrogen increased transpiration (larger leaf canopy), it reduced evaporation from the soil surface, in frequently irrigated plots, by similar amounts. As a result, in well-irrigated crops, the ETc/ETp ratio averaged 0.85 over the season, regardless of nitrogen level. Evaporation from the soil surface represented 15-25% of total water use by well-fertilised plants, but as much as 30-50% from the sparse stands of unfertilised crops. The proportion of water extracted from each depth increment of the silt-loam soil declined logarithmically, from the surface to 1.1 m depth, the maximum measured, for irrigated crops, and linearly when rain-fed. The ETc/ETp ratio fell below unity when 25-30% of the available water in the top metre had been depleted, equivalent to soil water deficits (SWDs) of 45-50 mm. By comparison, ETc declined to zero when 75-90% of the available water had been extracted, corresponding to actual deficits of 135-150 mm. Peak ETc rates reached 12-13 mm per day on days immediately following irrigation, nearly twice ETp (possibly due to the influence of advection) but then declined logarithmically with time to about 3 mm per day within 5 days. Using the same data, a companion paper reports the influence of climatic conditions on the yield responses to water of potato crops grown in the region. (C) 2002 Elsevier Science B.V. All rights reserved.