ABSTRACT
The partitioning of the precipitation at the soil surface into its components (infiltration, evapotranspiration, and runoff) during a period of time is called the water balance. This water exchange at the soil surface is conditioned by soil physical properties, vegetation characteristics, and the climate pattern shown by the dry and wet period distribution. Their respective influence on the processes involved is not independent, due to the close relationship between the physical characteristics of the soil surface and the vegetation cover distribution and conditions, which, in turn, are related to climatic and meteorological parameters. Since vapor fluxes return water to the atmosphere, water and energy balances are closely linked at the soil surface. In a similar fashion, the incoming energy at the soil surface, net radiation, is distributed into soil and air heat storage, and evaporation into the atmosphere. The resulting balance
equations can be written in terms of a physical description of all these water and energy fluxes, which add up to zero because the surfaces have no storage capacity. The balance equations at the soil surface are:
P ¼ Iþ Rþ E ð4:1Þ
Rng ¼ Gþ lEþH ð4:2Þ
where P, I, R, and E are precipitation, infiltration, runoff, and evaporation fluxes, respectively, in the water balance equation, Rng, G, E, and H are net radiation, soil heat flux, latent heat flux, and sensible heat flux, respectively, in the energy balance equation and l is the latent heat of vaporization for water.
