Figure from Rodriguez et al. (2010).

Water balance

A useful source is Nadia Christina Sabeh’s PhD thesis, 2007.

\[E_V + E_C + E_S + E_T = 0\]

Energy balance

\[Q_R + Q_G + Q_V + Q_S + Q_P + Q_L + Q_H = 0\]

Evaporative Cooling

Potential Evapotranspiration

Penman (1948)

This equation accounts for two main terms:

  • the first is the available energy flux density,
  • the second term is the drying power of the air.
\[PE = \frac{\Delta(R_n-G) + \gamma\cdot 2.6 (1+0.536 u_2)\text{VPD}}{\lambda(\Delta+\gamma)}\]

More about this here.

PyET

Luckily, we don’t have to work very hard to estimate the potential evapotranspiration. We will use the Python library pyet:

Using PyET is quite straightforward:

pyet.pm_fao56(tmean,               # mean temperature
              wind,                # wind speed
              rn=rn,               # net radiation
              tmax=tmax,           # max temperature
              tmin=tmin,           # max temperature
              rh=rh,               # relative humidity
              elevation=elevation) # elevation above sea level