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International audience; The Penman-Monteith equation (PM) provides a direct and logical route to explaining rates of crop water consumption without the need of resorting to the artificiality of a reference crop and ill-defined crop factors. However as an operational irrigation tool, the PM is often impractical because of uncertainties about stomatal behaviour and turbulent transport. While recent progress is providing better descriptions of these processes, it is questionable whether such detail is always warrented for practical irrigation. Another option for evaporation prediction is the Priestley and Taylor equation (PT). This approach enjoys reasonable empirical support in humid regions and our efforts here are devoted to better defining its limits of applicability. We begin by presenting previously published and new evidence in support of Priestley and Taylors' value of 1.26 for the coefficient α indicating proportionality between evaporation rate and available energy. While α may depart from this value depending on windspeed and saturation deficit, it is shown to be fairly insensitive to both over a reasonable range of conditions. A daytime mean saturation deficit of 10 g m–3 is suggested as a likely upper limit for the range of applicability of the PT; below this limit, the extra data inputs required for the PM will not be rewarded with a better estimate of evaporation. In arid regions and under stable conditions, fluxes well beyond the PT estimate have been measured and no simple prediction alternative to the PM exists.

The transpiration of 2 kiwifruit vines grown within sheltered orchards in the Nelson locality was estimated in late summer by measuring their water uptake for 2 days following excision. Total daily transpiration by vines with vertically projected canopy areas of 16–17 m2 varied between 80 and 100 liters of water per day (4.8–6.1 mm/day) and ranged from 1.6 to 2.4 times the equilibrium evaporation rate. Stomatal conductances on excised vines were lower than those of adjacent uncut vines after the first day. Since transpiration estimates using the Penman-Monteith formula agreed well with the measured water uptake by cut vines, this equation was used to predict the transpiration of adjacent intact vines. Advective enhancement of evaporation contributed between 21 and 49% of the daytime water use despite the presence of multiple, closely spaced shelterbelts. Transpiration continued throughout the night as a consequence of incomplete stomatal closure, so that in total, advected energy was responsible ...

An in situ device for determining the soil water potential by measuring the dewpoint temperature depression is described. A saturated ceramic plug is used to provide hydraulic contact between the soil and an air-tight measuring chamber housing a four-terminal thermojunction. The ceramic and the thermojunction references are thermally clamped to the copper body of the instrument in order that temperature differences between the vapor source (the internal evaporating surface of the ceramic) and the reference junctions are minimal. Thus, the device reduces the major source of error that plague conventional soil psychrometers in the presence of temperature gradients. A hygrometer placed perpendicular to an imposed thermal gradient in a soil column recorded a maximum zero offset voltage of 0.22 µV as compared with 130 µV from a Wescor psychrometer under the same conditions. Provided the ceramic remains saturated, the response of the hygrometer to changes in soil water matric potential is extremely rapid. Two bean plants (Phaseolus vulgaris L., c.v. “Tender-green”) growing in a 2-liter container of silt loam soil were subjected to wetting and drying cycles in the laboratory. Simultaneous measurements of leaf and soil water potential illustrate the capacity of the soil hygrometers to track changes in soil water matric potential. In the presence of solutes, the instrument is diffusion limited. The time constant for salt equilibration, with our geometry and using KCl as the solute was 23 hours. This is consistent with a diffusion coefficient of 2 to 3 × 10⁻⁶ cm³ sec⁻¹. For this reason the application of this design should be restricted to circumstances where the osmotic contribution to the total water potential is either small or not changing rapidly. These criteria are generally satisfied in the humid regions.

Losses of export-quality kiwifruit (Actinidia chinensis Planch.) attributable to wind damage increased with distance downwind of both natural and artificial windbreaks. These effects occurred on standard ‘T-bar’ trellised vines during a particularly windy season, within shelter spacings which the literature suggest should achieve adequate reductions in windspeed. Distance from the nearest upwind natural shelterbelt explained 57%of the variation in exportable yield per vine. Major losses resulted from frictional marks caused by fruit rubbing against each other and against plant canes. Mean proportions of exportable sized fruit rejected for this reason varied from 3–10%(for rows adjacent and to the lee of natural windbreaks) to 30–44%(3–4 rows downwind of artificial shelter). Trends in foliar damage and vegetative growth were similar to those observed in wind-rub damage. On rows partially shaded by natural shelter, up to 40%of the total fruit load did not reach export size because of poor pollinati...

The rooting patterns of mature kiwifruit (Actinidia chinensis Planch.) vines were observed in 2 soils of contrasting texture — Ohinepanea sand and Levin silt loam. Near the surface of both soils, roots extended laterally 2.2-2.4 m from the base of the stem. In the Bay of Plenty, rooting depths of 3 m have been observed and in the Ohinepanea sand significant moisture extraction occurred to depths exceeding 2.4 m. By contrast, the roots in the Levin soil were restricted to the top 70 cm because of unfavourable subsoil conditions. Some hydraulic characteristics of kiwifruit were measured and provided a basis for examining the relative contributions of plant and soilto- root resistances in the water uptake process. Large xylem vessels up to 0.5 mm in diameter in both roots and stem provide a very low resistance pathway for water movement within the vine. The measured hydraulic conductivities, which varied between 2.7 and 7.2 X 10-7 m2/Pa per s, may be the highest ever reported. Positive root pressure...

An irrigation experiment was conducted on young kiwifruit vines over two seasons to examine effects of water stress on fruit development. Vines were grown outdoors in a sandy, rooting medium enclosed within a polythene-lined trench with removable surface covers to enable strict control of the water supply. Measurements of fruit growth, leaf water potential, and stomatal conductance were made throughout the season in conjunction with periods of water stress imposed at different times, and for varying durations. Fruit development was very responsive to water stress with mean fruit size per vine at harvest varying from 60 to 130 cm3 as a result of various stress treatments. Fruit expansion ceased when predawn leaf water potentials fell below −0.1 MPa. Upon rewatering, leaf turgor was regained within 24 h even after severe, prolonged stress. Any turgor loss associated with fruit softening was quickly made up, and thereafter fruit growth continued at the same rate concurrently exhibited on continuously well-watered vines. Suggesting that stomatal conductance did not follow the rapid recovery of leaf water potentials and fruit expansion may be more closely linked to water supply than to the concurrent rate of photosynthesis. Despite the large range in mean fruit size, the shape of the fruit size distribution at harvest was not affected by water stress and it is concluded that harvest yields can be adequately modelled by assuming a normal distribution with a fixed standard deviation.