ABSTRACT

Hydrological processes depend not only on the water budget but also on the surface energy budget, which affects, in particular, the transfer ofwater back to the atmosphere as evaporation and transpiration and also the way in which precipitation as snow builds up into a snow pack and later melts. Evapotranspiration requires energy to change the phase state of water from liquid to vapour; snowmelt requires energy to change the phase state of water from ice to liquid (or sometimes directly to vapour, a process called sublimation). In both cases, the amount of energy required to change the phase of water is known (these are the latent heat of melting and latent heat of vaporisation). The difficulty is in estimating how much energy will be available from the energy budget in different circumstances. The energy budget involves several variables that are difficult to estimate by measurement, particularly at the catchment scale.