Controls on Interannual Variation in Evapotranspiration and Water Use Efficiency in a Mature, Furrow-Irrigated Peach Orchard.

Evapotranspiration (ET) and water use efficiency (WUE) in peach orchards has previously been observed in young (less than 5-8 years old), drip irrigated orchards using micrometeorological techniques such as Eddy Covariance or large-weighing lysimeters.  However, no work has been reported on ET and WUE in mature orchards nearing the end of their commercial lifetimes.  In this study, we established an Eddy Covariance (EC) tower in a commercial, mature (14 year old), large stature (canopy height 4-5.3 m), furrow irrigated cling peach orchard in the San Joaquin Valley, California, USA.  We observed ET, net ecosystem productivity (NEP), Gross Primary Productivity (GPP), ecosystem respiration (Re) and radiative fluxes with EC tower instrumentation. LAI and fractional canopy cover was assessed using non-destructive optical ground observations and satellite imagery.  Annual ET (~1400 mm/year) and maximum seasonal crop ET coefficients (>1.2) were higher than previous multi-annual lysimeters based studies in the San Joaquin Valley; seasonal crop Kc also rose to higher values (~1) earlier in the growing season (~Day of Year [DOY] 100) and remained high until leaf senescence (~DOY 300) .  Maximum canopy light interception peaked at over 80% with growing season leaf area index (LAI) ranging from 2-3.  Mean annual ET did not show significant interannual variation.  NEP was significantly higher in 2013 than 2012 (867 vs. 745 g C m^-2), but GPP (2,640 vs. 2,440 g Cm^-2) and Re (1,870 vs. 1560 g C m^-2) were significantly higher in 2012 than 2013.  Physiological WUE (GPP/ET) was significantly higher in 2012 than 2013 (1.99 vs. 1.83 g C m^-2 mm^-1 ET).  WUE variation seemed most strongly related to soil moisture especially in the period between harvest and senescence.  The results indicate the potential for mature peach to have very high ET, particularly in cling groves where production volume for canning is emphasized.