Scientific Investigations Report 2006–5318
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006–5318
potet_dpm.f
Calculate potential evapotranspiration; after Bauer and Vaccaro (1987) and Bauer and Mastin (1997).
Module to calculate potential evapotranspiration using either the Jensen-Haise (Jensen, 1974; Jensen and others, 1990) or Priestly-Taylor (Priestly and Taylor, 1972) method. Jensen-Haise incorporates coefficients, tx and ct for a HRU that are a function of long-term average July minimum and maximum temperatures (warmest month of the year) and altitude. Priestly-Taylor uses net radiation calculated in netrad_dpm.f module based on formulations from Bauer and Mastin (1997).
July, 2004
ct (calculated)
Jensen-Haise potential evapotranspiration equation coefficient, in 1/degrees.
tx (calculated)
Jensen-Haise potential evapotranspiration equation coefficient, in degrees.
petmin
Minimum daily potential evapotranspiration rate for each month, in inches.
hru_elev
Mean elevation for each HRU, in feet. [basin]
cov_type
HRU cover type: land use/cover type, from 1-31, no units. [basin]
hru_potet
Potential evapotranspiration for HRU, in inches.
hru_tmnjuly
HRU mean minimum July temperature used for potential evapotranspiration calculations, in degrees Fahrenheit. [grid]
hru_tmxjuly
HRU mean maximum July temperature used for potential evapotranspiration calculations, in degrees Fahrenheit. [grid]
hru_netrad
Daily net radiation for each of HRUs with cover types = 1, 2, 3, 10, 13, and 16, in cal/cm2/day. [netrad]
hru_solrad
Daily incoming solar radiation for each of the HRUs, in langleys. [grid]
tavf
Daily average temperature for each of the HRUs, in degrees Fahrenheit. [grid]
Potential evapotranspiration for a HRU is calculated using either the Priestly-Taylor method (Priestly and Taylor, 1972) for land use/cover of forest, grass, sage, water, bare soil, and impervious, or the Jensen-Haise method (Jensen, 1984; Jensen and others, 1990) for all other covers.
For the Priestly-Taylor method, the saturation vapor pressure at the average daily temperature is calculated as
satvp = e ( (16.78*tavc-116.9)/(tavc+273.3))
where
satvp is the saturation vaporation pressure at the average temperature, in kilopascals, and
tavc is the average daily temperature, in degrees Celsius.
The slope of satvp is then calculated as
slpvp = 4098. * satvp / (tavc+272.3)2
where
slpvp is the slope of the saturation vaporation pressure at the average temperature, in kilopascals per degree, and
tavc is the average daily temperature, in degrees Celsius.
Atmospheric pressure is calculated based on altitude,
prsr = 101.3 – 0.003215*alt
where
prsr is the atmospheric pressure, and
alt is the altitude of the HRU, in feet.
Calculations for the latent heat of vaporization and the pschometric constant are
hvap = 2501.0 – 2.361 * tavc
psycnst = 1.6286 * prsr/hvap
where
hvap is latent heat of vaporization, in kilojoules/kilogram,
psycnst is the pschometric constant for altitude=alt, in kilopascals, and
other variables as defined above.
The ratio used in the Priestly-Taylor method is
ratio = slpvp / (slpvp + psycnst)
where
variables as defined above.
Last, after conversions to appropriate units, equivalent evapotransipiration (eeq) is calculated as
eeq = ratio * slrnet / hvap
where
slrnet is the net solar radiation for the HRU (obtained from hru_netrad), and
other variables as defined above.
Potential evapotranspiration (hru_potet) is then set equal too eeq after conversion from centimeters to inches of water.
The Jensen-Haise method is used for all other land use/cover types and it written as
hru_potet(i) = ct(i) * (tav – tx(i)) * hru_solrad(i) * 0.000673
where
nhru is the number of HRUs,
i is the index for the HRU, from 1 to nhru,
hru_solrad is the daily incoming solar radiation, in langleys per day,
0.000673 converts langleys per day (cal/cm2/day) to equivalent depth by dividing by the latent heat of vaporization and assuming a constant temperature of 68 degrees Fahrenheit, and
other variables as defined above.
REFERENCES
Bauer, H.H., and Mastin, M.C., 1997, Recharge from precipitation in three small glacial-till mantled catchments in the Puget Sound Lowlands: U. S. Geological Survey Water-Resources Investigations Report 96-4219, 119 p.
Bauer, H.H., and Vaccaro, J.J., 1987, Documentation of a deep percolation model for estimating ground-water recharge: U. S. Geological Survey Open-File Report 86-536, 180 p.
Jensen, M.E., ed., 1974, Consumptive use of water and irrigation water requirements: New York, American Society of Civil Engineers, Irrigation and Drainage Division, 215 p.
Jensen, M.E., Burman, R.D., and Allen, R.G., eds., 1990, Evapotranspiration and irrigation water requirements: A.S.C.E. Manuals and Reports on Engineering Practice No. 70, 332 p.
Priestly, C.H.B., and Taylor, R.J., 1972, On the assesment of surface heat flux and evaporation using large scale parameters: Monthly Weather Review, v. 100, p. 81-92.
Henry H. Bauer and John J. Vaccaro
U.S. Geological Survey
Washington Water Science Center
934 Broadway, Suite 300
Tacoma, WA 98402
Modified by:
John J. Vaccaro
U.S. Geological Survey
Washington Water Science Center
934 Broadway, Suite 300
Tacoma, WA 98402
Telephone: 253-552-1620
Fax: 253-552-1581
Email: jvaccaro@usgs.gov
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