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Estimating Summer Evapo-Transpiration Losses in a Pennsylvania Scrub Oak Forest¹

ABSTRACT

Fiberglas soil-moisture units were placed to a depth of 2 feet in the different soils underlying a scrub oak forest in Pennsylvania. Successive daily readings on these units indicated the amount of water lost or gained in the horizon.

To determine evapo-transpiration losses, daily soil-moisture losses were analyzed for periods during the summer months when the soil was at or below field capacity. These losses were then related to several factors. Atmospheric saturation deficit was the only climatic element that was statistically related to evapotranspiration losses. Other climatic elements studied (air temperature and wind velocity) bore no relation to the soil moisture losses. In addition, soil moisture content and length of time elapsed since a storm had no relation to these losses. Further analysis revealed that evapo-transpiration losses start at a minimum saturation deficit of approximately 0.020 inches of mercury, and that the value of the saturation deficit regression coefficient is directly related to the total sand content in the soil profile.

Thus, for areas with vegetation similar to scrub oak, it may be possible to determine the evapo-transpiration regression equation by making a mechanical analysis of the underlying soil. Since this work was done on a watershed in which an attempt is made to balance the water economy, it was possible to check the calculated evapo-transpiration losses. The check was favorable.

¹ Presented before Division V-A Soil Science Society of America, Cincinnati, Ohio, November 21, 1952.

² Soil scientist in the Northeastern Forest Experiment Station, Forest Service, U.S. Department of Agriculture. Mr. Bethlahmy is stationed at the Delaware Basin Research Center, Bethlehem, Pa., which is carrying on forest-influences studies in cooperation with Lehigh University and the Pennsylvania Department of Forests and Waters.

Received for publication December 6, 1952.