Hydrological Processes in a Pre-montane Tropical Forest

Hydrological processes in pre-montane tropical forests are largely understudied. As climatic changes raise the orographic cloud bank, transitional swatches with characteristics of both cloud forests and rainforests are increasing in number and subsequently changing the hydrology of mountainous watersheds in the tropics. In this study, we focus on the connection between groundwater flow and surface runoff in a 10-hectare transitional forest catchment. The study site, part of the NSF REU program, is located on the Caribbean side of the Cordillera Tilarán mountain range in Costa Rica. A v-notch weir was constructed at the outlet of a first-order stream; three piezometers and five soil moisture sensors were installed along the adjacent slope. Groundwater elevation, soil moisture, and stream flow data were collected over a 3-month period during the wet season of 2012 (Figure 1). Precipitation data were collected every five minutes by two tipping bucket rain gages. A survey of the watershed mapped its topography, boundaries, vegetation characteristics (DBH, basal area, height), and hydrologic features (stream path, headwater springs). Analysis of the watershed's response to these rain events generated time to concentration and rainfall-runoff curves. Results show that precipitation and streamflow are tightly coupled, indicating the importance of overland runoff in the watershed's response to storm events. Rapid drainage of groundwater combined with the low permeability of the site's silty clay loam suggest that the majority of groundwater transport is due to macropore flow and horizontal fractures and indicates a high degree of connectivity between groundwater and surface water processes. Figure 1: Stream and groundwater response to precipitation events during July 2012 rainy season.