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In 2000, new sites were established in Shark Valley Slough, to test transferability of models developed using 1996-97 data, and to refine the understanding of factors related to Et. As of January 2003 there were five continuous Et sites in the Everglades National Park.
The original sites were selected to provide a network representative of the non-forested portion of the Everglades ecosystem in terms of plant communities, duration of water inundation (hydroperiod), and geographic coverage. Other factors in site selections were security and logistics. Sites in areas open to hunting and air boating were located in relatively remote locations and not on major air boat trails. Each site was located at the center of a circle of relatively uniform vegetative cover with a radius of at least 100 times the height of the upper air temperature/humidity sensor.
Stations were instrumented to provide data for: determination of total energy available for Et (latent heat flux) and convection (sensible heat flux); determination of the Bowen ratio (the ratio sensible heat flux/ latent heat flux), so that the amount of the total available energy that was utilized for Et could be determined; and characterization of meteorological conditions and Et-model development using ancillary data.
The array and arrangement of data sensors at the sites were dependent on whether the site was in open water or in dense, emergent vegetation. The major difference between open-water sites and vegetated sites is the method of determining the air-temperature and humidity differential with height, which is necessary for computation of the Bowen ratio. At the two open-water sites (sites 2 and 3), the air temperature and humidity differentials were measured from the water surface to a point 3-4 feet above the water surface. At the seven vegetated sites (sites 1, 4-9) the differentials were measured between two points in air, 3-5 feet apart.
At each site, sensor measurements were made automatically every 30-seconds and these measurements were averaged and stored onsite at 15- or 30- minute intervals. These data were then transmitted daily by cellular telephone to computer storage in the office. Data were reviewed on a daily basis to detect equipment breakdown and sensor malfunction. Site visits were made at approximately monthly intervals for routine scheduled maintenance and cleaning, or more frequently when malfunctions occurred.
Data were collected from January 1996 through December 1997 for sites 1, 2, 3, and 5. Data were collected from January 1996 through December 1999 for site 4, from December 1996 through December 1998 for site 6, and from January 1996 through December 2000 for sites 7 and 8. Site 9 was installed in January 1997 to increase representation of drier parts of the Everglades; site 9 furnished data from January 1997 through December 1997. Only data that passed screening tests for accuracy were used to develop the models of Et. The screening tests were based on range limits, visual inspection of plotted net radiation, temperature and humidity readings to eliminate periods when sensors were obviously malfunctioning, and on criteria given by Ohmura (1982). These criteria specified that flux calculations are inappropriate if the calculated latent heat flux is in the opposite direction from the observed vapor-pressure vertical difference. Such a situation would indicate an error in determination of either the energy budget or the vapor-pressure or temperature vertical differences. Ohmura also recommended that Bowen-ratio calculations be rejected if temperature or vapor-pressure vertical differences are at or less than sensor resolution limits. Resolution limits for this study are 0.013 degree Celsius for vertical temperature differences and 0.003 kilopascal (kPa) for vapor-pressure differences. These screening criteria eliminated about one-half of the available data from model development, mostly because of sensor failure and resolution limits. Most of the data rejected because of resolution limits or flux directions were for night-time hours, when energy inputs, air-temperature vertical differences, and vapor-pressure vertical differences are all relatively low.
Sites were visited at 4-6 week intervals for inspection and maintenance. Maintenance generally included the following items:
Ventilator fans - Clean and replace, if not operating Net radiometer domes - Clean and replace, if damaged. Replace radiometer if water damaged Radiation shields (air temperature and humidity) - Clean Air temperature and humidity sensors - Clean, replace sensors, if necessary. Water-level sensor - Raise float and check for proper response. Rain gage - Check for obstructions, clear if necessary; test calibration periodically. Water temperature sensors - Check for proper position and reading. Net radiometers and pyranometer - Check for level, adjust if necessary. Sensor exchange mechanism - Check for smooth operation, replace as necessary. Field verification of air temperature, relative humidity, wind speed, and wind direction using handheld meters.
The net radiometer domes required the most frequent maintenance. These domes, made of soft transparent polyethylene, shield the sensors from moisture, wind, or debris that could affect sensor performance. Problems encountered included crushing by hail, pecking by birds, and gradual deterioration of the polyethylene. Domes were changed at 3-month intervals, or sooner if damage occurred. If the domes were cracked, punctured, or there was evidence of water penetration into the sensor, the entire net radiometer was replaced.
Air temperature and humidity sensors failed frequently during the first year of operation, due to corrosion of electrical contacts. A change in sensor design resulted in much-improved service life of these sensors during the second year of operation. The sensor exchange mechanisms were subject to occasional failure, generally due to mechanical wear or water penetration into the control circuitry.
Net radiation is measured directly by the net radiometers, but the measured value is affected by wind speed and must be corrected. The wind correction factor was calculated from wind measured at the sites using procedures described by C. Fritchen of REBS, Inc. in a personal communication. Soil heat flux was measured at all vegetated sites, but was not measured at the open-water sites because these sites were always covered by water, generally to a depth of more than 1 ft. At the vegetated sites the soil heat flux was determined from the sum of heat flux measured by a heat-flux plate buried 5 centimeters (cm) below the land surface and the change in heat stored in the soil profile above the plate. Water heat storage was calculated at all sites whenever water was standing on the water surface. At open-water sites with little or no emergent vegetation, the air-temperature and vapor-pressure differentials necessary for the Bowen-ratio determination are determined from measurements of water temperature at the water surface and air temperature and vapor pressure at a point 3 to 4 ft above the water surface. The water-surface temperature is measured by using a float -mounted thermocouple, and is assumed to represent the air temperature at the water-air interface. The vapor pressure at that point is assumed to be equivalent to 100 percent relative humidity. Because the differences between water surface and air are much greater than differences in the air over similar distances, the effect of air and vapor pressure sensor bias is negligible. Therefore, the sensor exchange mechanism is not required and only one air temperature /vapor pressure sensor is needed at such sites.
See WRIR 00-4217 (<http://fl.water.usgs.gov/PDF_files/wri00_4217_german.pdf>) for more detail and the formulas used in the calculations.
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Data analysis and modeling
Data processing will be completed for all sites, to provide actual Et data, together with related meteorological data.
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