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\begin{document}

\title{Chirrip\selectlanguage{ngerman}ó Hydrological Research Site: advancing stable isotope hydrology
in the Central American Páramo\selectlanguage{english}}



\author[1]{Germain Esquivel-Hernández}%
\author[1]{Ricardo Sanchez-Murillo}%
\author[2]{Enzo Vargas-Salazar}%
\affil[1]{Universidad Nacional de Costa Rica}%
\affil[2]{Costa Rican National System of Conservation Areas}%


\vspace{-1em}



  
  \date{June 18, 2024}


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\textbf{1. Introduction}

Tropical mountainous ecosystems are recognized as providers of valuable
ecological and hydrological services (Viviroli et al, 2007). In Central
America, the P\selectlanguage{ngerman}áramo, a high\selectlanguage{english}-elevation tropical grassland ecosystem,
extends over \textasciitilde{} 200 km\textsuperscript{2} in Costa Rica
and Panama, with \textasciitilde{}50\% of this area located within the
Chirripo National Park between 3,100 and 3,820 m asl (-83.49deg, 9.46deg).
Vegetation mostly consists of 0.5 to 2.5 m tall bamboo dominated
(\emph{Chusquea subtessellata} ) grasslands, covering up to 60\% of the
total Paramo area in Costa Rica (Fig.1a). The climate is controlled by
the northeast trade winds, the latitudinal migration of the
Intertropical Convergence Zone (ITCZ), cold continental outbreaks (i.e.,
northerly winds), and the seasonal influence of Caribbean cyclones.
These circulation patterns produce two rainfall maxima on the Pacific
slope, one in June and one in September, which are interrupted by a
relative minimum between July-August, known as the Mid-Summer Drought,
due to intensification of trade winds over the Caribbean Sea (Magana et
al., 1999; Waylen, 1996). The wettest season extends from May to
November (contributing up to 89\% of the annual precipitation), whereas
the driest season is from December to April (Fig. 2a; Esquivel-Hernandez
et al., 2018). The surface water system of Chirripo is characterized by
a lake district which comprises approximately 30 lakes of glacial origin
and streams flowing down the Caribbean and Pacific slopes (Fig 1b). Lake
catchments are characterized by steep slopes that promote rapid
hydrological responses such as fast water\selectlanguage{english}-level changes. Input of water
to these glacial lakes is mostly controlled by the seasonal inputs of
rainfall, which mix up with stream and subsurface waters. In April 2015,
the Chirripo Hydrological Research Site (CHRS) was installed with the
goal of advancing the understanding of the hydrological functioning in
the Central American Paramo using environmental tracers (i.e., water
stable isotopes) in combination with hydrometric data. A detailed map of
CHRS is available in Esquivel-Hernandez et al. (2019).

\textbf{2. Methods}

\textbf{2.1. Collection of precipitation and surface water samples}

Daily precipitation samples (N = 512) were collected at the Base
Crestones shelter (-83.480deg, 9.460deg, 3,400 m asl) on the Pacific slope
of Chirripo using a passive collector (Palmex Ltd., Croatia). Samples
were collected manually on every rainy day at 07:00 hr (\selectlanguage{english}-06:00 GMT).
Lake water samples were collected at Lake Chirripo (-83.494deg, 9.483deg,
3,524 m asl, N= 48), Lake Ditkevi (-83.481deg, 9.469deg, 3,506 m asl, N=
66), and Morrenas Lakes (-83.486deg, 9.492deg, 3,495 m asl, N= 38) following
a roughly biweekly sampling plan. Lake water samples were manually
collected from about 20 cm below the surface and \textasciitilde{}25 m
away from surface stream inlets. Stream water (N= 74) was collected at
several sites where surface water was flowing into the lakes or into the
main channelized stream of CHRS (Talari River, -83.503deg, 9.458deg, 3,352 m
asl).

\textbf{2.2. Stable isotope analysis}

Stable isotope analyses of precipitation and surface water samples were
conducted at the Stable Isotopes Research Group facilities of
Universidad Nacional (Heredia, Costa Rica) using a cavity ring down
spectroscopy water isotope analyser L2120-i (Picarro, USA) and a
LWIA-45-EP water isotope analyzer (Los Gatos,
USA).\textsuperscript{18}O/\textsuperscript{16}O
and\textsuperscript{2}H/\textsuperscript{1}H ratios are presented in the
established delta notation \selectlanguage{greek}δ \selectlanguage{english}(%0), with reference to the VSMOW-SLAP
scale. Deuterium excess was calculated as \emph{d} -excess =
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{2}H-8\selectlanguage{ngerman}·\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O (Dansgaard, 1964). The
analytical long-term uncertainty is \selectlanguage{ngerman}±0.5\selectlanguage{english}%0 (1\selectlanguage{greek}σ\selectlanguage{english}) for \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{2}H
and \selectlanguage{ngerman}± 0.1\selectlanguage{english}%0 (1\selectlanguage{greek}σ\selectlanguage{english}) for \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O.

\textbf{2.3. Meteorological data and lake water temperature}

Due to the remoteness of CHRS, there is a limited access to internet
resources. Between April-August 2015, we only registered precipitation
amounts (mm/day) using a tipping bucket (Oregon Scientific, USA)
installed at the Base Crestones shelter. Between September 2015 and
December 2017, relative humidity (\%), air temperature (\selectlanguage{ngerman}°C), and
precipitation amounts (mm/hr) were recorded using a LW301 weather sensor
kit (Oregon Scientific, USA). In January 2018, when the Internet
connectivity in the shelter was improved, a Vantage Pro2 weather station
(Davis Instruments, USA) was installed to register the meteorological
conditions at CHRS. Between September 2015 and November 2020, the lake
water temperature was recorded at 15\selectlanguage{english}-min intervals by an Onset HOBO
water temperature data logger installed at the west bank of Lake
Ditkevi. Water temperature was also manually registered for some lake
and stream water samples collected between September 2015 and December
2018 using a field portable tester (Hanna Instruments, USA).

\textbf{3. The need to advance the hydrological research in the Central
American Paramo}

High\selectlanguage{english}-elevation tropical ecosystems like the Paramo are susceptible to
potential impacts of changes in climate and land cover which could
affect the variability in their hydrological conditions and
ecohydrological functional properties (Esquivel-Hernandez et al., 2017;
Wright et al., 2017). Overall, the information about the importance of
Paramo ecosystem was identified much earlier in South America than in
Central America which led to a more structured research in the Andes
compared to Central America (Correa et al., 2020). Therefore, the
isotopic and hydrometric information collected at CHRS can help define
and promote future hydrological research in tropical montane ecosystems
and can endorse adaptation policies to alleviate the impacts of
potential changes in the water balance of the Paramo and the lowlands
that rely on its water supply.

\textbf{4. Research findings and applications}

To our knowledge, CHRS is the only hydrological research site situated
above the tree timberline (i.e., \textgreater{}3,000-3,100 m asl) in the
Central American and Caribbean region. The Central American region is
transected by a NW--SE mountain range that divides the region into the
Caribbean and Pacific slopes with similar precipitation regimes
(Sanchez-Murillo et al., 2020a). The unique geographic location of Costa
Rica within this mountainous region, with moisture inputs from the
Caribbean Sea (windward) and the Pacific Ocean (complex leeward
topography), offers an ideal scenario for the use water stable isotopes
to develop hydrological research (Sanchez-Murillo et al., 2020b).

Combined analysis of air mass back trajectories and the temporal
precipitation isotopic variations for CHRS revealed the effective
contribution of maritime moisture from the Caribbean Sea (90\%
frequency) and the Pacific Ocean (10\% frequency), with the preferential
contribution from the Caribbean due the influence of the north\selectlanguage{english}-east
trade winds travelling over the central and south\selectlanguage{english}-eastern Caribbean Sea
towards CHRS (Fig 2b,c; Esquivel-Hernandez et al., 2019). CHRS data are
also key to identify inputs from the central Pacific Ocean and local
evapotranspiration fluxes. Overall, as water vapour encounters the main
mountain range, orographic distillation and convergence increase,
resulting in depleted precipitation arriving from the Pacific domain
(Fig 2b; Sanchez-Murillo et al., 2020a). The lesser contribution of
recycled water vapour arriving at CHRS is indicated by the\emph{d}
-excess variations in precipitation (\emph{d-} excess
values\textgreater{}10\selectlanguage{english}%0, Fig. 2c).

The first isotopic characterization of the surface water system of CHRS
indicate that the evaporation conditions in the glacial lakes are highly
influenced by input from precipitation that is in isotopic equilibrium
with local water vapour, yielding low evaporation to inflow ratios
(\textless{} 20\%, Esquivel-Hernandez et al., 2018). Due to their
position near the equator and high elevation, lake water temperature of
Lake Ditkevi shows seasonal variations but weak thermal stratification
(Fig. 2a). Overall, the attenuation effect observed in the isotopic
composition (\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O) and \emph{d} -excess of lake water
most likely resulted from the relatively long residence times of
precipitation, stream, and subsurface water (Fig. 2b,c). However, when
compared to precipitation, lake and surface water reflects the
evaporation effects as the corresponding evaporation lines (ELs) diverge
from the local meteoric water line (LMWL) of the study region (Fig. 3).
Therefore, because of the isotopic enrichment, the slopes of the ELs
fall between the slope of the LMWL and the slope of local evaporation
line (LEL) calculated using pan evaporation data (Fig.3;
Esquivel-Hern\selectlanguage{ngerman}ández et al., 2018).

The intensity of sources or sinks of methane remain critically
understudied in high elevation biomes and scarce data are currently
available in the literature for the neotropical Páramo ecosystems. CHRS
meteorological data were essential to identify key drivers of methane
fluxes in soils of the Chirripó National Park. Overall, the identified
soil and vegetation gradients revealed that sites without
drainage-impeding soil layers or with the facilitating presence of
vigorous \emph{Chusquea} vegetation led to relatively stronger sinks of
methane as compared to fields with wetter, less aerated soils (Chai et
al., 2020). CHRS data has also served as baseline information to improve
the limited paleoclimate data in Central America and to provide reliable
estimates of hydroclimate and environmental change in this region (Wu et
al., 2019a,b).

\textbf{5. Data availability statement}

The hydrologic data of the Chirripó Hydrological Research Site collected
between April 2015 and November 2020 are publicly available at
http://hdl.handle.net/11056/18825. Summary statistics of the dataset are
shown in Table 1. For more information on the available datasets, please
communicate with the lead author germain.esquivel.hernandez@una.ac.cr.

\textbf{6. Acknowledgements}

The authors would like to thank the administration and park rangers of
the Chirripó National Park (Costa Rican National System of Conservation
Areas, SINAC) for their valuable help with the collection of daily
precipitation samples and weekly surface water samples; the access to
the research site; and the logistical support. GEH and RSM. also thank
the Research Office of the National University of Costa Rica through
Grants SIA\selectlanguage{english}-0482\selectlanguage{english}-13 and SIA\selectlanguage{english}-0101\selectlanguage{english}-14, and the support by the International
Atomic Energy Agency Grant CRP\selectlanguage{english}-19747 under the initiative ``Stable
isotopes in precipitation and paleoclimatic archives in tropical areas
to improve regional hydrological and climatic impact models''.

\textbf{7. References}

Chai, L.L., Hernandez-Ramirez, G., Hik, D.S., Barrio, I.C., Frost, C.M.,
Chinchilla-Soto, C., Esquivel-Hernandez, G. (2020). A methane sink in
the Central American high elevation paramo: Topographic, soil moisture
and vegetation effects. Geoderma, 362, 114092.
https://doi.org/10.1016/j.geoderma.2019.114092

Correa, A., Ochoa\selectlanguage{english}-Tocachi, B.F., Birkel, C., et al. (2020). A concerted
research effort to advance the hydrological understanding of tropical
paramos. Hydrological Processes, 34, 4609-4627.
https://doi.org/10.1002/hyp.13904

Dansgaard, W. (1964). Stable isotopes in precipitation. Tellus, 16(4),
436--468. https://doi.org/10.3402/tellusa.v16i4.8993

Esquivel-Hernandez, G., Sanchez-Murillo, R., Birkel, C., Good, S. P.,
Boll, J. (2017). Hydroclimatic and ecohydrological resistance/resilience
conditions across tropical biomes of Costa Rica. Ecohydrology. 10 (6),
e1860. https://doi.org/10.1002/eco.1860

Esquivel\selectlanguage{english}-Hernandez, G., Sanchez\selectlanguage{english}-Murillo, R., Quesada\selectlanguage{english}-Roman, A.,
Mosquera, G.M., Birkel, C., Boll, J. (2018). Insight into the stable
isotopic composition of glacial lakes in a tropical alpine ecosystem:
Chirripo, Costa Rica. Hydrological Processes, 32, 3588--3603.
https://doi.org/10.1002/hyp.13286

Esquivel-Hernandez, G., Mosquera, G. M., Sanchez-Murillo, R.,
Quesada-Roman, A., Birkel, C., Crespo, P., \ldots{} Boll, J. (2019).
Moisture transport and seasonal variations in the stable isotopic
composition of rainfall in Central American and Andean Paramo during El
Nino conditions (2015--2016). Hydrological Processes, 33, 1802--1817.
https://doi.org/10.1002/hyp.13438

Magana, V., Amador, J. A., \& Medina, S. (1999). The midsummer drought
over Mexico and Central America. Journal of Climate, 12(1967),
1577--1588.
https://doi.org/10.1175/1520\selectlanguage{english}-0442(1999)012\textless{}1577:TMDOMA\textgreater{}2.0.CO;2

Sanchez-Murillo, R., Esquivel-Hernandez, G., Corrales-Salazar, J. L.,
Castro-Chacon, L., Duran-Quesada, A. M., Guerrero-Hernandez, M.,
\ldots{} Terzer-Wassmuth, S. (2020a). Tracer hydrology of the
data-scarce and heterogeneous Central American Isthmus. Hydrological
Processes. 34, 2660--2675. https://doi.org/10.1002/hyp.13758

Sanchez-Murillo, R., Esquivel-Hernandez, G., Birkel, C., Correa, A.,
Welsh, K., Duran-Quesada, A.M., Sanchez-Gutierrez, R., Poca, M. (2020b).
Tracing Water Sources and Fluxes in a Dynamic Tropical Environment: From
Observations to Modeling. Frontiers in Earth Science, 8, 438.
https://doi.org/10.3389/feart.2020.571477

Viviroli, D., Durr, H. H., Messerli, B., Meybeck, M., and Weingartner,
R. (2007). Mountains of the world, water towers for humanity: Typology,
mapping, and global significance. Water Resources Research, 43, W07447.
https://doi.org/10.1029/2006WR005653

Waylen, M. E. (1996). Interannual variability of monthly precipitation
in Costa Rica. Journal of Climate, 9, 2606--2613.
https://doi.org/10.1175/1520\selectlanguage{english}-0442(1996)009\textless{}2606:IVOMPI\textgreater{}2.0.CO;2

Wright, C., Kagawa\selectlanguage{english}-Viviani, A., Gerlein\selectlanguage{english}-Safdi, C., Mosquera, G. M.,
Poca, M., Tseng, H., Chun, K. P. (2017). Advancing ecohydrology in the
changing tropics: Perspectives from early career scientists.
Ecohydrology, 11, e1918. https://doi.org/10.1002/eco.1918

Wu, J., Porinchu, D.F., Horn, S.P. (2019a). Late Holocene hydroclimate
variability in Costa Rica: Signature of the terminal classic drought and
the Medieval Climate Anomaly in the northern tropical Americas.
Quaternary Science Reviews, 215, 144-159.
https://doi.org/10.1016/j.quascirev.2019.04.023

Wu, J., Porinchu, D.F., Campbell, N.L., Mordecai, T.M., Alden, E.C.
(2019b). Holocene hydroclimate and environmental change inferred from a
high-resolution multi-proxy record from Lago Ditkebi, Chirripo National
Park, Costa Rica. Palaeogeography, Palaeoclimatology, Palaeoecology,
518, 172-186. https://doi.org/10.1016/j.palaeo.2019.01.004

Table 1. Summary statistics of environmental conditions (Lake Ditkevi
average daily water temperature, average daily air temperature, and
daily precipitation amount) and stable isotope composition of
precipitation, lake water and stream water for the period April
2015-November 2020.\selectlanguage{english}
\begin{longtable}[]{@{}lllllll@{}}
\toprule
& Environmental conditions & Environmental conditions & Environmental
conditions & Environmental conditions & Environmental conditions &
Environmental conditions\tabularnewline
\midrule
\endhead
Parameter & N & Average & Median & Minimum & Maximum & SD\tabularnewline
Lake Ditkevi water temperature (\selectlanguage{ngerman}°C) & 1881 & 11.9 & 11.9 & 7.8 & 15.9 &
1.5\tabularnewline
Rainfall (mm/day) & 1948 & 6.1 & 1.6 & 0.0 & 189.5 & 11.2\tabularnewline
Air temperature (\selectlanguage{ngerman}°C) & 1825 & 11.0 & 9.6 & 2.0 & 25.8 &
3.6\tabularnewline
Relative humidity (\%) & 1825 & 77 & 83 & 19 & 99 & 17\tabularnewline
& \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O (%0) & \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O (%0) &
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O (%0) & \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O (%0) &
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O (%0) & \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O (%0)\tabularnewline
Type of sample & N & Average & Median & Minimum & Maximum &
SD\tabularnewline
Precipitation & 512 & -10.60 & -10.36 & -22.40 & -2.24 &
4.22\tabularnewline
Lake Chirrip\selectlanguage{ngerman}ó & 48 & -9.84 & -9.78 & -16.90 & -5.64 &
1.56\tabularnewline
Lake Ditkevi & 66 & -9.06 & -9.21 & -13.13 & -5.77 & 1.67\tabularnewline
Lake Morrenas & 38 & -9.06 & -9.65 & -12.33 & -1.88 &
2.31\tabularnewline
Stream water & 74 & -11.15 & -11.19 & -14.56 & -5.00 &
1.55\tabularnewline
& \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{2}H (%0) & \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{2}H (%0) &
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{2}H (%0) & \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{2}H (%0) &
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{2}H (%0) & \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{2}H (%0)\tabularnewline
Type of sample & N & Average & Median & Minimum & Maximum &
SD\tabularnewline
Precipitation & 512 & -73.98 & -71.56 & -165.05 & -4.68 &
34.10\tabularnewline
Lake Chirrip\selectlanguage{ngerman}ó & 48 & -71.78 & -70.34 & -118.49 & -46.08 &
10.39\tabularnewline
Lake Ditkevi & 66 & -64.89 & -66.11 & -94.34 & -41.90 &
11.22\tabularnewline
Lake Morrenas & 38 & -66.23 & -69.49 & -86.43 & -26.14 &
14.76\tabularnewline
Stream water & 74 & -78.08 & -78.34 & -101.71 & -38.10 &
10.15\tabularnewline
& \emph{d}-excess (%0) & \emph{d}-excess (%0) & \emph{d}-excess (%0) &
\emph{d}-excess (%0) & \emph{d}-excess (%0) & \emph{d}-excess
(%0)\tabularnewline
Type of sample & N & Average & Median & Minimum & Maximum &
SD\tabularnewline
Precipitation & 512 & 10.79 & 11.12 & -10.87 & 27.94 &
5.48\tabularnewline
Lake Chirrip\selectlanguage{ngerman}ó & 48 & 6.91 & 5.99 & -0.97 & 16.71 & 3.77\tabularnewline
Lake Ditkevi & 66 & 7.57 & 8.04 & -0.36 & 16.33 & 3.70\tabularnewline
Lake Morrenas & 38 & 6.28 & 7.62 & -11.08 & 15.55 & 5.84\tabularnewline
Stream water & 74 & 11.17 & 11.00 & -8.20 & 20.04 & 4.97\tabularnewline
\bottomrule
\end{longtable}\selectlanguage{english}
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