Interbasin groundwater flow and groundwater interaction with surface water in a lowland rainforest, Costa Rica: A review

Journal of Hydrology 320 (2006) 385–399 www.elsevier.com/locate/jhydrol

Interbasin groundwater flow and groundwater interaction with surface water in a lowland rainforest, Costa Rica: A review David P. Genereuxa,*, Michael Jordanb,1 a

Marine, Earth, & Atmospheric Sciences, Campus Box 8208, Jordan Hall, North Carolina State University, Raleigh, NC 27695-8208, USA b Arcadis G&M of North Carolina, Inc., 801 Corporate Center Drive, Suite 300, Raleigh, NC 27607-5073, USA Received 10 May 2005; revised 23 May 2005

Abstract This paper reviews work related to interbasin groundwater flow (naturally occurring groundwater flow beneath watershed topographic divides) into lowland rainforest watersheds at La Selva Biological Station in Costa Rica. Chemical mixing calculations (based on dissolved chloride) have shown that up to half the water in some streams and up to 84% of the water in some riparian seeps and wells is due to high-solute interbasin groundwater flow (IGF). The contribution is even greater for major ions; IGF accounts for well over 90% of the major ions at these sites. Proportions are highly variable both among watersheds and with elevation within the same watershed (there is greater influence of IGF at lower elevations). The large proportion of IGF found in water in some riparian wetlands suggests that IGF is largely responsible for maintaining these wetlands. d18O data support the conclusions from the major ion data. Annual water and major ion budgets for two adjacent watersheds, one affected by IGF and the other not, showed that IGF accounted for two-thirds of the water input and 92–99% of the major ion input (depending on the major ion in question) to the former watershed. The large (in some cases, dominating) influence of IGF on watershed surface water quantity and quality has important implications for stream ecology and watershed management in this lowland rainforest. Because of its high phosphorus content, IGF increases a variety of ecological variables (algal growth rates, leaf decay rate, fungal biomass, invertebrate biomass, microbial respiration rates on leaves) in streams at La Selva. The significant rates of IGF at La Selva also suggest the importance of regional (as opposed to small-scale local) water resource planning that links lowland watersheds with regional groundwater. IGF is a relatively unexplored and potentially critical factor in the conservation of lowland rainforest. q 2005 Elsevier B.V. All rights reserved. Keywords: Groundwater; Watershed; Tracer; Rainforest; Streamflow; Wetland; Tropical

1. Introduction * Corresponding author. Tel.: C1 919 515 6017; fax: C1 919 515 7802. E-mail addresses: [email protected] (D.P. Genereux), [email protected] (M. Jordan). 1 Tel.: C1 919 854 1282.

0022-1694/$ - see front matter q 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2005.07.023

Naturally occurring interbasin groundwater flow (groundwater flow beneath surface topographic divides) can be an important control on water and

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chemical fluxes to surface water, with significant implications for watershed science and management. Often considered a complicating ‘problem’ in watershed hydrologic studies, interbasin groundwater flow (IGF) has been detected at sites around the world (Genereux et al., 2002 and numerous references therein), and is an expected feature of some groundwater flow systems. IGF is possible even in homogeneous isotropic geological materials, given the right combinations of topography and length/ depth ratio of the groundwater system (To´th, 1963). In more realistic heterogeneous and anisotropic geological settings, IGF may follow structural and lithostratigraphic controls (e.g. Thyne et al., 1999; Hudson and Mott, 1997; Parker et al., 1988; Eakin, 1966). IGF is a groundwater process with important links to surface water. Most obvious is the fact that IGF will generally diminish surface water discharge from some watersheds (those in which IGF originates) and augment discharge from others (those receiving IGF). Also, watersheds receiving IGF are expected to have complex discharge zones characterized by mixing of groundwaters of very different age and chemistry: old high-solute IGF from a deeper regional groundwater system and much younger lower-solute groundwater from a smaller, shallower, locally recharged system. Noting the much shorter subsurface residence times in small local systems, To´th (1963) predicted large spatial heterogeneity in the chemistry of mixed groundwater in these areas. This chemical heterogeneity has been detected and quantified at a number of field sites (e.g. Thyne et al., 1999; Johannesson et al., 1995, 1997; Naff et al., 1974; Maxey, 1968), including the Costa Rican lowland rainforest site that is the subject of this paper. Major ion concentrations in surface water and groundwater suggest mixing between local and regional groundwaters at this site (Genereux et al., 2002; Genereux and Pringle, 1997; Pringle et al., 1993); this is supported by physical hydrologic data (Genereux et al., 2005; Jordan, 2003) and 18O data (Genereux, 2004). In addition to its fundamental hydrologic significance, the long-distance subsurface water and chemical movement represented by IGF may also have important implications for water resource or ecosystem management. Our goals here are to: (1) summarize the data (chemical, isotopic, and hydrologic) suggesting IGF occurs and placing

constraints on its magnitude at our lowland rainforest site, and (2) discuss the implications of IGF for watershed hydrology, water quality, ecology, and management, at our site and others.

2. Study site The Cordillera Central of Costa Rica is a northwest–southeast trending range of volcanic peaks associated with subduction of the Cocos plate as it moves to the northeast and beneath the Caribbean plate (e.g. Ludington et al., 1996). Our study site, La Selva Biological Station, is north of the Cordillera Central, in the transitional zone between the steep foothills and the Caribbean coastal plain. La Selva is a 1536-hectare research and education preserve owned and operated by the Organization for Tropical Studies (OTS). La Selva lies at the downslope end of a tract of primary rain forest that extends 35 km to the south (Fig. 1), through Braulio Carrillo National Park and up the north slope of Volcan Barva (elevation 2906 m) and smaller nearby peaks (Pringle et al., 1990). Other large peaks in the region include Volcan Poas (about 15 km northwest of Barva) and Volcan Arenal (about 60 km northwest of Poas). Volcanic activity in the area is ongoing and more intense toward the northwest (OVSICORI-UNA, 2003; Pringle et al., 1993). Geology of the region consists of Quaternary volcanic rocks (mainly andesitic lavas, ignimbrites, volcanic tuffs and breccias) originating from Barva and other major volcanic edifices, with interbedded mudflow deposits, ash, and paleosols (Alvardo Induni, 1990; Parker et al., 1988; Foster et al., 1985; CastilloMunoz, 1983; Bourgeois et al., 1972). Lavas generally function as aquifers and ignimbrite beds as aquitards (Parker et al., 1988). Based on boulders, lithic fragments in soils, and limited outcrops in streams, Alvardo Induni (1990) recognized three fairly distinct lava flows at La Selva, most likely all Pleistocene. A radiometric date (1.2 million years) is available only for the youngest flow. One or all of these lava flows may underlie the younger surficial alluvium which covers the northern third of La Selva, along the Rio Sarapiqui and Rio Puerto Viejo. All three lavas are porphyritic (2–35% phenocrysts, mainly of plagioclase) and have abundant plagioclase

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Entisols (!0.5% of land area) are found near the Rio Sarapiqui. On most ridges a thick clayey saprolite several meters thick lies between the overlying soil (roughly a meter in thickness) and the underlying bedrock. Elevation at La Selva ranges between approximately 35 and 130 m. From 1958 to 2002 annual precipitation at La Selva averaged 4242 mm; average monthly precipitation during the same period ranged from 155 mm (February) to 527 mm (July) (OTS, 2003; Sanford et al., 1994). February, March, and April are the driest months, averaging about 180 mm per month. Annually, evapotranspiration accounts for about half of precipitation (Loescher, 2002; Luvall, 1984). The Sura and Salto are the major streams draining La Selva (Fig. 2), and they (together with their tributaries and adjacent riparian groundwaters) have been the subjects of most of the water quality work at La Selva. This work was initiated to investigate the interactions of stream water nutrient concentrations and stream ecological processes. The large spatial variation observed in stream water phosphorus and major ion concentrations led to the discovery and further study of IGF.

3. Work related to interbasin groundwater flow (IGF) at La Selva

Fig. 1. Location of La Selva Biological Station (shaded area within the dotted line) in Costa Rica (after Pringle et al., 1990). La Selva sits at the northern end of Braulio Carrillo National Park; the dashed line shows the park boundary. Guacimo Spring is marked by the ‘x’ about 1.5 km southeast of La Selva.

with minor olivine, pyroxene, opaque minerals, and vesicles (one also contains glass). The major soil orders at La Selva are Ultisols (45% of area; mainly Typic Tropohumults) and Inceptisols (55% of area; various suborders) (Sollins et al., 1994). Inceptisols are found in the valley bottoms and on the alluvium occupying the northern portion of La Selva, Ultisols are found in other areas. Small areas of

Pringle et al. (1990) reported dissolved phosphorus (P) and nitrogen (N) concentrations for water samples from 75 sites within La Selva and adjacent upgradient areas in Braulio Carrillo National Park. Several sites at lower elevation showed elevated P levels more typical of sewage-contaminated waters than of forest streams (100–250 ppb soluble reactive phosphorus, SRP). SRP concentration was positively correlated with dissolved concentrations of several major ions (NaC, KC, Mg2C, Ca2C, ClK, and SO2K 4 ), suggesting that the P and major ions were derived together from weathering of volcanic rock. Pringle et al. (1990) point out that high dissolved P levels have been found in other volcanic areas. In this pioneering work, Pringle et al. (1990) hypothesized that groundwater acquires a high solute load by interaction with volcanic fluids and subsequent weathering beneath Volcan Barva and then flows downgradient as IGF to discharge into lowland watersheds and streams at La

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Fig. 2. La Selva Biological Station showing streams, trails, and surface water sampling locations.

Selva (e.g. their Fig. 5). The observed association of high concentration with low elevation is consistent with all later work at La Selva and with the conceptual model that ascribes high P and major ion concentrations at the site to upward discharge from a regional groundwater system underlying the lowland watersheds. Pringle (1991) found high P and specific conductance in a number of surface water sites at La Selva (including the Salto and Arboleda streams, and small riparian seeps and rivulets draining to these streams). These sites were said to be modified by ‘geothermal’ inputs or processes. No waters of elevated temperature were (or have been) found at

La Selva; the term ‘geothermal’ was introduced to reflect the potential involvement, farther upslope in the Cordillera, of high temperatures and/or volcanic fluids in creating the high solute water observed at La Selva. While Barva has not had a magmatic eruption since 1867, it has continued to show sulfurous fumarole activity and at least one spring high in sulfuric acid (Pringle et al., 1993). Geothermal processes (incorporation of volcanic fluids into groundwater and enhanced chemical weathering due to elevated temperatures) can clearly increase groundwater solute concentrations. Pringle and Triska (1991) found high-solute water in riparian wells along the Rio Salto, the first

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documentation that such water occurs in the subsurface as well as in streams at La Selva (as would be expected if it were IGF discharged from a regional groundwater system below La Selva). They also made the connection between this high-solute water and stream ecology by showing that three separate nutrient additions (N, P, and the 3 micronutrients Zn, Mo, and Co together) had no effect on algal growth in a high-P stream (the Salto), but P addition did increase algal growth in a low-P stream (the Pantano). This indicates P limits growth (at least at some times) in the low-P Pantano but not the high-P Salto. This is one example of how differences in groundwater flow and discharge to surface water can lead to different controls on stream ecological processes in adjacent rainforest streams. Pringle et al. (1993) provided a regional perspective by showing that water quality data from Volcan Barva and La Selva were similar in some respects to water quality data from nearby more active volcanoes (Poas and Arenal), and by further exploring the link between ‘geothermally modified’ waters and surface water microbial ecology (mainly algal community composition and standing crop). The conceptual model for IGF into La Selva (Pringle et al., 1990) was compared with conceptual models for occurrence of geothermally modified waters at Poas and Arenal (Fig. 6 in Pringle et al., 1993). Genereux and Pringle (1997) used a two-solute concentration plot (Fig. 3) to demonstrate that dryseason dissolved Cl and Na data from La Selva were consistent with a two-end-member mixing model. Most of the large spatial variation in concentration among streams and riparian seeps (a factor of 14 for Cl and 26 for Na) could be explained by mixing of two distinct waters: high-solute ‘geothermal groundwater’ (thought to represent IGF into the lowland watersheds at La Selva) and low-solute ‘local water’ (derived from precipitation onto, and draining from hillslopes within, the La Selva watersheds). The samples used to define geothermal groundwater (upper right in Fig. 3) were from Guacimo Spring, a large perennial spring on the left (northwestern) bank of the Guacimo River about 1.5 km south of the southeastern corner of the La Selva boundary (Fig. 1). Guacimo Spring is the highest concentration site that has been found in work at and near La Selva (Genereux et al., 2002; Genereux and Pringle, 1997; Pringle et al., 1990). The chemistry

389

Fig. 3. Na vs. Cl concentration for 1994 dry season samples from La Selva (after Genereux and Pringle, 1997). Low-solute ‘local water’ plots in the lower left, high-solute ‘geothermal groundwater’ from Guacimo Spring in the upper right. Geothermal groundwater was referred to as bedrock groundwater in later papers (Genereux, 2004; Genereux et al., 2002, 2005). Similar linear behavior is seen in more recent two-solute plots with other major ions and hundreds more analyses (Genereux et al., 2002).

of local water was defined by the samples of lowest concentration, from small streams at La Selva (lower left in Fig. 3) (Genereux and Pringle, 1997). More recent work has documented similar low major ion concentrations in some riparian wells (Genereux, 2004; Jordan, 2003; Genereux et al., 2002). The fraction of geothermal groundwater in these surface water samples (based on mixing calculations with both dissolved Cl and Na) varied from 0 to 0.85, with the highest values in small riparian seeps along the lower swampy portion of the Rio Salto (Genereux and Pringle, 1997). The large values found at the mouths of some streams in 1994 (0.33 at the Sura, 0.5 at the Arboleda, 0.57 at the Salto) suggested that IGF of high-solute water accounted for a large proportion of total discharge from these watersheds. While the data were from the time of year expected to have the highest proportion of geothermal groundwater (the late dry season, when local water discharge would likely be lowest), they also suggested that the absolute volume of discharge of geothermal groundwater was high. Given that annual discharge of local water was expected to be about 2 m (about 4 m of rainfall minus

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2 m of ET), the proportions of geothermal groundwater found at the outlets of some watersheds suggested at least 1–2 m of geothermal groundwater IGF into these watersheds. Genereux et al. (2002) carried out further mixing model calculations for a much larger sample set that included both groundwater and surface water, and spanned 4.5 years: (1) detailed dry season sampling (27 stream and riparian seep sites) in 1994, 1997, and 1998, (2) monthly stream and riparian seep samples (9 sites), August 1993 to December 1997, (3) nearstream groundwater samples (25 wells), October 1993 to December 1994 (monthly) and March 1998, and (4) monthly Guacimo Spring samples, January 1995 to March 1998. As with the earlier work (Genereux and Pringle, 1997), most of the variability in major ion concentrations in this larger sample set could be explained by mixing of two chemically and hydrologically distinct waters: high-solute bedrock groundwater (representing IGF), and low-solute local water draining from hillslope soils within the study watersheds (Table 1). The term ‘bedrock groundwater’ was used in place of the earlier term ‘geothermal groundwater’ to emphasize the geological medium in which the high-solute groundwater flow most likely occurs rather the potential role of geothermal processes in creating high solute concentrations. It is useful to distinguish between (1) the hydrological process (IGF), and (2) controls on the chemistry of the water involved in that process. IGF is controlled by the hydrogeological features of the system (its inputs via recharge, its size and shape and distribution of permeabilities in three dimensions). Geothermal processes may have influenced the chemical character of the water involved in the IGF, but the IGF would

occur (and carry hydrologic significance) with or without this influence. Also, long subsurface rock– water contact time alone, without the added influence of geothermal processes, increases groundwater solute concentrations. Groundwater major ion concentrations equal to or greater than those in bedrock groundwater at La Selva (Table 1) have been found in other basaltic rocks without influence from geothermal activity (e.g. Locsey and Cox, 2003; Edmunds et al., 2002; Herczeg and Edmunds, 2000; Hem, 1985). Both geothermal processes and long rock– water contact time may contribute to the high solute concentrations observed at La Selva; the relative significance of these two factors has not been quantitatively determined. The fraction of bedrock groundwater (fWATER) ranged from zero to about 0.46 in La Selva streams monitored during 1993–1997 (Table 2), based on Cl concentrations in individual stream samples and mean concentrations in the two end members, bedrock groundwater and local water (Genereux et al., 2002). fWATER values were even higher (up to 0.84) for small riparian seeps and shallow groundwater near the Salto stream. Inputs of bedrock groundwater by IGF were even more significant for major ions than for water itself. fWATER values of 0.49 and 0.84 correspond to fCl values of 0.92 and 0.99, respectively, indicating that bedrock groundwater contributed up to 92% of the Cl in streamwater samples and 99% of the Cl in shallow riparian groundwater (fCl, the fraction of the chloride in a water sample that is due to bedrock groundwater, is approximately equal to the fraction of all major ions contributed to the sample by bedrock groundwater, given the observed linear correlations between Cl and other major ions). fWATER and fCl of streams and riparian seeps varied on both long

Table 1 Mean concentrations (Conc), with corresponding standard deviations (SD) and coefficients of variation (CV), for six major ions in the two end-member waters at La Selva (after Genereux et al., 2002) Solute

Na KC Mg2C Ca2C ClK SO2K 4 C

Local water (99 samples)

Bedrock groundwater (56 samples)

Conc. (mM)

SD (mM)

CV (%)

Conc. (mM)

SD (mM)

CV (%)

0.094 0.016 0.054 0.036 0.072 0.005

0.032 0.006 0.027 0.019 0.009 0.010

34 38 50 53 13 200

1.92 0.226 1.56 0.725 0.903 0.126

0.22 0.012 0.11 0.098 0.046 0.009

11 5 7 14 5 7

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Table 2 Average contributions of bedrock groundwater to stream and riparian seep sampling sites at La Selva, August 1993–December 1997 Site

3 5 5b 6 9 22 26

fwater

All months

fwater

n

fwater

n

dry-wet

fwater

fCl

n

0.14 0.33 0.74 0.84 0.16 0.46 0.034

14 11 10 11 11 11 11

0.091 0.23 0.70 0.83 0.11 0.41 0.021

38 29 28 29 30 29 29

0.05 0.10 0.04 0.01 0.05 0.05 0.013

0.10 0.26 0.72 0.83 0.12 0.42 0.024

0.58 0.82 0.97 0.98 0.63 0.90 0.24

52 40 38 40 41 40 40

Dry months

Wet months

Site locations are in Fig. 2. During this 53-month period, average monthly precipitation was 119 mm for the 15 dry months, 432 mm for the 38 wet months, and 344 mm for all 53 months. The number of samples on which each fwater and fCl value is based is given as n. fwater and fCl are defined in the text. The difference between the dry month and wet month fwater averages for each site is shown in the 6th column; the average difference was 0.045. (After Genereux et al., 2002).

(monthly/seasonal) and short (storm event) time scales, in each case decreasing as conditions at La Selva became wetter. The high fWATER and fCl values found in riparian groundwater and seeps indicate that local water and bedrock groundwater derived from IGF mix in the shallow subsurface at La Selva, not just in stream channels. The highest fWATER values (up to 0.84) were found in the swampy area between the Salto and Pantano streams, suggesting that this area of riparian wetland may be maintained largely by IGF. If so, water quantity and quality in these wetlands would be largely controlled by conditions in a regional groundwater system and would be less sensitive to conditions in the watershed in which they reside. The two-end-member mixing model discussed above was used to generate five predictions concerning d18O in groundwater and surface water at 13 sites in the Taconazo and Arboleda watersheds (two small watersheds that represent part of the larger Sura watershed at La Selva, Fig. 4), on the premise that the mixing model would be supported if d18O data agreed with the predictions (Genereux, 2004). Overall, d18O data from March to July 2000 were consistent with the five predictions, supporting three and being neutral toward (neither supporting nor rejecting) the other two. The five predictions were: Prediction 1. Bedrock groundwater at Guacimo Spring should be isotopically lighter (lower d18O) than low-Cl local water because it is recharged at a higher elevation than local water, upslope to the south in the Cordillera Central.

Prediction 2. The 18O content of streamwater at the Arboleda weir, and groundwater at nearby Wells 1 and 2, should all be similar and consistent with significant fractions of both bedrock groundwater and local water. Prediction 3. 18O at Well 3 should indicate a smaller but non-zero amount of bedrock groundwater (less than at the Arboleda weir, Well 1, and Well 2). Prediction 4. The 18O contents of water at the other sampling sites (the Taconazo weir, stream sites Taco8 and Arbo4, and groundwater Wells 4–8), should be similar and consistent with nearly pure local water (no significant contribution from bedrock groundwater). Prediction 5. The 18O content of bedrock groundwater should be less variable over time than the 18O content at low-Cl sites that are mostly or completely local water. Prediction 1 follows from: (a) the interpretation of bedrock groundwater as IGF (which would necessarily have a distant recharge area at a higher elevation than La Selva), and (b) the well-known ‘elevation effect’ by which the 18O content of precipitation is lower at higher elevation (e.g. Lachniet and Patterson, 2002; Ingraham, 1998). Predictions 2–4 simply state that, with regard to mixing between local water and bedrock groundwater, results from d18O should mirror those from Cl. Prediction 5 is based on the expectation that the output from a large, deep, regional groundwater flow system will show less temporal variability than output from local groundwater flow systems fully within the small study watersheds.

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Fig. 4. Streams, wells, and weirs in the Arboleda (right) and Taconazo (left) watersheds at La Selva Biological Station.

d18O data gave reasonably clear support for Predictions 1, 2, and 5 above (Genereux, 2004): the bedrock groundwater end member was both isotopically lighter and less variable in time than the local water end member, and the lowest-elevation surface water and groundwater sites in the Arboleda watershed showed significant and very similar contributions from both end members (Fig. 5). d18O data neither clearly supported nor ruled out Predictions 3 and 4. The large intra-site temporal variability in d18O precluded conclusive confirmation or rejection of these latter predictions. This large variability seems ultimately related to the even larger temporal

variability in the d18O of precipitation at the study site (Fig. 5). For d18O, variability within the two end members (local water and bedrock groundwater) is large relative to the difference between the two endmember waters. This results in a large uncertainty in d18O-based mixing calculations. Mixing calculations based on Cl have much lower uncertainties, mainly because, relative to d18O, Cl exhibits much smaller variability within end members and a larger difference in end-member concentrations (Genereux, 2004). Thus, in this case Cl is the superior tracer for quantitative mixing calculations that show the proportion of IGF in streams and shallow riparian

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Fig. 5. Mean Cl concentrations and d18O values (with standard deviations), March-July 2000, for 14 sampling sites at La Selva: rainfall, Guacimo Spring (GS), and 8 wells and 4 stream sites in Fig. 4. ‘Arbo’ and ‘Taco’ refer to the Arboleda and Taconazo weirs, respectively. d18O is shown increasing downward (higher d18O values are lower on the graph’s vertical axis) to illustrate the similar relative pattern of inter-site variability for the two tracers.

groundwaters at La Selva. However, d18O data offer support for the two-end-member mixing model and the presence of IGF by supporting (or in two cases, not refuting) the five predictions above. The interpretation outlined above for La Selva geochemical data, from Pringle et al. (1990) forward, suggests that IGF is responsible for significant inputs of water and solutes to lowland rainforest watersheds. These inputs seem to significantly augment stream discharge and stream water major ion concentrations, as indicated by the large fWATER and fCl values found by Genereux et al. (2002). A direct quantitative measure of the significance of these inputs for lowland watersheds was provided by a paired-watershed study (Genereux et al., 2005; Jordan, 2003) to determine annual water and major ion budgets (inputs, outputs, changes in storage) for a watershed significantly

393

affected by IGF (the 50 ha Arboleda) and an adjacent watershed having little or no influence from IGF (the 26 ha Taconazo). As adjacent small watersheds (Fig. 4), the Arboleda and Taconazo are identical or nearly so in many respects (vegetation, relief, soil, rainfall, temperature, ET). However, Genereux and Pringle (1997); Genereux et al. (2002) found very different stream chemistry on these watersheds; high solute concentrations in the Arboleda suggested significant IGF of bedrock groundwater, while low concentrations in the Taconazo suggested none. Chemical (major ion) data and physical hydrologic data were used to compute annual water budgets and chemical (NaC, KC, Mg2C, Ca2C, ClK, and SO2K 4 ) budgets for the Arboleda and Taconazo watersheds (Genereux et al., 2005; Jordan, 2003). Water budgets were computed for four ‘budget years’, 12-month periods starting on December 1 (in 1998, 1999, 2000, and 2001); chemical budgets were computed for the latter two budget years. All annual input and output fluxes were normalized by watershed area (water fluxes were expressed in meters of water, chemical fluxes in mol/ha). Standard methods of error propagation (e.g. Genereux, 1998; Kline, 1985; Taylor, 1982) were used to estimate uncertainty in all budget terms; details are in Jordan (2003). IGF has a major effect on the water budget of the Arboleda and is responsible for the large difference between the water budgets of the two watersheds (Fig. 6). Over the four budget years, the Arboleda

Fig. 6. Water budgets for the Arboleda and Taconazo watersheds, December 2001 through November 2002, in meters of water (water volumes normalized by watershed area). The four bars show inputs and outputs for the Arboleda (‘Arbo In’ and ‘Arbo Out’) and inputs and outputs for the Taconazo (‘Taco In’ and ‘Taco Out’).

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received an average of 9957 mm of water per year by IGF, twice the average annual rainfall of 4973 mm. In other words, IGF accounted for about 2/3 of the water input to the Arboleda. Total stream discharge was far in excess of rainfall, providing unambiguous confirmation of IGF to the Arboleda. The water budget for the Taconazo suggested a small input of local water by IGF to this watershed (chemical data indicate no IGF of high-solute bedrock groundwater to the Taconazo); this input was marginally distinguishable from zero and was a minor part of the Taconazo water budget. The overall importance of IGF to the water budget of the Arboleda, and the lack of importance of IGF to the Taconazo water budget, are consistent with the prior chemical results (e.g. Genereux et al., 2002; Genereux and Pringle, 1997). The significant input of local water to the Arboleda by IGF was not (and could not have been) suggested by prior chemical data. This new discovery was brought to light only by combining chemical data with physical hydrologic data (the latter allowed determination of total IGF to the Arboleda watershed, while chemical data allowed total IGF to be separated into bedrock groundwater and local water). The presence of both local water and bedrock groundwater in IGF to the Arboleda watershed may reflect dispersion at the boundary of the local and regional groundwater systems. Dispersion (mixing) across this boundary would produce intermediate major ion concentrations in the flowpaths responsible for IGF. As a result, both local water and bedrock groundwater would contribute to the net watershed input by IGF. If such dispersion is the reason that local water contributes to IGF, it must take place outside the boundaries of the Arboleda watershed, upgradient to the south toward the Cordillera Central (dispersion within the boundaries of the Arboleda would likely involve local water that fell as precipitation onto the Arboleda watershed, which would not show up as IGF in the water budget calculation). IGF to the Arboleda watershed has an even greater effect on chemical budgets than on water budgets (Fig. 7), because the solute concentrations in bedrock groundwater are so much higher than those in local water (Table 1). Averaged over the six major ions (NaC, KC, Mg2C, Ca2C, ClK, and SO2K 4 ), IGF of bedrock groundwater accounts for 92.4% of major ion inputs to the Arboleda (Table 3); IGF of local water accounts for 4.5%, and the remaining 3.1% is due to

Fig. 7. Chloride, calcium, and magnesium budgets for the Arboleda and Taconazo watersheds, December 2001–November 2002, with fluxes in moles of solute per hectare of watershed area. The four bars are analogous to those in Fig. 6.

atmospheric inputs. Thus total IGF (local water plus bedrock groundwater) accounts for about 97% of major ion inputs. IGF to the Arboleda completely dominates the major ion chemistry of the stream water and riparian groundwater at lower elevation (it has not been detected at sites above about 50 m elevation). The exact reason for the difference between the two watersheds (why IGF is so important to the Arboleda and not to the Taconazo) may rest with some unobserved local difference in geology that

D.P. Genereux, M. Jordan / Journal of Hydrology 320 (2006) 385–399 Table 3 Relative magnitudes of three different chemical inputs to the Arboleda watershed, for six major ions, during a 24-month period (12/00–11/02) Solute

Atmos. Input (%)

BGW IGF (%)

LW IGF (%)

NaC KC Mg2C Ca2C ClK SO2K 4 Mean

2.3 1.5 0.3 1.2 4.7 8.5 3.1

93.5 93.8 98.7 97.0 87.4 84.2 92.4

4.2 4.7 1.0 1.8 7.9 7.3 4.5

The three inputs are atmospheric deposition, IGF of high-solute bedrock groundwater (BGW), and IGF of low-solute local water (LW). Each row sums to 100%, and the last row gives mean values for the six solutes.

allows regional groundwater discharge to one watershed but not the other. On the other hand, To´th (1963) has shown that even in homogeneous isotropic materials, a small watershed receiving no IGF may be found adjacent to another with deep groundwater inputs by IGF (it is not necessary to rely on heterogeneity or anisotropy to explain such a finding). While full budgets are not available for watersheds larger than the 50 ha Arboleda, chemical data from larger streams suggest the importance of IGF is not limited to this scale. Dry season fWATER values, averaged over samples from 1994, 1997, and 1998, were 0.49 for the Arboleda, 0.27 for the Sura (210 ha), and 0.41 for the Salto (1250 ha) (Genereux et al., 2002). This suggests a significant influence of IGF in watersheds about 4! and 25! the size of the Arboleda watershed. In any problem involving IGF there may be a scale at which IGF becomes unimportant (a scale that is large enough to include both the recharge and discharge areas for the deep regional groundwater system). If this scale exists for our study site, it is most likely tied to the roughly 35 km distance between La Selva and Volcan Barva (in terms of an area, the scale would be roughly on the order of 352Z1225 km2).

4. Discussion, significance Chemical, isotopic, and physical hydrological data suggest that interbasin groundwater flow (IGF) is an

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important hydrogeological process and a major influence on interaction of groundwater and surface water at La Selva Biological Station in Costa Rica. While it is by definition a groundwater process, IGF at La Selva has been extensively studied through it is effects on surface water. IGF greatly augments volumetric stream discharge and stream water solute concentrations at La Selva. About two-thirds of annual water inputs to the Arboleda watershed (roughly 10 m out of a total of 15 m) are due to IGF. Annual stream discharge from the Arboleda is about 13 m while discharge from the adjacent Taconazo watershed, which is not affected by IGF, is about 3 m. IGF of high-solute groundwater has a major (in some cases, dominating) influence on the major ion concentrations in stream water and riparian groundwater. About 97% of major ions in the Arboleda stream are due to IGF; the proportion is likely similar in other larger streams in the area, based on their elevated major ion concentrations (detailed budget studies have been done only for the Arboleda and Taconazo). Though chemical data suggest IGF influences many streams in the area, some streams are not affected (e.g. the Taconazo); also, IGF appears to be restricted to elevations below about 50 m. Thus, the interaction between surface water and groundwater changes greatly both (1) among watersheds and, (2) with elevation within some watersheds. IGF represents a hydrologic and geochemical influence superimposed to varying degrees (from zero to dominating) on other local ‘within-watershed’ controls on the interaction of groundwater and surface water. This finding has important implications in hydrology, ecology, and land-water management. Documenting and quantifying the long-distance subsurface movement of water and solutes, the mixing between different groundwater systems, and the influence of groundwater on surface water quantity and quality are of fundamental significance in hydrogeology and hydrology. Also, small watershed studies (including budget studies) are important tools in hydrology, geochemistry, and ecology (e.g. Johnson and Van Hook, 1989; Bruijnzeel, 1991; Likens and Bormann, 1995). IGF has been frequently cited as a complicating factor in such studies (e.g. Bruijnzeel, 1991). It is difficult but not impossible to explicitly account for IGF in watershed studies (Genereux et al., 2005;

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Jordan, 2003), and it is important to do so when IGF has the potential to influence, through addition or removal of water and/or solutes, a watershed process under study (e.g. interaction of groundwater and surface water). The early work related to water quality at La Selva (1990–1993 papers cited above) focused mainly on the nutrients nitrogen (N) and phosphorus (P) in stream water, because of their importance to stream ecological processes. The discovery of stream water high in P and having major ion concentrations positively correlated with P led to the original hypothesis concerning input of high-solute groundwater to lowland rainforest watersheds at La Selva (Pringle et al., 1990). IGF of high-solute groundwater is one control on stream ecological processes at the study site. Pringle and Triska (1991) documented P limitation of algal growth in a low-P stream not affected by IGF (the Pantano), and a lack of such limitation in a high-P stream receiving inputs of highsolute groundwater by IGF (the Salto). Apparently the high P levels in the Salto led to limitation of algal growth by another factor (probably light). Also, Pringle et al. (1993) found different algal species and standing crops in low-P and high-P streams. These results indicate that differences in IGF led to different controls on algal growth and different algal communities among otherwise similar lowland rainforest watersheds. Rosemond et al. (2002) found that leaf decay rate, fungal biomass, and invertebrate biomass in La Selva streams increased, up to a point, with stream water SRP concentration. The functional form of the dependence was given by the Michaelis-Menten equation, RZ Rmax ½SRP
=ð½SRP
C Km Þ, where in this case R is one of the three parameters listed above (leaf decay rate, fungal biomass, or invertebrate biomass), Rmax is the maximum value of R at high [SRP], [SRP] is the concentration of soluble reactive phosphorus in the stream water, and Km is the ‘halfsaturation’ constant (the [SRP] value at which RZ 0.5Rmax). Values of Km were 7–13 ppb (parts per billion). Thus, each dependent variable increased rapidly with [SRP] below about 30 ppb, and then slowly leveled off to a plateau (e.g. for KmZ10 ppb, R/Rmax is 0.5, 0.75, and 0.91 at [SRP] values of 10, 30, and 100 ppb, respectively). Ramı´rez et al. (2003) found that microbial respiration rates on leaves in

streams at La Selva showed a similar dependence on [SRP], with half-saturation constants of 3–7 ppb. These studies show that IGF at La Selva, because of its high P content, increases a variety of ecological variables (leaf decay rate, fungal biomass, invertebrate biomass, microbial respiration rates on leaves) in addition to algal growth rate. Some riparian areas at La Selva have significant amounts of high-solute groundwater from IGF (Pringle and Triska, 1991; Genereux et al., 2002), but no work has been done yet on the significance of this for the terrestrial ecology of these areas (e.g. effects on soil microbes or plants). Pringle et al. (1993) speculate on one potential link between riparian forest and stream ecology, for streams receiving high-solute (high-P) IGF. Primary production in at least some of these streams is lightlimited. If that constraint is removed (e.g. through clearing forest to create pasture), algal growth in these streams could be stimulated, with potentially significant subsequent effects on other aquatic ecological processes. This is not likely to occur in streams lacking inputs of high-P groundwater from IGF because of P limitations on growth, both before and (most likely) after forest clearing, in those streams. The scenario above is one example of how the presence of IGF raises considerations for land/water management. This example suggests that receiving IGF may make surface water conditions in a watershed more sensitive to land use within the watershed, compared to a condition without IGF. However, absent the ecological effect discussed above, it could be that IGF makes surface water quality less sensitive to land use within a watershed. If a large portion of stream discharge arises through groundwater input from outside the watershed, then stream water is somewhat ‘buffered’ against changes in quality and quantity that might arise from internal changes in land use or other factors within the watershed. While IGF may be helpful in this regard, it also complicates the management of surface water quality and quantity by making them somewhat dependent on conditions in a regional groundwater system whose size, boundaries, and recharge area(s) may be poorly known (as at La Selva). Clearly, water resource planning that is regional (as opposed to very local) is important in the presence of significant IGF. Also, significant differences in IGF among watersheds

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(as between the Arboleda and Taconazo at La Selva) make it difficult to generalize about watershed-scale controls on chemical fluxes in a given landscape. This could be a source of concern with some contemporary modeling approaches in which stream export variables (e.g. nutrient concentrations or loadings) are regressed against watershed characteristics to obtain relationships that are then applied outside the study watershed. Our existing data and understanding of IGF at La Selva raise a number of water resource management questions, for example: (1) What is the best approach to protecting a lowland rainforest watershed that receives two-thirds of its water and about 97% of its major ions by upward flow of deep groundwater from below? (2) If a stream ecosystem is light-limited because it receives high-P IGF, will clearing forest and increasing light greatly alter stream ecology? (3) Given the watershed budget results showing IGF of both bedrock groundwater and local water (perhaps by mixing across the subsurface interface between the two), how closely coupled are local and regional groundwater systems, and what are the expected effects on one by use of the other? (4) When and to what extent would alterations in a distant recharge area for bedrock groundwater affect water quantity and quality in the lowland watersheds receiving IGF? IGF involves active pathways for long-distance subsurface transport, linking watersheds (through groundwater) that lack direct surface water connections. IGF holds obvious significance for applied hydrologic problems (e.g. contaminant transport) related to the watersheds involved and the interposed deep groundwater system. The receiving watershed has an extra input to account for in a hydrologic and/or geochemical watershed study or management plan; the losing watershed that supplies the IGF has an extra loss. This complicates watershed-based analysis and management but probably not to the extent that a watershed-based approach should be abandoned. Rather, the approach can be modified to consider the possibility of gain or loss through IGF. At least three major priorities can be outlined for improved management of water quantity and quality in the presence of IGF: 1. Find and quantify the IGF. Including IGF in water resource planning obviously requires that one be

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aware of the IGF. Physical hydrologic data (in the context of water budgets) and data on naturally occurring chemical tracers are useful in this regard. Advanced tracer methodologies (including groundwater age-dating) also have the potential to improve detection and quantification of IGF for receiving watersheds. For example, IGF would be suggested by reliable findings of strongly bimodal groundwater age distributions, with some ages typical of those found in small local groundwater systems (several decades or less) and others much greater (e.g. many thousands of years, as is typical of larger regional systems). Improvements in the accuracy and ease of field estimation of actual ET at the watershed scale could contribute to construction of accurate watershed-scale water budgets, thereby facilitating detection and quantification of IGF for both losing and receiving watersheds. 2. Find/protect the recharge area for the IGF. IGF is often detected and quantified well downgradient of the recharge area for the groundwater system involved in the IGF, leaving open the question of the exact location of this recharge area. This is the case at La Selva; it is clear that the recharge area for bedrock groundwater must be to the south in the Cordillera Central, at elevations higher than those at La Selva, but exactly where is not clear. d18O data suggest the recharge area lies about 700 m above sea level, but this may represent some sort of average (perhaps weighted by recharge rate) across a broad recharge zone, and has not yet been corroborated by other data (Genereux, 2004). Nonetheless, tracer-based estimates of groundwater recharge elevation (from stable isotopes, trace gases, or other approaches) may be useful in at least placing constraints on the location of the recharge area for IGF. Traditional subsurface hydrogeologic mapping and collection of hydraulic head data may also be used, and have not at La Selva only because of the extremely poor access and impracticality of this approach in the dense national park jungles upslope of La Selva. Similar constraints may apply in other remote and undeveloped areas, forcing a greater reliance on measures that can be used in the lowland discharge areas (e.g. tracers). 3. Develop predictive hydrologic models. This may be the most challenging component as it requires

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models capable of propagating effects through the linked groundwater-watershed system, to predict the downgradient effects of an upgradient disturbance. New stresses on a regional groundwater system supplying IGF (e.g. a change in land use in the recharge area or in groundwater pumping rate or location) could ultimately be reflected in water quantity and quality in lowland watersheds receiving IGF. Predicting those effects is a difficult but important practical goal. Major challenges include defining appropriate model structures and appropriate values for the many model parameters that would likely be needed. Work at La Selva has largely accomplished item 1 above, has partially accomplished item 2 (more work is under way), and item 3 remains a longer-term goal.

Acknowledgements This work was supported by grants EAR-9800129, EAR-9903243, and EAR-0049047 from the US National Science Foundation. The authors gratefully acknowledge helpful collaborative inputs to some of the work discussed here from Catherine Pringle and Carol Kendall, and key logistical assistance from the Organization for Tropical Studies.

References Alvardo Induni, G.E., 1990. Caracteristicas geologicas de la Estacion Biologica La Selva, Costa Rica. Tecnologica En Marcha 10 (3), 11–22. Bourgeois, W.W., Cole, D.W., Riekerk, H., Gessel, S.P., 1972. Geology and soils of comparative ecosystem study areas, Costa Rica. Report of College of Forest Resources, University of Washington, in cooperation with OTS, p. 41. Bruijnzeel, L.A., 1991. Nutrient input-output budgets of tropical forest ecosystems: a review. Journal of Tropical Ecology 7, 1–24. Castillo-Munoz, R., 1983. Geology. In: Janzen, D.H. (Ed.), Costa Rican Natural History. University of Chicago Press, Chicago, pp. 47–62. Eakin, T.E., 1966. A regional interbasin groundwater system in the White River area, southeastern Nevada. Water Resources Research 2 (2), 251–271. Edmunds, W.M., Carrillo-Rivera, J.J., Cardona, A., 2002. Geochemical evolution of groundwater beneath Mexico City. Journal of Hydrology 258, 1–24.

Foster, S.S.D., Ellis, A.T., Losilla-Penon, M., Rodriguez-Estrada, H.V., 1985. Role of volcanic tuffs in groundwater regime of Valle Central, Costa Rica. Ground Water 23 (6), 795–801. Genereux, D.P., Wood, S., Pringle, C.M., 2002. Chemical tracing of interbasin groundwater transfer in the lowland rainforest of Costa Rica. Journal of Hydrology 258, 163–178. doi:10.1016/ S0022-1694(01)00568-6. Genereux, D.P., Pringle, C.M., 1997. Chemical mixing model of streamflow generation at La Selva Biological Station, Costa Rica. Journal of Hydrology 199, 319–330. doi:10.1016/S00221694(96)03333-1. Genereux, D.P., 1998. Quantifying uncertainty in tracer-based hydrograph separations. Water Resources Research 34 (4), 915–920. Genereux, D.P., 2004. Comparison of naturally-occurring chloride and oxygen-18 as tracers of interbasin groundwater transfer in lowland rainforest, Costa Rica. Journal of Hydrology 295, 17–27. doi:10.1016/j.jhydrol.2004.02.020. Genereux, D.P., Jordan, M.T., Carbonell, D., 2005. A pairedwatershed budget study to quantify interbasin groundwater flow in a lowland rainforest, Costa Rica. Water Resources Research, 41: W04011. doi:10.1029/2004 WR 003635. in press. Hem, J.D., 1985. Study and Interpretation of the Chemical Characteristics of Natural Water. US Geological Survey Water Supply Paper 2254, C3 plates, 263 pp. Herczeg, A.L., Edmunds, W.M., 2000. Inorganic ions as tracers. In: Cook, P., Herczeg, A.L. (Eds.), Environmental Tracers in Subsurface Hydrology. Kluwer Academic Publishers, Boston, pp. 31–78. Hudson, M.R., Mott, D.N., 1997. Faulting and coincident interbasin ground-water flow in a karst aquifer, Buffalo National River region, northwestern Arkansas. Geological Society of America Abstracts with Programs 29 (6), 181–182. Ingraham, N.L., 1998. Isotopic variations in precipitation. In: Kendall, C., McDonnell, J. (Eds.), Isotope Tracers in Catchment Hydrology. Elsevier Science Publishers, Amsterdam, pp. 87–118. Johannesson, K.H., Zhou, X., Stetzenbach, K.J., 1995. Rare earth element distributions in groundwaters and evidence of interbasin flow in the desert southwest. Geological Society of America Abstracts with Programs 27 (6), A96. Johannesson, K.H., Stetzenbach, K.J., Hodge, V.F., 1997. Rare earth elements as geochemical tracers of regional groundwater mixing. Geochimica et Cosmochimica Acta 61, 3605– 3618. Johnson, D.W., Van Hook, R.I., 1989. Analysis of Biogeochemical Cycling Processes in Walker Branch Watershed. SpringerVerlag, New York, USA (401 pages). Jordan, M., 2003. Effects of interbasin groundwater transfer on water and chemical budgets in lowland rainforest watersheds, La Selva Biological Station, Costa Rica. MS Thesis, Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, North Carolina, USA, 203 pp. Kline, S.J., 1985. The purposes of uncertainty analysis. Journal of Fluids Engineering 107, 153–160.

D.P. Genereux, M. Jordan / Journal of Hydrology 320 (2006) 385–399 Lachniet, M.S., Patterson, W.P., 2002. Stable isotope values of Costa Rican surface waters. Journal of Hydrology 260, 135–150. Likens, G.E., Bormann, F.H., 1995. Biogeochemistry of a Forested Ecosystem, second ed. Springer-Verlag, New York 160 pp. Locsey, K.L., Cox, M.E., 2003. Statistical and hydrochemical methods to compare basalt- and basement-rock-hosted groundwaters: Atherton Tablelands, northeastern Australia. Environmental Geology 43, 698–713. Loescher, H.W., 2002. Ecosystem-level responses of carbon and energy from a tropical wet forest in Costa Rica. PhD Dissertation, University of Florida, Gainesville, FL, 90 pp. Ludington, S., Castillo, R., Azuola, H., 1996. Geology of Costa Rica. In: Schruben, P.G. (Ed.), Geology and Resource Assessment of Costa Rica at 1:500,000 scale: a Digital Representation of Maps of the US Geological Survey’s 1987 Folio, I-1865. US Geological Survey, Reston, Virginia, USA. Luvall, J.C., 1984. Tropical deforestation and recovery: the effects on the evapotranspiration process. PhD Dissertation, University of Georgia, Athens, Georgia, USA. Maxey, G.B., 1968. Hydrogeology of desert basins. Ground Water 6 (5), 10–22. Naff, R.L., Maxey, G.B., Kaufmann, R.F., 1974. Interbasin groundwater flow in southern Nevada. Nevada Bureau of Mines and Geology Report 20, Reno, Nevada, USA, 28 pp. OTS, 2003. Organization for Tropical Studies, Duke University, Durham, North Carolina, USA. Meteorological data for La Selva Biological Station, available at http://www.ots.duke.edu/ en/laselva/metereological.shtml. OVSICORI-UNA, 2003. Observatorio Vulcanologico y Sismologico de Costa Rica - Universidad Nacional, Heredia, Costa Rica. Monthly summaries of volcanic and seismological activity in Costa Rica, available at www.ovsicori.una.ac.cr. Parker, J.M., Foster, S.S.D., Gomez-Cruz, A., 1988. Key hydrogeological features of a recent andesitic complex in Central America. Geolis II (1), 13–23. Pringle, C.M., Triska, F.J., Browder, G., 1990. Spatial variation in basic chemistry of streams draining a volcanic landscape on Costa Rica’s Caribbean slope. Hydrobiologia 206, 73–85. Pringle, C.M., Rowe, G.L., Triska, F.J., West, J., 1993. Landscape linkages between geothermal activity and solute

399

composition and ecological response in surface waters draining the Atlantic slope of Costa Rica. Limnology and Oceanography 38 (4), 753–774. Pringle, C.M., 1991. Geothermally modified waters surface at La Selva Biological Station, Costa Rica: volcanic processes introduce chemical discontinuities into lowland tropical streams. Biotropica 23 (4b), 523–529. Pringle, C.M., Triska, F.J., 1991. Effects of geothermal groundwater on nutrient dynamics of a lowland Costa Rican stream. Ecology 72, 951–965. Ramı´rez, A., Pringle, C.M., Molina, L., 2003. Effect of stream phosphorus levels on microbial respiration. Freshwater Biology 48, 88–97. Rosemond, A.D., Pringle, C.M., Ramı´rez, A., Paul, M.J., Meyer, J.L., 2002. Landscape variation in phosphorus concentration and effects on detritus-based tropical streams. Limnology and Oceanography 47 (1), 278–289. Sanford Jr.., R.L., Paaby, P., Luvall, J.C., Phillips, E., 1994. Climate, geomorphology, and aquatic systems. In: McDade, L.A., Bawa, K.S., Hespenheide, H.A., Hartshorn, G.S. (Eds.), La Selva: Ecology and Natural History of a Neotropical Rainforest. The University of Chicago Press, Chicago, Illinois, USA, pp. 19–33. Sollins, P., Sancho, F., Mata, R., Sanford Jr.., R.L., 1994. Soils and soil process research. In: McDade, L.A., Bawa, K.S., Hespenheide, H.A., Hartshorn, G.S. (Eds.), La Selva: Ecology and Natural History of a Neotropical Rainforest. The University of Chicago Press, Chicago, Illinois, USA, pp. 34–53. Taylor, J.R., 1982. An Introduction to Error Analysis: The Study of Uncertainty in Physical Measurements. University Science Books, Mill Valley, California, USA. Thyne, G.D., Gillespie, J.M., Ostdick, J.R., 1999. Evidence for interbasin flow through bedrock in the southeastern Sierra Nevada. Geological Society of America Bulletin 111 (11), 1600–1616. To´th, J.A., 1963. A theoretical analysis of groundwater flow in small drainage basins. Journal of Geophysical Research 68 (16), 4795–4812.