Data

Metadata

Format

Methodology

Sites Listing

Related Project:

ET Measurements and Modeling

SITE LOCATIONS AND BRIEF DESCRIPTIONS:

The number in parentheses is site/map number used in WRI-00-4217. Lat and long are latitude and longitude in degrees, minutes, and seconds. The following abbreviations are used in the comments column. SFWMD, South Florida Water Management District; ENR, test area for the Everglades Nutrient Removal Project; WCA1, Water Conservation Area 1, also known as Loxahatchee National Wildlife Refuge; WCA2, Water Conservation Area 2; WCA3, Water Conservation area 3; ENP, Everglades National Park; C111, a drainage canal.

SITE MODELS:

All site models are modified Priestley-Taylor models of the following form:

Le is evapotranspiration expressed in latent heat (watts/meter*meter),
alpha is the Priestley_Taylor coefficient (dimenionless),
delta is the slope of the saturation vapor-pressure curve, in kilopascals per degree Kelvin,
gamma is the psychrometric constant, in kilopascals per degree Celsius, and
A is the measured available energy (sum of net solar radiation, soil heat flux, and water heat storage)

Models are developed using least-squared regression of measured Le and A data at 30-minute intervals, representing alpha as a function of water depth in feet (D) and incoming solar radiation in watts/meter*meter (Rs) as follows:

The regression coefficients C0, C1, C2, and C3 are given in the table below, together with the coefficient of determination (R^2), and the coefficient of variation (CV), the standard error or regression, in percent of mean Le). The coefficient C3 is in scientific notation, where "E" represents 10 raised to the power indicated by the following digits.

The site models listed here were used to calculate Et for intervals where input data did not pass all screening criteria.

PERFORMANCE OF REGIONAL MODELS GIVEN IN WRI 00-4217:

The regional models of alpha and A described in WRI 00-4217 were developed using data for 1996-97 from 9 sites. The regional model for estimating alpha is:

where C0 = 1.1263, C1 = 0.1156, C2 = -0.0271, C3 = -0.000821, and C4 = 3.95e-7 for vegetated sites.

The regional model for estimating available energy is:

where C0 = 5.171, C1 = 0.2165, and C2 = 0.0654 for vegetated sites, T2.5 is the mean air temperature for the previous 2.5 hours (Celsius) and Tair is the mean air temperature for the previous 15 minutes (Celsius).

Estimates of latent heat can be made if available energy measurements are available by using equation (2) to estimate alpha for use with equation (1). If no available energy data are available, the available energy can be estimated using equation (3). Using the estimated available energy and the estimated alpha in equation (1) provides a method of estimating latent heat from water depth, solar intensity, and air temperature data.

The models for vegetated sites were tested using the additional data for 1998 - 2003, both at sites used to develop the regional models and at other sites not used in model development. The comparison between measured and estimated et is given in the table below, where Et is the measured Et sum (inches), Rm1 is the estimated Et sum obtained from measured A and alpha estimated from equation (2), Rm2 is the estimated Et sum from alpha estimated from equation (2) and A estimated from equation (3), dif1 is the difference between measured Et and Rm1, dif2 is the difference between measured Et and Rm2, and Yrpct is the fraction (in percent) of the year with data. All Et sums are normalized to a full year of data. Thus for some years (for example 1999 at F4) Et data and data necessary for estimating Et were only measured for about 8 months (63.3 percent of the year), and the averages of the measured Et values were used to estimate the totals for the year.

ERRATA

The following errors are noted for printed copies of WRI 00-4217:

Page 18, Table 5: The coefficient C2 for Site 1 should be -0.000574.

Page 23, Equation 14: The equation should be: W = 709.5 S (T2.5 - Tair). This formulation is consistent with the formulation of the energy budget given on page 8, equation 1, in which an increase in water temperature during a computation interval represents an increase in the water-heat storage term (W), resulting in a lower amount of available energy than if water temperature remained constant.

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