The quotidian cycle of evapotranspiration from saltcedar

This paper reports quotidian (daily) evapotranspiration measurements for 1999 at a dense saltcedar stand on the floodplain of the Rio Grande near Soccoro, NM (34°N, 107°W). The experimental plot was about 350 m wide by 1000 m long, at an elevation of 1374 m. Height of the dense saltcedar ranged from 5 to 7 m. A north (n) tower and a south (s) tower were placed about 685 m apart on the N-S centerline of the plot to support flux sensors at 15 m height. Each 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 is obtained as a residual in the surface energy balance G=Q+H+LE, with all 4 fluxes positive upward.

Basic flux data were obtained at 10 Hz averaged over 30-min periods for each of the 365 days of the year. All valid 30-min data were integrated into daily energy flux totals (MJ m^-2 d^-1). Those days with complete data on the north tower were used to establish the validity of the OPEC sensible heat flux measurements (nH1) by comparison against a CSI 3-dimensional sonic eddy correlation system (nH3). One hundred fourteen days of this comparison were obtained from day of year (DOY) 112 through DOY 271. The linear relation between the two systems was excellent for daily totals: nH3=1.01nH1 + 0.37, sd=0.66, and R²=0.96, with data ranging from –5 to 13 MJ m^-2 d^-1. Similar excellent comparisons were obtained for daily on-tower agreement (nH1 v. nH2, and sH1 v. sH2), and for between-tower agreement (nHi v. sHi, nQ v. sQ, and nG v. sG). These relations were used to fill data gaps in order to extend flux estimates over the full year 1999. Q, G and H from individual sensors at the two towers were averaged to obtain best estimates for the saltcedar stand; these averages were used to calculate mean daily LE.

The quotidian energy fluxes at the saltcedar site were integrated over 365 days to yield annual totals. These in MJ m^-2 yr^-1 are Q=4540; G=-3; H=1315; and LE=3222. LE total is equivalent to ET of 1315 mm yr^-1 of water. Saltcedar is deciduous, and at this site bud burst occurs around DOY 107 (April 17). By DOY 121 about 10 percent of annual ET has occurred, and leaf senescence takes place about DOY 305 (Nov. 1) by which date 95 percent of annual ET has occurred. Thus about 15 percent of annual ET occurs during the 6-month leafless period, and 85 percent occurs during the 6-month, physiologically active, in-leaf period. This data set will be useful for modeling saltcedar ET.