On quantifying soil water deficit of a partially wetted root zone by the response of canopy or leaf conductance

Quantifying the soil water deficit (SWD) and its relation to canopy or leaf conductance is essential for application of the Penman-Monteith equation to water-stressed plants. As the water uptake of a single root depends on the water content of the soil in its immediate vicinity, the non-uniform distribution of water and roots in the soil profile does not allow simple quantification of SWD from soil-based measurements. Using measurements of stem sap flux (with a heat pulse technique), soil evaporation (with micro-lysimeters) and meteorological parameters the canopy conductance was obtained through inversion of the Penman-Monteith equation. SWD was evaluated by averaging the soil water content profile of the root zone (monitored by layers with the TDR sensors) weighted by root distribution of the layers. The average canopy conductance at midday (11:00-15:00, Israel Summer Time), denoted as G _noon, was computed for each day of the experimental period. Stable summer weather, typical of the Mediterranean region, and the fully developed crop canopy, made water stress the only plausible cause of a G_noon decline. However, the daily decline of G_noon did not occur at the same weighted average soil water content during the successive drying cycles. For the cycle with less irrigation, the decline in G_noon occurred at higher soil moisture levels. Alternatively, when SWD was determined from the water balance, i.e., by defining water deficit as irrigation minus accumulated evapotranspiration, the G_noon decline occurred at the same value of water deficit for all irrigation cycles. We conclude that a climate-based soil water balance model is a better means of quantifying SWD than a solely soil-based measurement.