The location of Costa Rica on the Central American Isthmus creates unique microclimate systems that receive moisture inputs directly from the Caribbean Sea and the Pacific Ocean. In Costa Rica, stable isotope monitoring was conducted by the International Atomic Energy Agency and the World Meteorological Association as part of the worldwide effort entitled Global Network of Isotopes in Precipitation. Sampling campaigns were mainly comprised of monthly-integrated samples during intermittent years from 1990 to 2005. The main goal of this study was to determine spatial and temporal isotopic variations of meteoric waters in Costa Rica using historic records. Samples were grouped in four main regions: Nicoya Peninsula (2H = 6.6518O − 0.13; r2 = 0.86); Pacific Coast (2H = 7.6018O + 7.95; r2 = 0.99); Caribbean Slope (2H = 6.9718O + 4.97; r2 = 0.97); and Central Valley (2H = 7.9418O + 10.38; r2 = 0.98). The water meteoric line for Costa Rica can be defined as 2H = 7.6118O + 7.40 (r2 = 0.98). The regression of precipitation amount and annual arithmetic means yields a slope of −1.6‰ 18O per 100 mm of rain (r2 = 0.57) which corresponds with a temperature effect of −0.37‰ 18O/ ̊C. A strong correlation (r2 = 0.77) of −2.0‰ 18O per km of elevation was found. Samples within the Nicoya Peninsula and Caribbean lowlands appear to be dominated by evaporation enrichment as shown in d-excess interpolation, especially during the dry months, likely resulting from small precipitation amounts. In the inter-mountainous region of the Central Valley and Pacific slope, complex moisture recycling processes may dominate isotopic variations. Generally, isotopic values tend to be more depleted as the rainy season progresses over the year. Air parcel back trajectories indicate that enriched isotopic compositions both in Turrialba and Monteverde are related to central Caribbean parental moisture and low rainfall intensities. Depleted events appear to be related to high rainfall amounts despite the parental origin of the moisture
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