Scientific Investigations Report 2006–5318
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006–5318
plantet_dpm.f
Calculates actual plant transpiration.
This module calculates the plant transpiration from available excess water, the saturated store, and the unsaturated store. Calculations are not completed for HRUs that have covers of water or impervious surfaces, are covered by snow, or the temperature is below freezing. Plant transpiration occurs first from excess water, then from the current root zone of the saturated store, and last from the current root zone of the unsaturated store. Transpiration can be calculated based on a soil-limiting value or based on soil texture relations using the ratio of actual to potential transpiration. The former is from modifications of Bauer and Mastin (1997) that used formulations in Giles and others (1984) and the latter is from Bauer and Vaccaro (1986) that used formulations in Leavesley and others (1983). Note that for agricultural crops, the crop coefficients used in the Jensen-Haise method account for soil evaporation, and thus the plant transipiration is representative of evapotranspiration.
July, 2004
slmfac
Soil limiting transpiration coefficient for a soil, in inches/day.
avlcap
Available water capacity for a soil association, no units-as decimal fraction. [soilms]
spcyld
Specific yield of soils for a soil association, no units. [soilms]
soil_type
Soil texture for a soil association, no units. [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]
iroot
Distribution of root mass in root zone: linear (=0) or exponential (=1), no units. [cropcof]
hru_petplnt
Potential plant transpiration, in inches.
hru_petcrop
Potential plant transpiration adjusted for crop growth, in inches.
hru_aetplnt
Actual plant transpiration, 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 capacity for a soil for a HRU, in inches. [soilms]
excess
Amount of water in excess of the saturated soil moisture store, in inches. [soilms]
coefs
Daily values of 6 parameters associated with a HRU’s crop-type, units vary. [cropcof]
tavf
Daily average temperature for each of the HRUs, in degrees. [grid]
hru_snow
Amount of snow-water-equivalent for a HRUs, in inches. [snowpck]
hru_adjustpet
Potential evapotranspiration for HRU adjusted as used, in inches. [intrcp]
hru_petloss
Amount of potential evapotranspiration loss that is not transferred to plant potential evapotranspiration, in inches. [transfr]
For each HRU, the variables hru_aetplnt and hru_petcrop are first set to 0.0; these variables are calculated in this module. The total available potential evapotranspiration (PET) for the plant (hru_petplnt) is set to the adjusted PET (hru_adjustpet), which represents the starting PET adjusted for use due to interception, sublimation, soil evaporation, and transference to plants. Next, calculations are skipped for a HRU if, 1) it has land covers of water or impervious surfaces (commercial, industrial, urban, or barren rock), 2) the temperature is below freezing, or 3) the HRU is snow covered.
Otherwise, after setting several local parameters, the crop PET is set as
hru_petcrop(i) = hru_petplnt(i) * cp
where
nhru is the number of HRUs,
i is the index for the HRU, from 1 to nhru,
cp is the crop coefficient for this day for this cover type for this HRU,
hru_petcrop is the PET adjusted for crop growth, in inches, and
hru_petplnt is the PET, in inches.
Note that for non-agricultural plants the crop coefficient (cp) is equivalent to the alfalfa coefficient in the Priestly-Taylor PET-AET method (Priestly and Taylor, 1972).
The local variable crpcur is set to hru_petcrop so that hru_petcrop retains its original value to later calculate changes in PET and crpcur in this module gets reduced due to use of PET. The total amount of water available to be transpired in the root zone is then calculated as the the sum of the current water in the saturated and unsatured stores and put into the local variable smtotroot.
The actual transpiration (AT) limited value is calculated based on the soil limiting transpiration factor (slm) as,
aetlmt = slf * smtotroot / (satmaxroot + unsatmaxroot)
where
aetlmt is the AT limited value, in inches,
slf (slmfac for this soil type) is the soil limiting transpiration coefficient for a soil, in inches/day,
smtotroot is the total current water in the root zone, in inches,
satmaxroot is the maximum amount of water can be be in saturated store in the root zone, in inches, and
unsatmaxroot is the maximum amount of water can be be in the unsaturated store in the root zone, in inches.
The module proceeds HRU by HRU and for each HRU, layer by layer in the current root zone for the land use/cover for the HRU. Extraction is first done from excess water, then from saturated water, and last from unsaturated water. Each of these extractions is based on the presence of water in each of these stores and the availability of PET.
For the case that excess water exists, the maximum pet (maxpet) is set to the maximum of aetlmt or crpcur, and plant transipiration (locally defined as trnspr) is set to the minimum of maxpet or excess. Excess, crpcur, and aetlmt are then reduced by trnspr.
Next, for the case of water in the saturated store and the slf is greater than 0 (that is: use AT limited value-aetlmt) then trnspr is set to the minimum of the limited value (aetlmt) or the current water in the saturated store for this layer as
trnspr = minimum (storpor(i,n), aetlmt)
where
nhru is the number of HRUs,
i is the index for the HRU, from 1 to nhru,
nlayer is the number of layers for the association for this HRU,
n is the index of layers, from 1 to nlayer, and
other variables as defined above.
Storcur, storpor, and aetlmt are then reduced by trnspr and trnspr is accumulated in hru_aetplnt (AT). If all the aetlmt is used, then end the rest of calculations, do the sink calculations for hru_petloss.
If slf is not greater than 0.0 (do not use AT limited value), then determine if if there is available PET (hru_petcrop=crpcur > 0.0). If there is, then trnspr is set to the minimum of the PET (crpcur) or the current water in the saturated store for this layer as
trnspr = minimum (storpor(i,n), crpcur)
where
nhru is the number of HRUs,
i is the index for the HRU, from 1 to nhru,
nlayer is the number of layers for the association for this HRU,
n is the index of layers, from 1 to nlayer, and
other variables as defined above.
Storcur, storpor, and crpcur are then reduced by trnspr and trnspr is accumulated in hru_aetplnt (AT). If all the PET (crpcur) is used, then end the rest of calculations, do the sink calculations for hru_petloss.
If there still is available PET and water in the unsaturated store, the unsaturated extraction proceeds under two possible methods. The first is that AT is limited by soil moisture according to Black and others (1984) or else AT is calculated based on soil texture and current mositure (which also can limited by soil moisture).
For the case of using the AT limited formulation for the unsaturated store, trnspr is set to the minimum of the limited value (aetlmt) or the current water in the unsaturated store for this layer as
trnspr = minimum (sms(i,n), aetlmt)
where
nhru is the number of HRUs,
i is the index for the HRU, from 1 to nhru,
nlayer is the number of layers for the association for this HRU,
n is the index of layers, from 1 to nlayer, and
other variables as defined above.
Unsatcur, sms, and aetlmt are then reduced by trnspr and trnspr is accumulated in hru_aetplnt (AT). The rest of calculations then skip to the sink calculations for hru_petloss.
For the case of using the soil-texture/soil moisture formulation for the unsaturated store, trnspr is based on three curves presented in Bauer and Vaccaro (1987) and Leavesley and others (1983). These curves reflect the ratio of AT/PET for different values of of the ratio of sms/smscapmax under the soil types of sand, loam, and clay (Saxton and others, 1974). In this formulation, based on the soil texture (soil_type that is set equal local variable st), either 1 or parts of 2 of the soil type extraction curves can be used. Extraction is based on the root density in the root zone. The root density can be linear or exponential based on the user input parameter iroot (defined above).
Unsatcur, sms, and crpcur are reduced by trnspr and trnspr is accumulated in hru_aetplnt (AT). The remaining calculations then skip to the sink calculations for hru_petloss.
Last, to preserve the calculated variables hru_adjustpet, sms, storpor, unsatcur, storcur, excess, and hru_petloss, a MMS function, putvar, is invoked to store these HRU values in the modules where they were declared.
Bauer, H.H., and Mastin, M.C., 1997, Recharge from precipitation in three small glacial-till mantled cathcments in the Puget Sound Lowland: 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.
Giles, D.G., Black, T.A., and Spittlehouse, D.L., 1984, Determination of growing season soil water deficits on a forested slope using water balance analysis: Canadian Journal of Forest Research, v. 15, p. 107-114.
Leavesley, G.H., Lichty, R.W., Troutman, B.M., and Saindon, L.G., 1983, Precipitation-runoff modeling system: User’s manual: U.S. Geological Survey Water-Resources Investigations Report 83-4238, 245 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.
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.
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
U.S.
Department of the Interior | U.S. Geological
Survey
Persistent URL: https://pubs.water.usgs.gov/sir20065318
Page Contact Information: Publications Team
Page Last Modified: Thursday, 01-Dec-2016 19:32:08 EST
