A surface energy‐balance computer

Abstract

AbstractA surface energy‐balance computer is described. This is fed with such information as dry‐ and wet‐bulb temperatures and temperature differences in the vertical (in the form of fine wire thermocouple outputs) and wind speed differences. The psychrometric equation is solved automatically to yield a vapour‐pressure difference and the latent‐heat term is then computed on the basis of an aerodynamic equation. The sensible‐heat term is similarly made available. The net flux of radiation and the soil‐heat flux are also recorded. A determination of each of these four fluxes is made once per minute and the values are integrated and presented on counters which are photographed at desired intervals, the smallest interval so far used being 20 min. Results indicate that for periods of an hour or more the ‘positive’ and ‘negative’ terms in the surface‐heat balance equation so obtained, agree to about 10 per cent except when the heat storage in the first two cm of soil, of which no account is taken, is obviously changing. The only complete 24 hr observational period so far covered indicates a daily balance to a fraction of a mw hr cm−2. Over approximately 80 hr observations, which were not continuous but contained about an equal number of daylight and dark hours, a balance to better than 1 per cent was obtained. A further result, which requires additional confirmation, is that the daily water loss from the centre of an extensive short‐grass area which has an ample water supply may be obtained for all practical purposes by measuring the total net radiation and assuming that all is used in the evaporative process. It becomes obvious that if hourly mean values of the profiles of temperature, humidity and wind are used to compute the evaporation then, in general, the results will be in error by up to about 20 per cent, although daily totals may be more accurate than this. The machine as it stands is not reliable for continuous operation owing to the use of a number of electro‐mechanical devices which are required to work under conditions for which they were not designed. Some improvements will be required before reliable continuous operation becomes possible.