Temperature Environment in the Air Layer over Azuki Bean and Pasture Fields

Abstract

The growth and development of crops depend heavily on the temperature environment surrounding them. Temperature environment in the surface air layer above a crop field is substantially affected by aerodynamical properties and heat balance characteristics of underlying surfaces as well as macro weather conditions. To investigate the influence of crop canopy with different canopy structure on the temperature regime of the surface layer, micrometeorological observations were made simultaneously at azuki bean and pasture fields (Fig. 1). The data obtained from field observations were analyzed on the basis of heat balance and surface air layer theories. The results can be summarized as follows:1. As can be seen in Fig. 4, the value of zero-plane displacement (d) of both fields decreased with increasing wind velocity, while the value of roughness length (z0) was an increasing function of wind velocity. The values of both z0 and d were found to be larger for the pasture field than those for the azuki bean field, respectively. The difference in the value of z0 and d between the two fields seems to be related with the difference in LAI between them (the values of LAI were 1.0 and 7.0 for azuki bean and pasture fields, respectively). The dependence of aerodynamical properties of the fields on wind was well expressed by Eqs. (5) and (6).2. Exchange velocity, which characterizes the exchange intensity of an entity in the air layer between Z1 and Z3, was calculated from the heat balance data obtained at two fields. It was found that the values of exchange velocity (D1-3) above the azuki bean field were somewhat smaller than those above the pasture field in a range of wind over 1.2m/sec, whereas the reverse was in the case in a range of wind below 1.2m/sec. The reverse of wind-exchange velocity relationship, as observed in a range of wind below 1.2m/sec, was assumed to be due to buoyant effect of air which becomes more intensive under low wind and high radiation conditions.3. Over the azuki bean field with sparse canopy, less heat was consumed for transpiration with proportionate increase of sensible heat loss, implying that thermal convection was more intensive over the azuki bean field than over the pasture field. The exchange velocity in the air layer over both fields was well expressed as a linear function of stability ratio, as shown in Fig. 6. The soil heat flux at the azuki bean field was somewhat larger than that at the pasture field (Fig. 7). This is presumably because more radiation can penetrate the canopy of azuki bean. The values of Bowen ratio were 0.35-0.45 and 0.2-0.3 at the azuki bean and pasture fields, respectively.4. Assuming that the value of air temperature measured at a reference height (Hp=about 20m) above the two fields is approximately the same, the following relation as to the difference in surface air temperature between them is given as:θA-θ′A=1/Cpρ(RN-lE-G/DA-Hp-R′N-lE′-G′/D′A-Hp)The assumption adopted to obtain the above relation was realized during daylight hours except extremely unstable air condition. Even though thermal stratification in the air was nearly neutral there was somewhat large difference in air temperature at the reference height between two fields at night. Under the conditions that the value of air temperature at the reference height was nearly the same between two fields, the values of temperature difference (θ1-θ′1) calculated from Eq. (3) fairly well agreed with observed ones (Fig. 7). The above relation indicates evidently that the difference in surface air temperature between two crop fields is determined in terms of {Q/D-Q′/D′}.5. In or