Numerical Modeling of Water Fluxes in the Root Zone of a Mature Pecan Orchard

Little is known about the root zone soil water dynamics in irrigated pecan [Carya illinoinensis (Wangenh.) K. Koch] within and outside tree canopies. Simulations were performed using the HYDRUS-1D model to quantify isothermal and thermal water fluxes in the unsaturated zone of a mature pecan orchard in Las Cruces, NM, with and without root water uptake. Simulated water contents and soil temperatures correlated well with measured data at each depth. Isothermal water flux dominated the soil water movement in bare soil immediately aft er irrigation, while the contribution of vapor flux increased with increasing soil drying because of upward isothermal and much smaller thermal water and vapor fluxes within the 20-cm depth. In contrast, isothermal water flux was predominant throughout the under-canopy soil profile. Actual evaporation from bare soil displayed two distinct stages, immediately aft er irrigation and aft er evaporation continued to fall off with soil drying. Immediately aft er 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 due to root water extraction substantially contributed to actual evapotranspiration. Relative evapotranspiration (actual/potential ratio) correlated (P < 0.05) with the pecan stem water potential. The root water uptake pattern followed the root length density distribution immediately aft er irrigation. Patterns of uncompensated (water stress index [omega] = 1.0) and compensated (0.0 < omega < 1.0) root uptake were similar during early periods aft er irrigation. Compensated uptake remained higher under water-stressed conditions, and the compensation from the deeper soil profile generally increased for lower values of., although not consistently for 0.1 < omega < 0.5.