Modeling Flow and Storage of Water in Irrigated Date Palm Trees By Means of Dynamic SPAC Approach.

Integrated Soil-Plant-Atmosphere Continuum (SPAC) models are able to increase our understanding of the dynamics of different components in the hydraulic path from soil through roots to stems, leaves and the canopy boundary layer. Date palm trees, which have a great economic importance to growers, are characterized by an exclusive canopy structure. The architectural pattern of a date palm orchard incorporates tall trees and a canopy concentrated at the edge of the trunk. The objective of the study was to increase the understanding the role of irrigated date palm trunk’s water storage.

We investigated flow, storage and potentials of water, by using CoupModel (Jansson and Karlberg, 2004), which is a one-dimensional process-based ecosystem model that simulates the coupled biological and physical processes in the soil-plant-atmosphere systems. Meteorological driving variables for model was available from an adjacent weather station. The model was calibrated with transpiration and sap flow measurements in date palms grown in lysimeters. Maximum water storage was described as a function of leaf area index and the tree’s height. A Monte Carlo based method  was used to  define a posterior ensemble of model candidates as constrained by the available measurements using various subjectively chosen performance indicators.  The prior uniform distribution of parameters values between the suggested maximum and minimum was transformed to posterior distributions for a number of  selected parameters.

The simulations confirmed the importance of the water storage in trees by regulating the transpiration fluxes, particularly under high evaporative demand, when water uptake rate by roots cannot meet the potential evapotranspiration demand. Followed by the assumption that the uptake rate is governed by a potential gradient, the refilling of the reservoir begins in the afternoon, when the vapor pressure deficit starts to decrease. Night water uptake ceased around midnight due to the minor potential gradients between soil and the tree.

The results emphasize the importance of proper irrigation timing, specifically under coarse grained soils, and the role of the water storage of date palms in carbon assimilation maintenance, under hyper-arid climate.