Seasonal isotope hydrology of a coffee agroforestry watershed in Costa Rica

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Precipitation (mm)

Streamflow (mm) Streamflow

Precipitation

Seasonal isotope hydrology of a coffee agroforestry watershed in Costa Rica

J. Boll ¹ , K. Welsh* ^1,2 , O. Roupsard ^2,3

1

University of Idaho, Moscow, ID, USA;

²

Centro Agronómico Tropical de Investigación y Enseñanza (CATIE), Turrialba, Costa Rica;

³

UMR Eco&Sols, CIRAD, Montpellier, France

H21F-0794

Introduction

In the tropics, the “amount effect” has been

recognized as one of the primary influences on stable isotope values in tropical precipitation for monthly samples (Figs 1 and 2). However, when examining event-based precipitation events at a regional scale, lifting condensation levels and

surface relative humidity have a greater influence (Sánchez-Murillo et al. 2014). The main goals of this research were to examine what microclimate factors influence local stable isotope (δ

¹⁸

O and δD) values and whether stable isotopes can be used to trace seasonality in this region at different temporal scales.

Methods

• Sampled precipitation (event-based samples),

groundwater (weekly samples), and stream water (weekly samples at 4 locations) for 2+ years.

• Micrometeorological data were collected at an eddy-flux tower located on site.

• Study Site: The Mejías watershed (Cartago, Costa Rica) is an agroforestry microwatershed (0.9 km

²

) located on the Aquiares Coffee Farm.

Results

Acknowledgments

This work was funded by an NSF IGERT (Integrative Graduate Education and Research Traineeship) and a U.S.

Borlaug Fellowship in Global Food Security. The CoffeeFlux project provided site infrastructure and the Aquiares Farm provided site access. The authors would like to thank Alvaro Barquero, Alejandro Barquero, Alexis Pérez, and Amilkar Moncada for field assistance.

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δ

18

O (‰)

Monthly δ

¹⁸

O values: Study data

MSD MSD

Fig. 5. Monthly averaged values for δ

¹⁸

O in precipitation at the study site.

Precipitation isotope values were correlated with air temperature, wind speed, and wind direction.

Fig. 6. The Local Meteoric Water Line (LMWL) for precipitation samples (collected between Aug 2011 and Dec 2013) for the watershed.

y = 8.4987x + 18.02

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δD (‰)

δ¹⁸O (‰)

LMWL

Precipitation Groundwater Streamwater -15

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Jan-02 Apr-02 Jul-02 Oct-02 Feb-03 May-03 Aug-03 Dec-03

δ

18

O (‰)

Monthly δ

¹⁸

O values: Historic GNIP data

MSD MSD

Dry Dry

Rainy Rainy

Dry Dry

Rainy Rainy

Fig. 4. Monthly averaged values of δ

¹⁸

O in precipitation from historic GNIP database for Turrialba. MSD = Mid-Summer Drought.

F. G´omez-Delgado et al.: Modelling hydrological behaviour of coffee agroforestry basin in Costa Rica 371 Figure 1

Fig. 1. (a) Location of Reventaz ´on river basin in Costa Rica, Central America. (b) Position of experimental basin inside of Reventaz ´on basin. (c) The “Coffee-Flux” experimental basin in Aquiares farm and its experimental setup to measure the water balance components.

In the present study we attempted to couple two lumped models into a new integrative one, chosen to be scalable and parsimonious: a basin reservoir model similar to the CREC model (Cormary and Guilbot, 1969) and employing the Diskin and Nazimov (1995) production function as pro- posed by Moussa et al. (2007a,b), and the SVAT model pro- posed by Granier et al. (1999). While the basin model was considered appropriate for its simplicity and capacity to sup- port new routines, the SVAT model was chosen for its par- simony (three parameters in its basic formulation), its ro- bustness (uses simple soil and stand data in order to pro- duce model runs for many years, avoiding hydraulic param- eters that are difficult to measure and scale up), its ability to quantify drought intensity and duration in forest stands, and for its successful past validation in various forest stands and climatic conditions, including tropical basins (Ruiz et al., 2010).

This paper aims to explain and model the hydrological behaviour of a coffee AF micro-basin in Costa Rica, as- sessing its infiltration capacity on andisols. The method- ology consists of experimentation to assess the main water fluxes and modelling to reproduce the behaviour of the basin.

First, we present the study site and the experimental design.

Second, we develop a new lumped eco-hydrological model with balanced ecophysiological/hydrological modules (that we called Hydro-SVAT model). This model was tailored to the main hydrological processes that we recorded (stream- flow, evapotranspiration, water content in the non-saturated

zone and water table level) and that are described in the sub- sequent sections. Third, we propose a multi-variable calibra- tion/validation strategy for the Hydro-SVAT model so we cal- ibrate using the streamflow in 2009 and validate using the re- maining three variables in 2009 and the streamflow in 2010.

Fourth, we make an uncertainty analysis to produce a confi- dence interval around our modelled streamflow values, and a sensitivity analysis to assess from which parameters this uncertainty might come. Fifth, we attempt to simplify the initially proposed model based on the results of the sensitiv- ity analysis. Finally, we discuss the main findings concerning the water balance in our experimental basin.

2 The study site

2.1 Location, soil and climate

The area of interest is located in Reventaz ´on river basin, in the Central-Caribbean region of Costa Rica (Fig. 1a and b).

It lies on the slope of the Turrialba volcano (central vol- canic mountain range of the country) and drains to the Caribbean Sea. The Aquiares coffee farm is one of the largest in Costa Rica (6.6 km²), “Rainforest Alliance^TM” certified, 15 km from CATIE (Centro Agron´omico Tropical de Investigaci ´on y Ense ˜nanza). Within the Aquiares farm, we selected the Mej´ıas creek micro-basin (Fig. 1c) for the

“Coffee-Flux” experiment. The basin is placed between the www.hydrol-earth-syst-sci.net/ 15/369/2011/ Hydrol. Earth Syst. Sci., 15, 369–392, 2011

Fig. 1. Location of study site. Source: Gómez-Delgado et al. 2011

References

Gómez-Delgado, F.., et al. 2011. Modelling the

hydrological behaviour of a coffee agroforestry basin in Costa Rica. Hydrology and Earth System Sciences 15(1):

369-392.

Sánchez-Murillo, R., et al. 2014. Spatial and temporal variation of stable isotopes in precipitation across Costa

Rica: An analysis of historic GNIP records. Open Journal of Modern Hydrology 3: 226-240.

Fig. 9. Hourly values of δ

¹⁸

O (‰) measured in stream water (11/26/12 – 12/1/12), compared with stream flow and precipitation levels. Both δ

¹⁸

O and δD are positively correlated with precipitation amount (p=0.02 and 0.00005).

Discussion

• For monthly averaged samples (Figs 4 and 5), dry season precipitation is enriched with respect to the rainy season, but at an hourly or daily

basis other trends are evident, such as

correlation with meteorological measurements and stream response to precipitation events (Fig 9).

• Air temperature, wind speed, and wind direction are all significantly correlated with δ

¹⁸

O and δD values in precipitation, which could be due to seasonal differences in air mass sources.

• Examining stream water δ

¹⁸

O and δD values at a finer scale (Fig. 9) shows how streams respond to precipitation, as the stream becomes more enriched with higher amounts of rain due to

source contribution of groundwater and overland flow from spring and roads (Figs. 10 and 11).

• Stable isotopes are useful for examining

seasonality and watershed dynamics when examined at different temporal scales. For

example, measuring isotopes in stream samples during storm events assists in our understanding of when storm flow reaches streams.

• Further work will include partitioning stream

water sources and quantifying spring contribution to storm flow.

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δ18 O (‰)

δ

¹⁸

O in groundwater

WTL-1 WTL-2 WTL-4 WTL-5

Fig. 7. δ

¹⁸

O values in groundwater (weekly samples) at four groundwater wells: WTL-1 and WTL-2 (low elevations), WTL-4 (mid-elevation), and

WTL-5 (high elevation).

*Contact: Kristen Welsh

wels8152@vandals.uidaho.edu

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δ

18

O (‰)

δ

¹⁸

O in stream water

Flume Lower Middle Upper

Fig. 8. δ

¹⁸

O values in stream water (weekly samples) at four locations in the watershed. Values generally become enriched in δ

¹⁸

O from upper elevations to lower elevations.

Fig 2. The land cover at the site is dominated by Coffea arabica coffee plants

interspersed with Erythrina poeppigiana shade trees.

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δ18 O (‰)

δ

¹⁸

O, streamflow, and precipitation (hourly)

O18

Fig 3. The study site is part of the global

network of FLUXNET micrometeorological sites.

Storm flow Low flow

Fig. 11. Low flow conditions compared with storm flow conditions at the flume.

Fig. 10. Intermittent springs (a) and road and path runoff (b) at the site contribute significantly to storm flow.

a b