Measurement and estimation of total radiation absorbed by a hedgerow in a coffee crop

All-wave net radiation absorbed by a canopy is an important variable influencing crop transpiration and photosynthesis. In discontinuous canopies, like orchards, vineyards and coffee plantations, the knowledge of total radiation absorbed by single trees or by the canopy of a hedgerow is necessary to estimate the plant transpiration as a component of the total crop water loss. However, few attempts have been done to measure net radiation directly in these conditions. Thorpe (1978), as example, measured the net exchange of radiation intercepted by the foliage of apple trees in a hedgerow orchard using eight linear net radiometers fixed with their long axes parallel to the rows and arranged around a row forming an imaginary cylindrical surface. McNaughton et al. (1992) computed the net radiation absorbed by a single tree using eight net radiometers attached to a frame that rotates around the tree, in a spherical geometry of measurement allowing the integration of the measurements in time and space.

In this paper we describe a technique for measurements of all-wave radiation absorbed by a hedgerow in a small coffee plantation (0.25 ha). The rows of coffee plants are oriented SSE-NNW, 2.5 m apart and with spacing between trees of 1.0 m, forming compact hedgerows of 2.5 m high and 1.6 m wide. The experiment was carried out in Piracicaba (22o42’ S, 47o30' W, 546 m altitude), São Paulo State, Brazil, from August to October 2002. The methodology was based on Thorpe (1978), with the stationary linear radiometers being replaced by eight punctual net radiometers model Q7.1 (Radiation and Energy Balance Systems, Seattle, WA) mounted at equi-latitudinal intervals of 45o on a vertical circular aluminum-built frame encompassing the canopy. The frame had a continuous and reversible movement of 4.28 m within the row, covering four plants, allowing a temporal and spatial integration of the measurements of all sensors. Each complete run of the system over the rail took 40 s. The arrangement of the sensors and the movement of the frame along the hedgerow formed a notional horizontal cylinder, being the integrated values calculated as proposed by Thorpe (1978). The signals of each radiometer were taken every 1 s and mean values were recorded every 15 min interval. In this study we discuss the mechanical performance of the device tested during several days. We also show diurnal course of net radiation absorbed by the hedgerow canopy in days with different weather (sky) conditions.

Data computed in different time-scales (15 min, 30 min, daytime, and 24 hr) were correlated to net radiation over turfgrass (Rng) and solar radiation (Rs), both measured in a standard weather station located at about 250 m from the experimental area. The results showed the possibility to estimate total radiation absorbed by the hedgerow canopy from Rng and Rs at different time-scales, by the use of specific coefficients for the studied coffee plantation. The values of total radiation absorbed computed at the different time-scales were also compared to those estimated by the use of the Beer's attenuation law, taking into account the radiation measured by a horizontal net radiometer above the crop, the estimated radiation extinction coefficient of the canopy and the leaf area index. A very good agreement between both types of determination was observed. The direct measurement of net radiation by the proposed technique appears to be satisfactory, being useful in testing models of radiation absorption by canopies planted in a hedgerow system. Improvements of the procedures here reported are suggested, like small modifications in the mechanical system.

The use of the proposed technique will be extended to other hedgerow coffee plantations with different geometrical characteristics, in order to test models of radiation absorption by canopies.