Year-to-year variability in hydrometeorology of a tropical montane rainforest

The rainforests of Costa Rica experienced an unusual drought beginning in November 2018 that delayed the onset of the rainy season through July 2019. Although there is considerable research on the effects of drought and plant health, few direct measurements have been previously available to characterize the effects of such a drought on a tropical pre-montane transitional forest. To better assess forest response to severe drought in tropical climates, more direct measurements are needed. We hypothesize that a decrease in precipitation led to an increase in light availability, enhancing the greenness of the forest canopy and decreasing flows in headwater and second-order streams. To test this hypothesis, long-term meteorological data was collected and analyzed at the Texas A&M Soltis Center site in the northern region of Costa Rica to understand the drought's impact. Flow was measured in two nearby streams, one headwater and one second-order. Canopy greenness was determined using the Green Chromatic Coordinate (GCC) index, as measured by a phenocam. Despite a 40%, or 1026 mm decrease in precipitation in 2019 compared to the average of the previous eight years and decreased cumulative rainfall in 2019 compared to 2018, the GCC slightly increased by 0.65% from 2018 to 2019. Average streamflow from headwater decreased by 0.001406 m^ 3 /s or 45%. Temperature, solar radiation, and precipitation were below average and relative humidity was above average for 2019 compared to the average of the previous eight years. These markers are opposite of expected during periods of reduced rainfall. Ongoing analysis of fog frequency, solar radiation and soil moisture will help us further understand the relationship between precipitation and forest health in drier periods.