Numerical Simulation of Water Fluxes In the Root Zone of An Irrigated Mature Pecan Orchard.

Open–canopies in mature pecan orchards have potential to modify tree microclimate and surrounding soil surface conditions, characterized by localized conditions of soil water and thermal regime within tree canopies and outside tree drip lines, both of which may influence non–isothermal soil water dynamics in unsaturated soils and are essential for a proper understanding of the soil–plant–atmospheric continuum. Yet, little is known about the root zone soil water dynamics in irrigated pecans within and outside tree canopies. Simulations were carried out using HYDRUS–1D model to quantify isothermal and thermal water fluxes in the unsaturated zone of a mature pecan orchard in Las Cruces, New Mexico, with and without root water uptake. Simulated water contents at 5, 10, 20, 40, and 60 cm and soil temperatures at 5, 10, 20, and 40 cm soil depths correlated well with the measured data at each depth. Isothermal water flux dominated the soil water movement in bare soils immediately after irrigation, while the contribution of thermal vapor flux increased with increasing soil drying because of upward isothermal and much smaller thermal water, and vapor fluxes within 20 cm depth. In contrast, isothermal water flux was predominant throughout the under–canopy soil profile. Actual evaporation rate at bare displayed two distinct stages, immediately after irrigation and after evaporation continued to fall off with soil drying. Immediately after irrigation, trends of under–canopy actual and potential evaporation rates were similar. With the depletion of surface soil water, evaporation losses were lower and actual transpiration rate due to root water extraction substantially contributed to actual evapotranspiration rate. Relative evapotranspiration (ratio of actual to potential) correlated (P<0.05) with pecan water stress indicator measured as stem water potential. Root water uptake pattern followed the root density distribution immediately after irrigation. Patterns of uncompensated [water stress index (w) = 1] and compensated (w = 0) root uptake were similar during early periods after irrigation. However, compensated uptake remained higher during water–stressed conditions in the top 20 cm, and the compensation from deeper soil profile generally increased for lower values of w (0<w<1.0), although not consistently for 0<w<0.5.