This study originated with the objective of parameterizing riparian evapotranspiration (ET) in the water budget of the Middle Rio Grande. We hypothesized that flooding and invasions of non-native species would strongly impact ecosystem water use. Our objectives were to measure and compare water use of native (Rio Grande cottonwood, Populus deltoides ssp. wizleni) and non-native (saltcedar, Tamarix chinensis & Russian olive, Eleagnus angustifolia) vegetation and to evaluate how water use is affected by climatic variability resulting in high river flows and flooding as well as drought conditions and deep water tables. Eddy covariance flux towers to measure ET and shallow wells to monitor water tables were instrumented in 1999. Active sites in their second decade of monitoring include a xeroriparian, non-flooding salt cedar woodland within Sevilleta National Wildlife Refuge (NWR) and a dense, monotypic salt cedar stand at Bosque del Apache NWR, which is subject to flood pulses associated with high river flows.
Three-dimensional eddy covariance: Measures fluxes of latent heat, sensible heat, and momentum, integrated over an area such as a vegetation canopy. High frequency measurements are made of vertical wind speed and water vapor, and their covariance over thirty minutes is used to compute latent heat flux, the heat absorbed by evaporation, from the canopy surface. Latent heat flux (LE) is converted to a direct measurement of evapotranspiration (ET). Simultaneous, high frequency measurements of temperature are used with vertical wind speed to compute the sensible heat flux (H), the heat transfer due to vertical temperature gradients. Measuring net radiation (Rn) and ground heat flux (G), allows the energy balance to be calculated (Rn = LE + H + G), providing a self-check for accuracy and closure error.
Sites: Two Rio Grande riparian locations in P. deltoides forests, two in T. chinensis forests. In each forest type, one of the two sites is prone to flooding from elevated Rio Grande flows, and the other site does not flood. A fifth site was located in a mix of non-native Eleagnus angustifolia (Russian olive) and native Salix exigua (coyote willow) prone to flooding.
Design: Eddy covariance systems were mounted on towers in the turbulent surface layer 2-2.5 m above the canopy. Measurement period was 10 Hz and the covariance period was 30 minutes. Additional energy fluxes were made at 1 Hz and averaged over 30 minutes.
Water table fluctuations were monitored at the sights with groundwater wells installed ~ 1 m below baseflow water table. Wells were constructed of 5 cm inner diameter PVC pipe with approximately 1 m screen lengths. Automated pressure transducers were deployed to measure water table elevations at 30-minute intervals.
Precision: Thirty minute average or total (e.g., precipitation) core data from field instruments and processed field data (thirty minute or daily average or total. Data are programmed for IEEE4 4 byte floating point output (~ 7 digits), but actual precision values are not apparent in the program or in many instrument manuals.
Missing Data: Direct-from-field data time stamps are excluded if data are missing.
Instrument Name: 3-D Sonic Anemometer
Manufacturer: Campbell Scientific, Inc. (Logan, UT)
Model Number: CSAT3
Instrument Name: CO2/H2O Analyzer
Manufacturer: Li-Cor, Inc. (Lincoln, NE)
Model Number: LI-7500
Instrument Name: Net Radiometer
Manufacturer: Kipp & Zonen (Delft, The Netherlands)
Model Number: CNR1
Instrument Name: Barometric Pressure Sensor
Manufacturer: Vaisala (Helsinki, Finland)
Model Number: CS105
Instrument Name: Temperature and Relative Humidity Probe
Model Number: HMP45C
Instrument Name: Wind Sentry (Anemometer and Vane)
Manufacturer: R.M. Young (Traverse City, MI)
Model Number: 03001
Instrument Name: Tipping Bucket Rain Gage
Manufacturer: Texas Electronics, Inc. (Dallas, TX)
Model Number: TE525
Instrument Name: Quantum Sensor (PAR)
Model Number: LI-190
Instrument Name: Water Content Reflectometer
Model Number: CS616
Instrument Name: Soil Heat Flux Plate
Manufacturer: Radiation and Energy Balance Systems, Inc. (Bellevue, WA)
Model Number: HFT3
Instrument Name: Averaging Soil Thermocouple Probe
Model Number: TCAV
Instrument Name: Measurement and Control System (Datalogger)
Model Number: CR5000
Instrument Name: Levelogger and Barologger (Water Table)
Manufacturer: Solinst Canada Ltd. (Georgetown, ON, Canada)
Model Number: 3001 LT M10 and 3001 LT M1.5
Instrument Name: Mini-Diver, Cera-Diver, and Baro-Diver (Water Table)
Manufacturer: Van Essen Instruments ((Delft, The Netherlands)
Model Number: DI501, DI701, and DI500
Instrument Name: Krypton Hygrometer
Model Number: KH2O
Model Number: Q-7.1
Instrument Name: Pyranometer
Model Number: CM3
Instrument Name: Micrologger
Model Number: CR23X
Instrument Name: Submersible Sensor Pressure Transducer (Water Table)
Manufacturer: Electronic Engineering Innovations (Las Cruces, NM)
Model Number: 2.0 (2 m) and 5.0 (4 m)
a] Before ET is computed from LE, various standard corrections are applied. These include: coordinate rotation to align the wind vector with the sonic anemometer, corrections developed from frequency response relationships that incorporate sensor line averaging and separation (Massman corrections), and corrections to account for flux effects on vapor density as opposed to mixing ratio measurements. Corrections are made in a data analysis (Perl) program. See Cleverly, et al., Hydrological Processes 20: 3207-3225, 2006 for more detail and references.
b] On days in which 1-4 of the 30 min LE values are missing, a general linear regression model between LE and Rn is used to estimate missing data whenever the regression coefficient was significantly different from 0 (i.e. p > 0.5). ET is not calculated from LE on days that do not match the above criteria.
c] Other missing data required for derived data values, as well as out of range data are filtered out in data analysis (Perl) programs.
d] Closure of the energy balance is achieved by adding the measured Bowen Ration (H/LE) components to H and LE. Closure represents the error introduced when applying the energy balance method to estimate ET: closure = Rn - LE - H - G. The measured Bowen Ratio, H / LE, is used to parse the closure value into component H and LE values.
e] Soil water content data are calibrated with soil water content (% vol) values measured from field samples by linear regression in a data analysis (Perl) program.
f] Well loggers are pressure transducers that measure absolute pressure (barometric plus water column pressures). An on-site barometric pressure transducer suspended above the water table is calibrated to quantify pressure in units of elevation head, which is subtracted from absolute head to arrive at the actual water level.
g] Well data are calibrated using periodic manual measurements of water table elevations.