A set of semi-continuous measurements of temperature, wind and moisture gradients as well as of net radiation and ground heat flux covering a period of about one and a half years has been analysed to give a corresponding set of complete surface energy balance data on an hourly basis. An analysis of the evaporation data so obtained is given.
It is shown that surface resistance r, exhibits a diurnal trend: values are smallest (ca. 150 s m-') a few hours before noon and increase to as much as 800 s m-' towards dusk. The minimum values tend to be higher during dry periods when the soil moisture is low. There is also some indication that r, decreases rapidly soon after rainfall.
An exponential relation is found between the fraction of available energy used as evaporative flux, a, and r, for values of rI/rs i 0.70, where rr is the climatological resistance. On the other hand, the ratio of r, to r, is linearly correlated with LY, implying that an equilibrium state is established between the grass surface and the atmosphere, at least from mid-morning to mid-afternoon when the leaves are dry. Near-noon values calculated by Stewart and Thorn for Thetford Forest also follow a linear trend.
The above two regression results (In (r,) versus a ; rl/rs versus a) are combined to obtain an empirical relation of the form rr = ma exp (a -6n) which is used to estimate evaporative flux. The estimates are found to be within 20% of calculated values.
List of Symbols
A B-l d e es e, G H k L P 4 ra rv b bv available energy = R -G. excess non-dimensional resistance (bulk parameter). zero-plane displacement. vapour pressure of the air at z = zx. mean vapour pressure on surface of vegetation. saturation vapour pressure. ground heat flux. sensible heat flux at z = zR. van Karman's constant; taken as 0.35. Monin-Obukhov stability length. atmospheric pressure. specific humidity. external or aerodynamic resistance between the crop surface and air. aerodynamic resistance to transpiration flux from vegetation to reference level zx. aerodynamic resistance to momentum transfer to the vegetation cover from the level zx. excess aerodynamic resistance experienced at the surface by water vapour over that encountered there by momentum. climatological (isothermal) resistance = pC,(ew(T)-e)/yA. surface resistance. bulk physiological (stomata]) resistance of the vegetation.