Image processing/Evapotranspiration
The radiation is coming from the planet's star, Earth has around 1800 W/m2 of incoming radiation from the sun (also called irradiance) at exo-atmospheric altitude.
Atmospheric transfer of irradiance is subject to atmosphere particles interactions, scattering, diffraction, reflection, etc.
When reaching the planetary surface, is it partially reflected in the shortwave by Albedo. Surface Albedo is the integrated 0.3-3 micro-meters reflectance, it reflects the shortwave energy coming from the sun by 5% on oceans, 15-25% on vegetation, 35-40% on sand/desert/beach, 60-80% on snow/clouds, roughly. This energy fraction is returned to atmosphere for complex interaction with gas particles again before leaving the atmosphere altogether.
As for the shortwave surface balance, the energy is also received in the longwave, but interacts with the grey-body characteristics of the surface elements. The blackbody to greybody fraction is ruled by emissivity, a hidden component in the thermal spectrum (search for Temperature- Emissivity Separation algorithms). On Earth, emissivity is always above 0.9 (mostly 0.96-0.98). The Stefan-Boltzman equation is dealing with blackbody energy, and multiplying it by Emissivity transforms it to greybody energy emitted.
Together, shortwave and longwave energy balance provide with the net radiation balance at the surface of the planetary body. This crucial term is the total energy available for thermodynamic fluxes to act on the surface of the planet. On a planet like Earth where there is a triple phase of a given molecule (H20, in gas, liquid, solid), energy available is used to transfer phases from lower energy to higher energy (sublimation, liquefaction, evaporation). Thermal transfers also happen as conduction (soil, rocks) and convection (atmosphere, oceans). The i.eb.* modules and models deal with conduction (i.eb.g0, for thermal conduction in soils), convection (i.eb.h_* especially, for thermal convection in atmosphere) and by residual method estimate the energy left for evaporation processes (i.eb.evapf, i.eb.eta are examples).
i.evapo.* modules/models are integrated models, from the oldest (Penman-Monteith i.evapo.pm, Priestley-Taylor i.evapo.pt, Hargreaves i.evapo.mh, etc) that are not based on energy balance, but are computing some form of evapotranspiration (ETo: Evapotranspiration for a reference of 20cm well-watered grass, see FAO no56 report of Richard Allen) to newer that area in some form or another linked to thermal processes (i.evapo.senay: uses a thermal index), biome thermal processes (i.evapo.zk: global model based on typical info from biomes Albedo/soil heat flux) or more thermodynamic processes (i.evapo.potrad: potential ET if not water stress, based on astronomical equations only). Most developed thermodynamic models are TSEB (i.evapo.tseb: Two Source Energy Balance, Schmugge, Kustas, etc., 2000), SEBS (i.evapo.sebs: Zhu et al, 2002), SEBAL (i.eb.h_SEBAL95, i.eb.g0, i.eb.evapfr, i.eb.eta: Bastiaanssen et al, 1998), and some newer models like RESET and others. The list of models using thermodynamic principles is large now, and any willing to submit a code for any of them is most welcome.