The Lower Rio Grande Valley in southern New Mexico is one of the major pecan producers in the nation. Pecan is known to consume large amounts of water. Irrigation depends on the availability of water for most of the growing season, particularly surface water from the Rio Grande and supplemental groundwater. Previous studies mainly focused on pecan ET and pecan water use efficiency.
Tree canopies at open-canopy mature pecan fields have potential to modify microclimate and soil conditions: under-canopy may influence evaporation, transpiration and root water uptake whereas outside tree drip line (bare, no roots) may influence water vapor flow and evaporation. Little is known about the root zone soil water dynamics in irrigated pecan within and outside tree canopies.

Objective

The objective of the study was to quantify different water and energy fluxes using the HYDRUS-1D model in the unsaturated zone of an irrigated mature pecan orchard in the Lower Rio Grande Valley, with (under-canopy) and without (outside the tree drip line or bare) root water uptake. The model was used to simulate different isothermal and thermal water fluxes under bare and under-canopy soil conditions, including actual and potential evaporation rates in bare and under-canopy soils, with evapotranspiration and transpiration rates as well as root water uptake pattern in under-canopy soils.

Summary of the Experiment

• Simulations were performed using the HYDRUS-1D model to quantify isothermal and thermal water fluxes in the unsaturated zone of a mature pecan orchard iwith and without root water uptake.
• Soil water content (?) at 5, 10, 20, 40 and 60 cm and T at 5, 10, 20, and 40 cm depths were measured and used as calibration targets to optimize water and heat transport parameters.
• The site was equipped with an automatic HOBO U30-NRC weather station which recorded the solar radiation, air temperature, barometric pressure, wind speed and direction, and relative humidity.
• The midday stem water potential (SWP) was monitored twice a week with a pressure chamber.

Results & Discussion

The HYDRUS-1D simulations were for a sandy loam flood-irrigated mature pecan orchard under under-canopy (with root water uptake) and bare (outside tree drip line or no roots) soil conditions.

These simulations during a 117-d period demonstrated that the model predicted the soil water contents and soil temperatures and their temporal variations at all depths of 5, 10, 20, and 40 cm to a reasonable accuracy.

Isothermal water flux dominated the soil water movement in bare soil immediately after 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 (Fig.1.). 

The simulated actual evaporation rate, which decreased between two successive irrigation events in proportion to the amount of water available near the bare soil surface, showed two distinct evaporation stages: immediately after an irrigation event when actual evaporation rates were almost equal to potential ones, and after actual evaporation rates began to fall off as the soil dried. Trends of actual and potential evaporation rates at under-canopy locations were also similar immediately after irrigation.

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 (Fig. 2).

For more information see the presentations and publications under Outreach