Weather Station-based Potential Evapotranspiration Computation Model
and Database for Central and South Florida
Wossenu Abtew, J. Obeysekera, A. Reardon and N. Duerr
South Florida Water Management District, West Palm Beach, FL
Evapotranspiration is one of the major parameters in south Florida hydrology. As
a major component of the hydrologic cycle, there is a need for reasonably
accurate estimates of evaporation from water bodies and evapotranspiration from
vegetation. It is a parameter whose measurement or estimation had been a
challenge. Evapotranspiration depends on the availability of energy, on the
mechanism of mass transfer, energy transfer and the availability of water.
Evaporation and evapotranspiration are functions of solar radiation, temperature,
wind speed, vapor pressure deficit, atmospheric pressure, characteristics of the
surrounding environment and type and condition of vegetation. South Florida is
an area of high rainfall, high humidity and generally low wind speed. Air
temperature is high with relatively warm winter months. Solar radiation is
abundant with seasonal and daily variation. Most of the variation of daily ET is
explained by solar radiation.
There are 25 weather stations in the South Florida Water Management District
monitoring network. Solar radiation, humidity, wind speed and air temperature
sensors are common equipment in the weather stations. Some are equipped with
net radiometers to measure net solar radiation. At the South Florida Water
Management District, a computer program, ET_SF, was developed to directly
access a meteorological database holding fifteen minute interval field
observations and then compute potential ETp using each of the six methods with
varying complexity. The program can access data from all the weather stations in
the network and computes ETp for the respective site. Figure 1 depicts graphic
interface of the ETp computation program. Daily potential ETp data is stored in
the corporate hydrometeorologic database, DBHYDRO, which is accessible
through the Web. Areal estimates of ETp can be generated from the point
estimates at the weather stations.
Annual average air temperature is 23.1 oC with monthly average temperature
increasing from 17.9 oC in January to 27.4 oC in August. The average wind speed
is 3.1 m s-1. The annual average relative humidity is 80.7% with annual average
minimum and maximum values of 67% and 92%, respectively. The area has
significant sunshine with an annual average solar radiation flux rate of 0.1908 kw
m-2. Table 1 depicts the monthly average meteorological parameters as computed
from weather stations with varying lengths of record from 1988 to 2002. The
areal average annual ETp is 130.1 cm.
Figure 1. Graphic Interface of the ETp Computation Program, ET_SF.
Table 1. District-wide average monthly weather parameters.
Parameter
Jan Feb Mar
Apr
May June July Aug Sept Oct Nov Dec
Tmean (oC)
17.9 19.0 20.6 22.7 25.0 26.6 27.3 27.4 26.3 24.8 21.5 18.8
Tmax (oC)
23.2 23.3 24.8 25.9 27.3 28.5 29.2 29.2 28.0 27.5 25.2 23.3
Tmin (oC)
9.7 12.0 14.7 18.0 21.8 24.3 24.7 24.5 23.8 20.3 16.1 10.5
Rhmin (%)
62.8 63.9 60.9 58.3 65.2 71.6 73.1 74.3 74.3 67.1 67.0 65.9
Rhmax (%)
94.0 92.2 91.3 89.6 90.1 90.9 90.8 91.2 92.7 92.5 93.9 95.0
WS@10m (m/s) 3.22 3.42 3.66 3.56 3.18 2.73 2.53 2.53 2.69 3.18 3.31 3.11
Rs (MJ M-2 d-1) 11.87 14.62 18.14 20.44 21.40 19.43 19.15 18.58 16.06 14.69 12.34 10.91
ETp (mm/d)
2.57 3.16 3.93 4.42 4.63 4.20 4.14 4.02 3.47 3.18 2.67 2.36
Rainfall (cm)
5.59 5.99 7.45 6.55 11.84 19.94 17.73 17.86 18.36 11.99 5.84 4.83
A network of weather stations and an ETp computation software developed inhouse is currently being used to develop a daily ETp database at the South Florida
Water Management District. A simple ETp estimation model that was previously
calibrated with lysimeter data is being applied.
Citation:
Abtew, W., 1996, Evapotranspiration Measurements and Modeling for Three
Wetland Systems in South Florida. Journal of the American Water Resources
Association. 32 (3): 465-473.
Reardon, A. and Abtew, W., 2002, Evapotranspiration Estimation for South
Florida-Documentation for Program ET_SF. Environmental Monitoring and
Assessment Department. South Florida Water Management District, West Palm
Beach, FL.
Contact: Wossenu Abtew, South Florida Water Management District, 3301 Gun
Club Rd., West Palm Beach, FL, 33406, Phone: 561-682-6326, Fax: 561-6816265, Email: wabtew@sfwmd.gov, Oral, Hydrology and Hydrologic Modeling