Plant water use efficiency (WUE) is defined as the net dry matter production (DM) per unit of
consumptive water use. It is a vital variable for plant growth, yield, and irrigation-management
models. Pecan [Carya illinoinensis (Wangenh.) C. Koch] WUE has been estimated for
above ground biomass growth using evapotranspiration (ET) and DM data measured separately
in different experiments. The WUE for whole pecan trees (including above and below
ground parts), a direct measurement (ET and DMmeasured in one experiment), and a simple
model forWUE based on weather conditions, are currently lacking.A 16.5mwalk-up tower in
a flood irrigated pecan orchard, located in the Mesilla Valley of NM, was instrumented with
energy budget and eddy flux sensors. Continuous, above canopy measurements of vertical
fluxes of sensible heat, H2O vapor, and CO2 were made by the eddy covariance technique in
growing seasons from 2002 to 2005. ET was calculated from vapor flux. DM production was
calculated from CO2 flux assuming that dry matter of pecan trees was 46.4% carbon.
During the growing seasons (May through November), the mean ET was 122.7 cm (48
inch) per season; the mean dry matter production for the whole trees was 22082.3 kg ha1
(19684.4 lb acre1) per season. The average seasonal water use efficiency for the whole trees
was 179.7 kg ha1 cm1 (406.5 lbs acre1 inch1). In ‘on’ years (high-yield years) 13.8% of the
dry matter produced was allocated to the harvested nut crop, while in ‘off’ years only 8.0%
was so allocated. Similarly, the nut WUE as nut yield per unit water used (as cm depth) was
higher in ‘on’ years, at 26.2 kg nuts ha1 cm1 versus 14.9 kg nuts ha1 cm1. A simple model
for monthly WUE (kg ha1 cm1) as a function of vapor pressure deficit (VPD) and relative
humidity (RH) was obtained. It is anticipated that the measured WUE, the simple WUE
model, and other data obtained in this study will be useful in developing and validating
pecan growth, yield, and irrigation-management models.