Select Page

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.