Studies of Energy and Gas Exchange within Crop Canopies (4)

Abstract

Biometrical data of the leaf area density and spatial distribution of the leaf area in corn crop fields were used to calculate penetration of the direct solar radiation into the canopy, the effective leaf area function, sunlit leaf area index and the intensity of the direct solar radiation falling on leaf surtace. The leaf orientation function was obtained with a silhouette method described in a previous paper (UDAGAWA et al., 1968).The relation proposed by Ross and NILSON (1965) was used to determine the penetration of the direct solar radiation from the data of the leaf orientation and the direction of the sun. For simplicity, the following relation was used to obtain the effective leaf area function:GL(w)=∑6j=1∑8k=1gL(w, θLj, φLk)|cosr0rL|jk, where gL(w, θLj, φLk) is the leaf normal distribution function, |cos r0rL|jk cosine of the angle between the leaf normal rL and the direction of the sun r0, and w the depth in the canopy. The mean extinction coefficient (kd) for attenuation of the direct solar radiation within the canopy was determined fromkd=cosech0GL, where GL is the mean effective leaf area function and h0 the sun altitude.The analysis indicates that the profile of the effective leaf area function changes with sun altitude. When the canopy was relatively sparse, the profile at the low sun altitude was found to be concave against z-axis, indicating that both the upper and lowest layers of the canopy were more penetrative compared with middle layers. On the other hand, the profile was convex when the sun altitude was higher than 45° This means that the direct solar radiation is strongly diminished in both the upper and lowest layers. When the sun altitude was between 30° and 40°, the profile became approximately constant and the value was about 0.5, implying that the canopy behaved to the direct solar radiation like a random orientation canopy in which the spatial distribution of leaves is non preferential as to both the inclination angle and azimuth angle. The diurnal change in the profile GL(w) fade gradually away with development of the canopy as can be seen in Fig. 2. The sun altitude relationship of the mean extinction coefficient kd was approximated bykd=(0.383+0.0035 h0) cosec h0 variety Ko-7, kd=(0.445+1.22×10-4h01.69) cosec h0 variety Ko-1.The difference in the sun altitude dependence of kd between two varieties of the crop seems to be due to difference in the orientation function of leaves (see Table 1 and Fig. 1).The mean extinction coefficient kd decreased sharply with sun altitude from about 2 for the sun altitude of 5-10°to about 0.6 for the sun altitude of 60°(see Fig. 4), while the value of GL increased with sun altitude. This is because of rapid decrease in the optical thickness of the canopy (cosec h0·Ft, where Ft is the leaf area index). Comparing these results it is known that the sun altitude dependence of the mean extinction coefficient for the corn canopy coincides well with that for a canopy of leaves with inclination angles of 40-50°.The leaf area exposed to the direct solar radiation was evaluated by making use of the data of penetration of the direct solar radiation and the leaf area density within the canopy. Percentage of the sunlit leaf area index to the leaf area index was found to be related to both the growing stage of crop and sun altitude. When the sun was higher the percentage decreased considerably