Estimating net radiation absorbed by orchard and isolated tree

One of the most difficult and limiting tasks in estimating either orchard or isolated tree transpiration is the determination of the absorbed net radiation. Field measurement is cumbersome because it is needed a set of net radiometers displayed around the tree. Besides the operational difficulties, the results represent only the sampled tree and cannot be taken to be representative of any other tree in the orchard. Reported examples of field measurements are very few indicating the practical difficulties of such endeavor.

The objective here is to present a simple approach to estimate the daily total net radiation absorbed by individual trees either isolated or in orchards based only on the conventional grass surface net radiation.

Two sets of net radiation absorbed by trees (Qn) were analysed: one for an isolated walnut tree, and another for a citrus tree in a orchard. "Measured" Qn were found to be directly proportional to the tree leaf area in a linear fashion. Standard weather station grass surface net radiation (Rng) was used to estimate Qn.

One approach is very simple and consists of taking 32% of Rng and multiplying it by the tree leaf area.

Another way is to multiply Rng by the ratio between the tree leaf area and the grass leaf area index. An almost perfect fit (1:1) was obtained for both trees. The grass leaf area index here assumed to be constant and equal to 3 resulted in an overprediction of 5% in the total tree net radiation. In practical terms, such small discrepancy compensates for not having to use any special equipment to "measure" the plant net radiation, which is applicable only to that specific tree.

The main source of error seems to be the leaf area, and the major problem now is to find an easy way to determine the leaf area of the tree.