#### 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.