Climate and moisture variability in a tropical forest : long-term records from Barro Colorado Island, Panamá

Windsor, Donald M. Climate and Moisture Variability in a Tropical Forest: Long-term Records from Barro Colorado Island, Panama^ Smithsonian Contributions to the Earth Sciences, number 29, 145 pages, 35 figures, 11 text tables, 40 appendix tables, 1990. —Long-term environmental monitoring results are presented documenting the seasonality experienced by the lowland tropical forest on Barro Colorado Island, Panama. A dry season has developed during each of the past 62 years, typically starting in late December or early January and ending with the first heavy rains in late April or early May. Solar radiation totals climb during dry seasons to monthly levels 50% higher than normal in the wet season. Average daily maximum temperatures increase by two degrees C while average daily minimum temperatures increase by roughly one degree C. Development of the dry season is accompanied by a doubling in average daily windspeed and a drop of ten percent in average midday relative humidity. Annual rainfall on Barro Colorado Island averages 2612 mm (1925-1989), 90% of which falls in the months of May through November. Rainfall on Barro Colorado Island and seven other sites in the middle of the isthmus has decreased significantly over time. The only long-term rainfall records without decreasing trend come from coastal sites, suggesting that convective, but not orographic, rainfall, has diminished during the last sixty years. Further, annual rainfall appears to be influenced by factors associated with El Nino events. Higher than normal rainfall tends to occur the year before and lower than normal rainfall tends to occur the year of such events. Dry-season forest and clearing temperatures on Barro Colorado Island were elevated during each of the three El Nino events occurring in the past 16 years. Storms that drop most of their moisture in a minute or two are common during the wet season on Barro Colorado Island. The soils of Lutz catchment are steep and clay rich. Most moisture in storage resides in the upper 10 cm. Available soil moisture is largely depleted by midto late dry season. Actual evapotranspiration, calculated as the difference between rainfall and runoff and changes in storage for each of 14 years averaged 64 percent of annual rainfall. Eighty-five percent (1534 mm) of annual rainfall was returned directly to the atmosphere as vapor in the dryest of the past 16 years (1976) while only 47% (1953 mm) was returned in the wettest year (1983). Comparison of average evapotranspiration and rainfall characteristics from 18 temperate and tropical hydrological studies suggests a loose positive relationship exists between these two quantities in forested ecosystems. OFFICIAL PUBLICATION DATE is handstamped in a limited number of initial copies and is recorded in the Institution's annual report, Smithsonian Year. SERIES COVER DESIGN: Aerial view of Ulawun Volcano, New Britain. Library of Congress Cataloging in Publication Data Windsor, Donald M. Climate and moisture variability in a tropical forest: long-term records from Barro Colorado Island, Panama" / Donald M. Windsor. (Smithsonian contributions to the earth sciences : no. 29) Includes bibliographical references. 1. Forest microclimatology—Panama—Barro Colorado Island. 2. Moisture—Panama—Barro Colorado Island. 3. Barro Colorado Island (Panama)—Climate. I. Title. II. Series. QE1.S227 no. 29 [SD390.6. P2] 550 s—dc20 [551.69727'5] 89-600304