The Rio Guacimal originates in a tropical montane cloudforest (TMCF) in Monteverde, located on the western side of the Tilaran mountain range, Costa Rica. A TMCF is characterized by frequent cloud cover and enhanced precipitation by canopy interception. The biological diversity in a TMCF attracts tourists to the region, and the success of ecotourism and agriculture is driving rapid growth of this rural community. This growth and decreased cloud formation in the region (Lawton et al., 2001) could stress the quality and quantity of water. The purpose of this study is to characterize the water chemistry and surface water hydrology of a 21 km² headwater watershed of the Rio Guacimal to predict impacts of land use changes.

Stream and precipitation samples were collected from March 2000 to September 2001. The sample sites were located in remote areas near the headwaters and downstream of businesses, residences, and agriculture. The water samples were analyzed for specific conductance (SC), pH, dissolved oxygen, acid neutralizing capacity (ANC), concentrations of Ca, Mg, Na, K, NH₄, SO₄, NO₃, Cl, PO₄ and dissolved silica. Stage of the Rio Guacimal was measured by a Campbell Scientific datalogger with pressure transducer. Throughfall (TF) and open (no canopy) precipitation were measured by tipping buckets and stemflow (SF) collectors in upper elevations. The data showed that SC, ANC, and base cation concentrations in the streams increased with land use. ANC increased from 212.0 畫q/L near the headwaters to 864.4 畫q/L downstream of land use on the Rio Guacimal. NH₄, SO₄, NO₃ concentrations (2.1 畫q/L, 117.1 畫q/L, 58.8 畫q/L respectively) were highest on a tributary flowing through the most densely populated area. The TF and SF precipitation data had higher ion concentrations than from open areas. The hydrograph of the Rio Guacimal showed distinctive and rapid fluxes in recorded stage associated with precipitation, and changes in mean stage that corresponded with the dry, transitional, and wet seasons. The average changes on the rising limb of stage were 0.43 ft for January-February, 0.21 ft for March-May, and 1.48 ft for June-December. Further study is needed to determine if the observed rise in stage from June to December is due to greater frequency and magnitude of storms, increased groundwater storage, or both.