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scikits.hydroclimpy.lib.evaporation.PotentialEvapoTranspiration.PenmanMonteithASCE

PotentialEvapoTranspiration.PenmanMonteithASCE(Rs=None, sunshinehours=None, a_s=0.25, b_s=0.5, Kt=None, tdew=None, RHmin=None, RHmax=None, RHmean=None, crop_ref='short', u2=2, albedo=0.23000000000000001)

Returns the potential evapotranspiration estimated by the Penman-Monteith equation, using the approximation of the ASCE [Allen_etal_1998]._

\lambda_H ET_o = \cfrac{\Delta (R_n-G) + \gamma \cfrac{C_n}{T_{avg}+273} u_2 (e_s^* - e_a)}{
                    \Delta + \gamma \left(1 + C_d u_2\right)}

where u_2 is the wind speed measured at an altitude of 2m [m.s-1], C_p and C_d two constants depending on the reference crop and the time interval: for daily to monthly time steps, C_n=900 and C_d=0.34 for clipped grass, and C_n=1600 and C_d=0.38 for alfalfa.

The values of these coefficients were determined by assuming a bulk resistance of 70 [s.m-1] for clipped grass of an height of 0.12 m, and of 45 [s.m-1] for alfalfa at a height of 0.50~m. A constant albedo \alpha \approx 0.23 is also assumed.

Parameters:

Rs : TimeSeries, optional

Incoming solar radiations [MJ.m-2.d-1]. If None, Rs is estimated from extraterrestrial solar radiations (Ra) and minimum and maximum temperatures as K_{t} \sqrt{\Delta\;T} Ra.

sunshineshours : TimeSeries, optional

Series of actual sunshine hours.

a_s : float

Regression constant.

b_s : float

Regression constant.

Kt : {None, float}, optional

Regression constant used to approximate solar radiations from temperatures. The default (K_t=0.171 corresponds to the standard value suggested by Hargreaves and Samani [Hargreaves_Samani_1985]. If None, K_t is evaluated from the temperatures range \Delta T as K_t = 0.00185 {\Delta T}^2 - 0.0433 {\Delta T} + 0.4023. [REF_TO_FIND_19xx]

crop_ref : {‘short’,’tall’}, optional

String giving the kind of reference crop. Use 'short' for clipped grass (default), 'tall' for alfalfa.

z : float, optional

Local elevation above the sea level [m].

u2 : float, optional

Mean wind speed at an elevation of 2m [m.s-1]

albedo : float, optional

Albedo. By default, a value of 0.23 is assumed for grass/alfalfa references.

RHmin : TimeSeries, optional

Minimum relative humidity [0-1].

RHmax : TimeSeries, optional

Maximum relative humidity [0-1].

RHmean : TimeSeries, optional

Mean relative humidity [0-1].