The Central Valley is a heavily populated area containing four major cities of Costa Rica located within its boundaries (~60% population) along with significant industrial activity. Weather conditions in the valley are mainly influenced by easterly Caribbean trade-winds during the dry season (February-April) and by frequent westerly continental air masses during the wet season (May-October). About ~50% of GNIP were located within the valley limits. In this region, mean annual precipitation ranges from ~3500 mm in the highlands to ~2000 mm within the foothills (Figure 2), and the area experiences 3 - 4 dry months over the summer. This inter-mountainous valley region has an elevation between 1000 and 2500 meters m.a.s.l, with average temperatures that range from 14˚C to 18˚C. The northern Pacific region of the Nicoya Peninsula receives on average 1500 mm of precipitation and suffers extensive dry periods that can reach up to 6 months. The Caribbean slope is characterized by a short dry season (1 - 2 months); mean annual precipitation in this region ranges from 4500 to 6000 mm.

We analyzed stable isotope records (n = 679) of 46 monitoring stations across Costa Rica obtained from the GNIP data base (Table 1, Figure 1). Station altitude ranges from 0 - 3000 m.a.s.l with a mean altitude of 930 m.a.s.l. Sampling frequency was typically monthly except for one station (Monteverde) where event-based sampling was conducted by Rhodes et al. (2006). The entire data set is comprised of three relative short-term sampling campaigns: 1990-1992, 1997-1998, and 2002- 2005. Weighted d¹⁸O and d²H means were calculated for 42 stations (87%). Linear regression analysis of elevation and arithmetic d¹⁸O values was conducted considering 18 stations which are unlikely to be influenced by evaporation enrichment: La Selva, Turrialba, Santa María, San Pablo, Santa Lucía, Birrí, Monteverde, Zurquí, Heinz Hoffman, Monte de la Cruz, Pacayas, Fraijanes, Vara Blanca, La Giralda, Sacramento, Paso Llano, Poás, and Irazú. Likewise, linear regression analysis of precipitation amount and d¹⁸O values was determined using data from 11 stations which are representative of all four regions: Turrialba, Puriscal, Fraijanes, Orotina, Santa Barbara, Rancho Redondo, Puntarenas, Hacienda Tempisque, and Sacramento.  

Spatial analysis was conducted using the Atlas of Costa Rica 2008 database [37]. The natural neighbor procedure in ArcGIS 9.3 was applied to calculate deuterium excess, d¹⁸O, and d²H interpolations. High station densities within the Central Valley and Nicoya Peninsula may overemphasize the isotopic signature of these regions. Data limitations constrained this and additional analyses. First, the majority of the stations (61%) generally reported isotopic values over a 1 year period with monthly values ranging from n = 4 to 28 with a mean of n = 15. Second, most of the reported stations were located within the Central Valley (the most populated area) and the North Pacific region (Figure 1). Therefore, a considerable gap exists along the Pacific and Caribbean shores, continental divide, and the northern and southern regions (Figure 1). Third, there was an omission of temperature values which prevented calculation of d¹⁸O and temperature correlation.

To complement the historic GNIP station data analysis, air parcel back trajectories were calculated for particular locations using the hybrid single particle Lagrangian integrated trajectory model (HYSPLIT) [38]. Isotopic values in precipitation from two distinct study locations, Monteverde (continental divide) and Turrialba (Caribbean Slope), were analyzed. Both a depleted and enriched isotopic event during the 2011 and 2012 water years were chosen from these two locations. Single (24 hours) five days back trajectories were calculated using the vertical velocity model calculation method. Trajectory ensembles were done using the GDAS1 meteorological database. Back trajectories were calculated starting at 2300 UTC. Rainfall rate, downward radiation,

relative humidity, and elevation of the mixing layer depth were also computed.

Precipitation was collected on event-basis. Monteverde sampling was carried out during July, August, and September of 2011. Turrialba’s sampling period used in this study was from September to December, 2012. The collector for individual events consisted of a 10.16 cm plastic funnel coupled with a metal filter mesh (i.e., to prevent external contamination). The funnel was connected to 4 L high density polyethylene (HDPE) container. An approximately 3 cm mineral oil layer was added to prevent fractionation according with standard sampling protocols. Later, mineral oil was separated using a 250 - 500 mL separatory funnel. Samples were stored upside down in a HDPE bottles with conic and polyseal inserts and parafilm seals until analysis. Stable isotope analyses were conducted at the Chemistry School of the National University (Heredia, Costa Rica) and the Idaho Stable Isotope Laboratory (Moscow, Idaho) using a Cavity Ring Down Spectroscopy (CRDS) water isotope analyzer L2120-I (Picarro, CA) and L1120-i (Picarro, CA), respectively. Ratios of d¹⁸O/d¹⁶O and d²H/d¹H are expressed in delta units (‰, parts per mil) relative to Vienna Standard Mean Ocean Water (V-SMOW). The current analytical analyzers precision is 0.1‰ d¹⁸O/d¹⁶O and 0.5‰ d²H/d¹H.

Precipitation variability in Costa Rica reflects regional and local complex dynamics. Figure 2 shows mean annual precipitation throughout Costa Rica and precipitation amounts recorded during isotopic sampling campaigns. GNIP precipitation records are consistent with the Atlas of Costa Rica reported values and ranged from 5074 mm (Zurquí) to 897 mm (Hacienda Tempisque) with a mean of 2479 mm (Table 1, Figure 2). The lowlands of the Nicoya Peninsula is the region with the lowest precipitation regime (<1500 mm) while the Talamanca Range and northeastern Caribbean lowlands received the greatest amount, between 4500 - 8000 mm

annually. These precipitation records provide a reliable basis for further stable isotope spatial analysis.

Stable isotope signatures in precipitation across Costa Rica were variable during the sampling campaigns. d¹⁸O composition ranged from −1.8‰ to −10.1‰ with an arithmetic mean of −6.7 ± 1.4 (‰) (Figure 3, Table 1). Weighted d¹⁸O values range from −3.2‰ to −11.1‰ with a mean of −7.4 ± 1.8 (‰) (Table 1). Deuterium composition varied from −17.3‰ to −70.4‰ with an arithmetic mean of −43.2 ± 10.3 (‰) (Figure 3, Table 1). Weighted d²H values ranged from −19.7‰ to −76.8‰ with a mean of −49.0 ± 13.7 (‰) (Table 1). Figure 4 shows the regional water meteoric lines for each of the four main regions for the GNIP Stations (Nicoya Peninsula, Pacific Coast, Caribbean Slope, and Central Valley). The best fit straight line to the data for the Nicoya Peninsula yields an arithmetic water line equation of d²H = 7.84d¹⁸O + 7.60 (Figure 4), whereas the weighted meteoric water line yields a distinct best fit straight line equation of d²H = 6.65d¹⁸O − 0.13 (Figure 4), likely resulting from small precipitation amounts. For the Pacific Coast, the best fit straight line to precipitation-weighted annual means is d²H = 7.60d¹⁸O + 7.95 (Figure 4), which is consistent with the Global Meteoric Water Line (GMWL) of d²H = 8d¹⁸O + 10 [39]. Interestingly, Pacific Coast stations are close to equilibrium conditions rather than exhibiting evaporation enrichment. The Caribbean Slope precipitation-weighted water line d²H = 6.98d¹⁸O + 4.97 (Figure 4) reflects orographic distillation of Caribbean-sourced moisture. Overall, the best fit line to the precipitation-weighted data for all regions combined yields a water line of d²H = 7.61d¹⁸O + 7.40 (r² = 0.98) which corresponds closely to the GMWL (Figure 5). Enriched values above the annual mean of −6.7‰ d¹⁸O were often observed during January to March. The greater variation occurred in May and June when the ITCZ starts its migration over Costa Rica. Towards the middle of the wet season (August, September, and October) d¹⁸O variation appears to decrease due to increase in precipitation amounts. The intensification of northeast trade winds in December produced a decrease in precipitation, and consequently, enrichment in isotopic values (Figure 5).

The variability in isotopic composition of precipitation with elevation is not consistent across the GNIP stations indicating a complex system. Figure 6 shows three lon-

gitudinal transects. For the Caribbean slope, there is an evident orographic distillation effect from Puerto Limón (0 m.a.s.l.) to Taus (900 m.a.s.l.) of −3.5‰ d¹⁸O/km. The ~75 km Pacific slope transect from Herradura (3 m.a.s.l.) to Paso Llano (2397 m.a.s.l.) represents a more complex system. For instance, a rainout effect is observed up to the Santa Bárbara station (1102 m.a.s.l.) (Figure 6). However, at the two higher elevation stations (Birrí and Heinz Hoffman) isotopic compositions are more enriched, suggesting another moisture source. A similar phenomena is also observed along the Nicoya Peninsula transect of ~66 km from the coast to the Rincón de la Vieja volcano (Figure 6).

Considering the 18 stations mainly from the Central Valley cluster, the correlation of elevation with arithmetic mean annual d¹⁸O is moderately strong (r² = 0.77). The best fit straight line has a slope of −0.2‰ d¹⁸O per 100 m, which is close to the range of 0.1‰ and 0.36‰ reported by Leibungdgut et al. (2009) [40]. Since the altitude effect is the combined result of the temperature effect and moisture depletion by adiabatic cooling, a temperature effect can be calculated by knowing the atmospheric lapse rate. Lachniet and Paterson (2002) [27] reported a lapse rate of −5.4˚C/km for the San José area. Therefore, an altitude effect of −0.2‰/km corresponds to −0.37‰/˚C, in agreement with temperature effects reported in the pioneer work done by Dansgaard [23].

One important factor controlling isotopic variations in the tropics is the “amount effect” [23,24,41]. This effect has been attributed to three main factors according to Scholl et al. (2009) [42]: 1) the isotopic composition of the condensation in a cloud decreases as cooling and “rainout” occurs; 2) smaller raindrops equilibrate to a larger degree with the water vapor and temperature conditions below the cloud; and 3) small raindrops evaporate more than larger raindrops on their way to the land

surface. The slope of the best fit straight line to precipitation amount and d¹⁸O from 11 stations, which are representative of the four main regions described previously, was found to be −1.6‰ per 100 mm decrease in rainfall (r² = 0.57). This effect is clearly present during the times of lowest precipitation (January to April).

The “amount effect” was also observed in the d-excess isoscape presented in Figure 7. D-excess composition ranged from 2.9‰ to 14.4‰ with an arithmetic mean of 10.8 ± 2.0 (‰) (Table 1, Figure 7). Weighted d-excess values range from 3.5‰ to 17.4‰ with a mean of 10.3 ± 2.1 (‰) (Table 1, Figure 7). Low d-excess (~3‰) is predominantly present within the Nicoya Peninsula and Caribbean lowlands and may also be a common phenomena across the northern lowlands and Nicaragua trough. High d-excess (>17‰) was found in the dense forested areas east of the Central Valley (Zurquí and Irazú mountain range) and the Pacific slope of the Monteverde tropical cloud forest (Figure 7), which may indicate successive evaporation and precipitation cycles. Based on d-excess values, Rhodes et al. (2006) [43] concluded that moisture flux from evaporation is an important input to the region during the transitional and dry seasons when trade winds from the Caribbean slope dominate. Therefore, it appears that intense moisture recycling processes govern isotopic variations within the

inter-mountainous regions of Costa Rica.

Figure 8 shows daily variation in isotopic composition in precipitation at Monteverde and Turrialba. For Monteverde, a remarkable depletion in δ¹⁸O was observed on 7/18/2011 and an enriched event was sampled on 7/26/ 2011 (Figure 8). For Turrialba, sampling coincided with the wet to dry season transition, but sampling events for the purposes of the air mass trajectories were selected