Reference Evapotranspiration ET0 with 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 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.
Reference Evapotranspiration ET0 with Priestley Taylor method

The Priestley-Taylor method (Priestley and Taylor, 1972) for the
calculation of daily ET0 (mm d-1) replaces the aerodynamic term of
Penman-Monteith equation by a dimensionless empirical multiplier (a,
Priestley-Taylor coefficient)