Scientific Investigations Report 2006–5318
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006–5318
soilevap_dpm.f
Calculates evaporation from bare soil after Bauer and Vaccaro (1987) and Bauer and Mastin (1997).
Actual soil evaporation is based on a relationship that is assumed to hold for all soils. The relationship uses the current percent volume of soil moisture in uppermost foot and remaining potential evapotranspiration (PET) that is left after evaporation of intercepted moisture (Saxton and other, 1984). PET is proportional to the amount of ground not shaded. Soil evaporation is only calculated for the non-agricultural land uses/covers. The module is based on Bauer and Vaccaro (1987) and Bauer and Mastin (1997).
July, 2004
avlcap
Available water capacity for a soil association, no units, as decimal percent. [soilms]
nlayer
Number of 6" layers for a soil association, no units. [soilms]
hru_soil
HRU soil type, no units. [basin]
cov_type
HRU cover type: land use/cover type, from 1-31, no units. [basin]
hru_pevsoil
Potential evaporation demand for soil evaporation, in inches.
hru_actevsoil
Actual evaporation from upper 1 foot of the soil column, in inches.
sms
Soil moisture in unsaturated soil moisture store, by layer, in inches. [soilms]
storpor
Soil moisture in saturated soil moisture store, by layer, in inches. [soilms]
unsatcur
Current available water capacity for a soil for a HRU, in inches. [soilms]
storcur
Current saturated water for a soil for a HRU, in inches. [soilms]
excess
Amount of water in excess of complete saturation of soils, in inches. [soilms]
hru_adjustpet
Potential evapotranspiration for HRU adjusted as used, in inches. [intrcp]
coefs
Daily values of 6 parameters for a crop-type, units vary. [cropcof]
hru_snow
Amount of snow-water-equivalent for a HRU, in inches. [snowpck]
The land use/cover for each HRU (cov_type) is checked for the type of cover. For water HRUs, the Priestly-Taylor (Priestly and Taylor, 1972) potential evapotranspiration (PET) is used as the actual evaporation (AEV),
hru_actevsoil(i) = 1.26 * hru_adjuspet(i)
hru_pevsoil(i) = hru_actevsoil(i),
where
nhru is the number of HRUs,
i is the index for the HRU, from 1 to nhru,
hru_actevsoil is the AEV from the water body, in inches,
hru_adjustpet is the available PET after being reduced due to the use of AET, in inches, and
hru_pevsoil is the potential soil evaporation, in inches.
If there is snowpack (hru_snow), then all calculations are skipped for this HRU and hru_pevsoil is set equal to hru_actevsoil=0.0. For the case of impervious covers, Priestly-Taylor PET is set to PE, and if excess water exists, AEV is set to potential evaporation (PE) and excess is reduced by AEV; the module then skips to the next HRU.
Soil evaporation is then calculated for all the non-agricultural land uses/covers because the PET/AET formulations for agricultural lands are for total water use that includes AEV. Thus, actual soil evaporation (AEV) is only calculated for forests, sage, and bare soils. The formulations used for native grasslands are based on agricultural lands and thus grasslands also are not included.
If there is saturated water in the upper 2 layers (1 foot), then for the non-agricultural lands, the soil type (hru_soil) is found; otherwise the saturated-store evaporation is skipped and the unsaturated (field capacity) store is analyzed. Based on the soil type, the number of 6-inch layers (nlayer), the available water-capacity (avlcap), and the total water stored in the saturated store in the upper 1-foot (storpor(HRU,1) + storpor(HRU,2) = sattot) are determined. The evaporation from the saturated store is then found as,
actevsat = minimum of hru_ pevsoil(i) or sattot
where
nhru is the number of HRUs,
i is the index for the HRU, from 1 to nhru,
actevsat is the AEV for upper 1 foot of the soil column from the saturated store, in inches, and
other variables as defined previously.
The water currently stored in each of the upper 2 layers of the saturated store (storpor) is then reduced by actevsat, and the total amount of water stored in the saturated store (storcur) is also reduced by actevsat.
If there is remaining PE and there is water above wilting point in the upper 2 layers (1 foot), then for the non-agricultural lands calculate AEV based on Saxton and others (1974); see also Bauer and Vaccaro (1987) for a graphical representation of the relations used. The calculations are based on the ratio of the total water stored in the field capacity store for the upper 1-foot to the total available water capacity for the same 1-foot of soils. If the ratio is greater than 0.25 (25 percent) one relation is used and if the ratio is greater that 0.40 (40 percent) then the AEV is set to the PE. For ratios less than 0.25, the water is assumed to be tightly enough bonded such that the evaporative demand cannot evaporate it (Saxton and others, 1974). Thus the AEV from the field capacity store is then found as,
actevcap = ratio * (hru_ pevsoil(i) - actevsat - actexcs)
where
nhru is the number of HRUs,
i is the index for the HRU, from 1 to nhru,
actevcap is the AEV for upper 1 foot of the soil column from the field capacity store, in inches,
actevsat is the AEV for upper 1 foot of the soil column from the saturated store, in inches,
actexcs is the AEV of the water in excess of saturated conditions, in inches, and
other variables as defined previously.
The water currently stored in each of the upper 2 layers of the field capacity store (sms) is then reduced by actevsat, and the current total amount of water stored in the field capacity store (unsatcur) is reduced by actevcap.
The total amount of water evaporated is calculated as,
hru_actevsoil(i) = actevcap + actevsat + actexcs
and the PET is reduced by the amount of energy used for evaporation,
hru_adjustpet(i) = hru_adjustpet(i) - hru_actevsoil(i)
where
nhru is the number of HRUs,
i is the index for the HRU, from 1 to nhru, and
other variables as previously defined.
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.
Saxton, K.E., Johnson, H.P., and Show, R.H., 1974, Modeling evapotranspiration and soil moisture: Transactions of the American Society of Agricultural Engineers, v. 17, No. 4, p. 673-677.
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
John J. Vaccaro and Henry H. Bauer
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
U.S.
Department of the Interior | U.S. Geological
Survey
Persistent URL: https://pubs.water.usgs.gov/sir20065318
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