Evapotranspiration
- interception_mm(P_24, vc, lai, int_max=0.2)
Computes the daily interception. The daily interception of a vegetated area is calculated according to Braden1 and Hoyningen-Huene2.
\[I^*=I_{max} \cdot I_{lai} \cdot \left(1-\frac{1}{1+\frac{c_{veg} \cdot P} {I_{max} \cdot I_{lai}}}\right)\]- Parameters
P_24 (float) – daily rainfall, \(P\) [mm day \(^{-1}\)]
vc (float) – vegetation cover, \(c_{veg}\) [-]
lai (float) – leaf area index, \(I_{lai}\) [-]
int_max (float) – maximum interception per leaf, \(I_{max}\) [mm day \(^{-1}\)]
- Returns
int_mm – interception, \(I^*\) [mm day \(^{-1}\)]
- Return type
float
- et_reference(rn_24_grass, ad_24, psy_24, vpd_24, ssvp_24, u_24)
Computes the reference evapotranspiration.
The reference evapotranspiration \(ET_{ref}\) is an important concept in irrigation science. The reference evapotranspiration can be inferred from routine meteorological measurements.
The reference evapotranspiration is the evapotranspiration of grass under well watered conditions. First the aerodynamical resistance for grass \(r_{a,grass}\) [sm \(^{-1}\)] is calculated:
\[r_{a,grass}=\frac{208}{u_{obs}}\]Then the reference evapotranspiration \(ET_{ref}\) [W m \(^{-2}\)] can be calculated as follows, with taking the default value for the grass surface resistance \(r_{grass}\) = 70 sm \(^{-1}\)
\[ET_{ref}=\frac{\Delta \cdot Q_{grass}^{*}+ \rho c_{p}\frac{\Delta_{e}}{r_{a,grass}}} {\Delta+\gamma \cdot \left(1+\frac{r_{grass}}{r_{a,grass}}\right)}\]The soil heat flux is assumed to be zero or close to zero on a daily basis.
- Parameters
rn_24_grass (float) – net radiation for reference grass surface, \(Q^{*}_{grass}\) [Wm-2]
u_24 (float) – daily wind speed at observation height, \(u_{obs}\) [m/s]
ad_24 (float) – daily air density, \(\rho_{24}\) [kg m-3]
psy_24 (float) – daily psychrometric constant, \(\gamma_{24}\) [mbar K-1]
vpd_24 (float) – daily vapour pressure deficit, \(\Delta_{e,24}\) [mbar]
ssvp_24 (float) – daily slope of saturated vapour pressure curve, \(\Delta_{24}\) [mbar K-1]
- Returns
et_ref_24 – reference evapotranspiration (well watered grass) energy equivalent, \(ET_{ref}\) [W m-2]
- Return type
float
- et_reference_mm(et_ref_24, lh_24)
Computes the reference evapotranspiration.
\[ET_{ref}=ET_{ref} \cdot d_{sec} \cdot \lambda_{24}\]where:
\(d_{sec}\) seconds in the day = 86400 [s]
- Parameters
et_ref_24 (float) – daily reference evapotranspiration energy equivalent, \(ET_{ref}\) [W m-2]
lh_24 (float) – daily latent heat of evaporation, \(\lambda_{24}\) [J/kg]
- Returns
et_ref_24_mm – reference evapotranspiration (well watered grass), \(ET_{ref}\) [mm d-1]
- Return type
float
- et_actual_mm(e_24_mm, t_24_mm)
Computes the actual evapotranspiration based on the separate calculations of evaporation and transpiration.
\[ET = E + T\]- Parameters
e_24_mm (float) – daily evaporation in mm, \(E\) [mm d-1]
t_24_mm (float) – daily transpiration in mm, \(T\) [mm d-1]
- Returns
et_24_mm – daily evapotranspiration in mm, \(ET\) [mm d-1]
- Return type
float
- 1
H Braden. Ein energiehaushalts-und verdunstungsmodell for wasser und stoffhaushaltsuntersuchungen landwirtschaftlich genutzer einzugsgebiete. Mittelungen Deutsche Bodenkundliche Geselschaft, 42(S):294–299, 1985.
- 2
JV Hoyningen-Huene. Interception of precipitation in agricultural crops. Verlag Paul Parey, Hamburg, Berlin, Germany, DVWK-Schrift, 1983.