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The compensation heat pulse method is widely used to estimate sap flow in conducting organs of woody plants. Being an invasive technique, calibration is crucial to derive correction factors for accurately estimating the sap flow value from the measured heat pulse velocity. We compared the results of excision and perfusion calibration experiments made with mature olive (Olea europaea L. 'Manzanilla de Sevilla'), plum (Prunus domestica L. 'Songal') and orange (Citrus sinensis (L.) Osbeck. 'Cadenero') trees. The calibration experiments were designed according to current knowledge on the application of the technique and the analysis of measured heat pulse velocities. Data on xylem characteristics were obtained from the experimental trees and related to the results of the calibration experiments. The most accurate sap flow values were obtained by assuming a wound width of 2.0 mm for olive and 2.4 mm for plum and orange. Although the three possible methods of integrating the sap velocity profiles produced similar results for all three species, the best results were obtained by calculating sap flow as the weighted sum of the product of sap velocity and the associated sapwood area across the four sensors of the heat-pulse-velocity probes. Anatomical observations showed that the xylem of the studied species can be considered thermally homogeneous. Vessel lumen diameter in orange trees was about twice that in the olive and plum, but vessel density was less than half. Total vessel lumen area per transverse section of xylem tissue was greater in plum than in the other species. These and other anatomical and hydraulic differences may account for the different calibration results obtained for each species.

During three consecutive years (1991-93), a field experiment was conducted in 3n intensively irrigated agricultural sail in SW Spain. The maio objective of tbis study was to determine the water flow and nitrate leaching below tbe root zone~ under an irrigated maize crop and after the growing season (bare soil and rainy periad). The experiment was carried out on a furrow irrigated maize crop using one of the highest nitrogen fertilizatían rates traditionally used by furmers in !he region [abont 500 kg N•(ha•yryl] and ano!her !hat represents one third of!he fonner [170 kg N•(ha.yryl] to provide data !hat can be used to propase modifications of nitro gen fertilizatian to maintain crap yield and to prevent tbe degradatian of the environment. 'Ibe tenns of water balance (crop evapotranspiration, drainage and soil water storage), .. and the nitrate leacbing were detenruned by intensive field monitoring of tbe soil water content, soil water potential and extracting the soil solution through use of a neutron probe, tensiometers and ceramic suction cups. Nitrogen uptake by tbe plant and N03-N produced by mineralization were also detennined. The results sbowed that, in terms of water balance, crop evapotranspiration was similar with botb N-fertilization rates used. During tbe irrigation period drainage below the root zone was limited. Dnly in .1992 tbe occurrence of rainfalls during tbe earIy growing period, wben tbe soil was wet from previous irrigation, caused a considerable drainage. Nitrate leaching during the entire experimental perlod amounted to 150 and 43 kg.ha-1 in the treatments with bigh and low N-fertilization, respectively. Tbis leaching occurred mainly during tbe bare soil and rainy periods, except in 1992 wben considerable nitrate leacbing was observed during tbe crap season owing to high drainage. Nitrate leacbing was oot so bigh duriog the bare soil penod as could be expected because of tbe drought during the experimental periodo A reduction of Nfertilization strongly decreased mtrate leacbing witbout decreasing yield.

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