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Module: potetjh

Description

The potential evapotranspiration for each HRU (potet) for each time period is computed by:

\(potet = jh\_coef * (tavgf − jh\_coef\_hru) * rin\)

where

  • \(jh\_coef\) = the monthly air temperature coefficient used in Jensen-Haise potential evapotranspiration computations,
  • \(jh\_coef\_hru\) = the air temperature coefficient used in Jensen-Haise potential evapotranspiration computations for each HRU,
  • \(rin\) = the daily solar radiation expressed in inches of evaporation potential, and
  • \(tavg\) = the average HRU temperature in °F

For aerodynamically rough crops, which are assumed to include forests, jh_coef can be computed each month for the watershed by:

\(jh\_coef=[C1+(13.0*CH)]^{-1}\)

C1 is an elevation correction factor computed by:

\(C1 = 68.0 − [3.6 * \frac {E1} {1000}]\)

where

  • \(E1\) = the median elevation of the watershed in feet.

CH is a humidity index computed by:

\(CH = 50.0 e2 − e1\)

where

  • \(e2\) = the saturation vapor pressure (mb) for the mean maximum air temperature for the warmest month of the year, and
  • \(e1\) = the saturation vapor pressure (mb) for the mean minimum air temperature for the warmest month of the year.

jh_coef_hru is computed for each HRU by:

\(jh\_coef\_hru = 27.5 − (0.25 * (e2−e1)) − \frac {E2} {1000} \)

where

  • \(E2\) = the median elevation of the HRU in feet.

The basin weighted average potential evapotranspiration, basin_potet, is also computed in this module.

References
  • Jensen, M.E. and H.R. Haise, 1963, Estimating evapotranspiration from solar radiation: Proceedings of the American Society of Civil Engineers, Journal of Irrigation and Drainage, v.89, no. IR4, p. 15-41.
  • Jensen, M.E., D.C.N. Rob, and C.E. Franzoy, 1969, Scheduling irrigations using climate-crop-soil data: National Conference on Water Resources Engineering of the American Society of Civil Engineers, New Orleans, LA, 1969, Proceedings, 20 p.
  • Leavesley, G.H., R.W. Lichty, B.M. Troutman, and L.G. Saindon, 1983, Precipitation-runoff modeling system–user’s manual: U. S. Geological Survey Water Resources Investigations report 83-4238, 207 p.