Potential evapotranspiration by Penman method

Potential evapotranspiration (PET): the amount of water evaporated and transpired per unit area per unit time from a short green grass cover (0.12 m height, well-watered, actively growing, completely shading the ground) with unlimited water supply.

Penman's combination method (steps)

Penman combines the energy-balance (radiation) and aerodynamic (wind) terms:

$$PET = \frac{\Delta H_n + \gamma E_a}{\Delta + \gamma}$$

where $PET$ in mm/day, $\Delta$ = slope of the saturation vapour pressure curve at air temperature (mm Hg/$^\circ$C), $\gamma$ = psychrometric constant (0.49 mm Hg/$^\circ$C), $H_n$ = net radiation (mm of evaporable water/day), $E_a$ = aerodynamic term (mm/day).

1. Mean air temperature $T$ and saturation vapour pressure $e_a$ (mm Hg) at $T$; get $\Delta$ from tables.
2. Actual vapour pressure $e_d = e_a \times RH/100$.
3. Net radiation:

$$H_n = H_a(1-r)\left(a+b\frac{n}{N}\right) - \sigma T_a^4(0.56-0.092\sqrt{e_d})\left(0.1+0.9\frac{n}{N}\right)$$

where $H_a$ = extraterrestrial radiation (table by latitude and month), $r$ = albedo (0.25 for grass), $n/N$ = ratio of actual to possible sunshine hours, $a = 0.18$, $b = 0.55$ (typical), $\sigma$ = Stefan-Boltzmann constant. 4. Aerodynamic term:

$$E_a = 0.35\left(1+\frac{u_2}{160}\right)(e_a - e_d)$$

with $u_2$ = wind speed at 2 m height (km/day). 5. Substitute into the Penman equation to get PET (mm/day). 6. Crop evapotranspiration: $ET_c = K_c\times PET$. 7. Multiply by the number of days of the month for monthly values.