Figure 1. Location of Blue Cypress Marsh site and Everglades site. [larger image]

Evaporation and plant transpiration (ET) are significant components of the water budget in south Florida. Water loss through ET can exceed rainfall during dry years. Recent advances in instrumentation and measurement techniques have made it possible to develop a better understanding of ET processes and to quantify ET rates. ET rates at two sites vegetated primarily by sawgrass, one in Blue Cypress Marsh near Vero Beach in the St. Johns River floodplain and the other in the southern Everglades of Everglades National Park (fig. 1), yield significantly different ET rates during drought conditions that occurred in January through June of 2000.

The Blue Cypress Marsh site (BCM) has dense sawgrass in a thick peat soil. At this site, the ET fraction (EF), which is the ratio of latent heat (the energy equivalent of ET) to the sum of latent heat and sensible heat (convective heat transport), was affected little by the change in water level even when nearly 3 feet below land surface. The Everglades National Park site (ENP) has a relatively sparse rush/sawgrass community in a thin marl soil. At this site, the EF decreased markedly as the water level dropped to about 2 feet below land surface.

The monthly total ET was greater at BCM than at ENP for each month except January (fig. 2). The largest differences in monthly ET rates (greater than 1 inch) occurred from March through July, when water levels were below land surface at one or both sites. Annually, the total ET was 55.7 inches at BCM and 43.5 inches at ENP. This difference in annual total ET is not explainable by differences in available energy (138 watts/m² at BCM and 132 watts/m² at ENP for 2000), and is the result of the differences in EF between the two sites.

The EF (and hence ET) apparently are related to water level at the ENP site but not at the BCM site. The reason for this difference in behavior of EF is not understood, but probably is related to the differences in plant cover and soil type between the two sites. The thick sawgrass cover at BCM apparently is able to transpire at maximum efficiency even when the water level is more than 2.5 feet below land surface. The thick peat soil layer may play a role in this high EF even during low-water conditions by providing a reservoir of soil moisture from which the sawgrass can draw. Additionally, the sawgrass coverage at BCM is relatively uniform and thick, and the incident solar radiation only penetrates the top of the sawgrass. At ENP the vegetative cover is thinner and less extensive. Heating of the sawgrass probably is less than heating of the land surface in exposed locations, so that the utilization of available energy for sensible heat transport likely is less at the BCM site than at the ENP. Less sensible heat transport relative to latent heat transport would cause a relatively high EF.

Figure 2. Monthly total evapotranspiration, January 2000 through December 2000. [larger image]

Contact: Edward R. German, U.S. Geological Survey, 224 W. Central Parkway, Altamonte Springs, FL, 32714
Phone: 407-865-7575, Fax: 407-865-6733, egerman@usgs.gov

(This abstract was taken from the Greater Everglades Ecosystem Restoration (GEER) Open File Report 03-54)

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