| Popis: |
The quantity of water needed to protect buds from frosts of specified severity can be estimated by setting up a heat balance equation for a single bud. To prevent bud temperature falling below the level that would cause damage to the tissue (which depends on development stage), heat is added in the form of latent heat of fusion as water turns to ice, at a rate of ) λf = 3.35 x 105 J kg-1. When water is applied, cooling by evaporation or heating by condensation will also occur, and a term for the latent heat (i.e. a loss for evaporation or a gain for condensation) must be included. The steady state heat balance equation for the protected bud, in terms of mean energy fluxes (Wm-2) per unit surface area, is thus: Hf = λfF=Rp+Hp+λeE, (42) where Hf is the rate of supply of heat of fusion, Rp is the net loss of heat by radiation and Hp is the loss of sensible heat by convection. F and E the rates of fusion and evaporation (or condensation) (kgm -2 S -1) whilst the latent heat values of fusion and evaporation (or condensation) are denoted by λf and λe, respectively (J kg -1). Equation (42) assumes that the loss of heat by conduction is small and the rate of change of stored heat is negligible compared to the above energy fluxes averaged over periods of 30 min (Hamer 1984a). |
