NIGHTTIME EXCHANGE PROCESSES NEAR THE SOIL SURFACE OF A MAIZE CANOPY

The exchange process in the lower region of a maize canopy is analyzed for two nights, It appears that during calm nights a free convection state develops in the lower region of the canopy. Convective heat is r eleased at the soil's surface and transported directly to the higher p ortion of the canopy. The released sensible heat at the soil's surface can be easily calculated by applying the Nusselt number for free conv ection. At night thermal energy is also released through cooling of th e canopy. The released heat from the stored canopy heat is of the same order of magnitude as all other energy terms. Conversely during dayti me for most agricultural crops this energy term is of minor importance in comparison to the other energy terms. The formation of dew at nigh t is an important process. A maximal possible estimate of dew accumula tion for a particular night can easily be made by using the potential dew. The potential dew can be deduced more accurately by taking into a ccount the released heat stored in the plant canopy.