Wednesday, 15 May 2002: 4:30 PM
THE QUOTIDIAN CYCLE OF EVAPOTRANSPIRATION FROM PECAN TREES
This paper reports quotidian (daily) evapotranspiration measurements for 2001 at a closed canopy pecan orchard on the floodplain of the Rio Grande located 7 km south of Las Cruces New Mexico The 5.1 ha pecan field was planted in 1970. Addition pecan orchards in all directions surrounded the orchard. The trees average height was 12.8m with a tree spacing of 9.7 m by 9.7 m. The average tree diameter was 30cm. A walk up tower was placed in the center of the orchard to support flux sensors at 16 m height. The instrument package included a CSI data logger, a radiometer for net radiation Q, discs for soil heat flux G, and two sets of one-propeller eddy correlation (OPEC) sensors. OPEC systems measure sensible heat flux H with a sensitive, vertically oriented, propeller anemometer and a fine-wire thermocouple. OPEC latent heat flux LE was obtained as a residual in the surface energy balance G=Q+H+LE, with all fluxes positive upward. The field was irrigated from two wells that pump into an irrigation ditch and then the water is discharged into the field through a high flow turnout. The farmer irrigated when a tensiometer reaches 65 mb at the 45 cm depth. Sparling meters were installed on the pumps and read before and after each irrigation. The total irrigation amount was 184 cm. The maximum daily evapotranspiration flux was 9.5 mm/day and the season accumulative evapotranspiration from March 10 to Nov 28 was 149.2 cm resulting in an irrigation efficiency of 81%. The crop coefficient (Daily measured Et/ reference Et) ranged from 0.2 to 1.25. Increase in evaporation due to irrigation was only detected for the April 9 irrigation. The season water use was 6 % higher than reported values in the literature but the daily values in April May and June were 30% higher and the July values were 13% lower that values reported in the literature. The difference in the literature reported crop coefficient and the measured crop coefficient using the OPEC system would change the timing of irrigations during those time periods when the differences were significant.