Reference Evapotranspiration ET0 on a daily time step

The FAO-56 Penman-Monteith equation (Allen et al., 1998) estimates the
reference crop evapotranspiration, ET0 (mm d-1)

Monteith, J.L., 1965. Evaporation and Environment. 19th Symposia of the
Society for Experimental Biology, University Press, Cambridge,
19:205-234.
Reference Evapotranspiration by the Hargreaves method

When solar radiation data, relative humidity data and/or wind speed data
are missing, they should be estimated using the procedures presented in
this section. As an alternative, ETo can be estimated using the
Hargreaves ETo equation

Hargreaves, G.L., Hargreaves, G.H., and Riley, J.P. 1985. Agricultural
benefits for Senegal River Basin. J. Irrigation and Drainage Engr., ASCE
111:113-124.
Reference Evapotranspiration ET0 with the Penman-Monteith Equation

The FAO-56 Penman-Monteith equation (Allen et al., 1998) estimates the
reference crop evapotranspiration, ET0 (mm d-1)

Monteith, J.L., 1965. Evaporation and Environment. 19th Symposia of the
Society for Experimental Biology, University Press, Cambridge,
19:205-234.