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scikits.hydroclimpy.lib.evaporation.PotentialEvapoTranspiration.PenmanMonteith

PotentialEvapoTranspiration.PenmanMonteith(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.

The Penman-Monteith equation is

\lambda_H ET = \cfrac{\Delta (R_n-G) + \rho_a c_p (e_s-e_a)/r_a}{
                      \Delta + \gamma \left(1 + \frac{r_s}{r_a}\right)}

where \lambda is the R_n = R_{ns}-R_{nl} the net radiation as the difference between the incoming shortwave radiation R_{ns} and incoming longwave radiation R_{nl}; G the soil flux density; \rho_a the mean air density at constant pressure; c_p the specif heat of air; e_s-e_a the vapor pressure deficit of air; r_s and r_a the bulk surface and aerodynamic resistances.

The net incoming shortwave radiation R_{ns} is calculated from the incoming solar radiations R_s as R_{ns} = (1-\alpha) R_s where \alpha is the albedo.

The aerodynamic resistance is calculated assuming a standardized measurement height for wind and humidity of 2m:

r_a = \cfrac{\ln{\left[\cfrac{2-2/3h}{0.123h}\right]} \ln{\left[\cfrac{2-2/3h}{0.0123h}\right]}}{(0.41)^2 u_2}.

where h is the crop height. For small crops (clipped grass), a value h=0.12 is assumed, with a bulk surface resistance r_s=70 [m.s-1]. For tall crops (alfalfa), a value h=0.5 is assumed, with a bulk surface resistance r_s=45 [m.s-1].

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, optional

Regression constant.

b_s : float, optional

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].