Functions to calculate potential evapotranspiration using Penman or Penman-Monteith.

penman(latrad, elevation, slorad, asprad, J,
            Tmin, Tmax, RHmin, RHmax, R_s,  u,
            z=2.0, z0 = 0.001, alpha = 0.08, windfun="1956")
penmanmonteith(rc, elevation, Tmin, Tmax, RHmin, RHmax,
               Rn, u = NA)

Arguments

latrad

Latitude in radians.

elevation

Elevation (in m).

slorad

Slope (in radians).

asprad

Aspect (in radians from North).

J

Julian day, number of days since January 1, 4713 BCE at noon UTC.

Tmax

Maximum temperature (degrees Celsius).

Tmin

Minimum temperature (degrees Celsius).

RHmin

Minimum relative humidity (percent).

RHmax

Maximum relative humidity (percent).

R_s

Solar radiation (MJ/m2).

u

With wind speed (m/s).

z

Wind measuring height (m).

z0

Roughness height (m).

alpha

Albedo.

windfun

Wind speed function version, either "1948" or "1956".

rc

Canopy vapour flux (stomatal) resistance (s·m-1).

Rn

Daily net radiation (MJ·m-2·day-1).

Details

The code was adapted from package `Evapotranspiration', which follows McMahon et al. (2013). If wind speed is not available, an alternative formulation for potential evapotranspiration is used as an approximation (Valiantzas 2006)

References

Penman, H. L. 1948. Natural evaporation from open water, bare soil and grass. Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences, 193, 120-145.

Penman, H. L. 1956. Evaporation: An introductory survey. Netherlands Journal of Agricultural Science, 4, 9-29.

McMahon, T.A., Peel, M.C., Lowe, L., Srikanthan, R., McVicar, T.R. 2013. Estimating actual, potential, reference crop and pan evaporation using standard meteorological data: a pragmatic synthesis. Hydrology and Earth System Sciences 17, 1331–1363. doi:10.5194/hess-17-1331-2013.

Value

Potential evapotranspiration (in mm of water).

Author

Miquel De Cáceres Ainsa, CREAF