Evapotranspiration and Energy Balances of Forest and Field

The energy and water balance components of a 70-year-old Norway spruce forest, a 2-year-old alfalfa field, and a potato field near Munich, Germany, were computed for the period May–October 1965 by using hourly averages of metorologic parameters. Turbulent diffusion and energy budget methods were used to determine evapotranspiration and convective exchange. The two methods yielded satisfactory, agreement for the low crops but not for the forest. The mean seasonal albedo values for the potatoes and the alfalfa were 0.18 and 022, respectively. The albedo of the forest was obtained for 1 day and averaged 0.05. The shortwave incoming radiation over the forest was 12% less than that over the fields. The net shortwave radiation over the forest was 2 and 7% greater than that over the potatoes and the alfalfa, respectively, whereas the long wave emission from the forest was 22 and 15% less than that from the potatoes and the alfalfa, respectively. The total net radiation over the forest was 20% greater than that over the alfalfa and 16% greater than that over the potatoes. Sensible heat flux from the forest was 2.6 and 1.6 times greater than that from the alfalfa and the potatoes, respectively. Evapotranspiration from the forest was 4% less than that from the alfalfa but 14% greater than that from the potatoes. The roughness lengths of the forest and the fields were about 0.1 of their heights. The zero plane displacements were 0.71 and 0.5–0.6 of the heights of the forest and the crops, respectively. The exchange coefficient at the top of the trees was about 100 times as large as that at the tap of the crops. The water supply in the soil was not limiting on any of the three experimental sites during the vegetation period.

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