Elsevier

Applied Geography

Volume 78, January 2017, Pages 94-103
Applied Geography

Exploring hydrologic connections between tropical mountain wetlands and glacier recession in Peru's Cordillera Blanca

https://doi.org/10.1016/j.apgeog.2016.11.004Get rights and content

Highlights

  • Land cover and environmental data from Peru were used in an econometric model.

  • The model assessed wetland hydrologic connectivity from 1987 to 1995.

  • Results show changing glacier area and stream discharge drive wetland change.

  • This approach is useful when altered hydrological and climate regimes cause change.

  • More integration of econometric methods may facilitate interdisciplinary research.

Abstract

Receding mountain glaciers affect the hydrology of downslope ecosystems with consequences for drinking water, agriculture, and hydropower production. Here we combined land cover derived from satellite imagery and other environmental data from the northern Peruvian Andes into a first differencing regression model to assess wetland hydrologic connectivity. Wetland area was considered the response variable and a variety of land cover, climatic, and stream discharge explanatory variables were tested to evaluate effects of possible hydrologic connectivity. The results indicate that there were two primary spatial driving forces of wetland change in Peru's Cordillera Blanca from 1987 to 1995: 1) loss in glacier area was associated with increased wetland area, controlling for other factors; while 2) an increase in mean annual stream discharge in the previous 12 months increased wetland area. The general approach we used expands the ways that connectivity between landscape changes and hydrologic and ecosystem processes can be assessed.

Graphical abstract

This research proposes relationships among peak water, glacier recession, changing glacier discharge, wetland area, and wetland ecosystem services with the approximate period of this study indicated by the grey shaded area.

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Introduction

Landscape change can result from shifting hydrological processes and, in turn, feedbacks through altered evapotranspiration, runoff, and water storage can further influence landscape arrangement and composition. Landscape changes detected by satellite observations provide insights into the hydrologic connectivity that characterizes these processes, although quantifying the effects is complicated by high natural variability, typically unmeasured groundwater interconnections, and incomplete data archives (DeFries and Eshleman, 2004, Wohl et al., 2012). In this research, we adapt an econometric model to examine the influence of glacier retreat on downslope wetland ecosystems. To do this, we utilized an historic stream discharge dataset for selected Santa River watersheds in the Cordillera Blanca of northern Peru and compared it to land cover change derived from satellite imagery. The general approach can be adapted to study many other situations where the relationship between hydrologic connectivity and environmental drivers of change is in need of assessment (e.g. Ponette-González et al., 2014).

In joining econometric modeling with spatial categorical data, the explanatory variables can be conceived of as spatial driving forces, with ancillary datasets representing variables such as precipitation, evapotranspiration, or topography that can further specify the model (Chowdhury, 2006). Response variables can similarly be extracted from spatial categorical data derived from remotely-sensed imagery. Multiple logistic regression is often used (Cui et al., 2014, Mertens and Lambin, 2000), which provides a probability estimate of the effect of the explanatory variables on the response variable (Wooldridge, 2009). Another technique involves panel data and fixed effects regression (Andreß, Golsch, & Schmidt, 2012), which relies on a simple design: make each observation its own control. In this case, for each watershed we can estimate the effects of explanatory variables and control for unobserved variables. This is useful in studies where a non-experimental design is the only available strategy to evaluate interconnections. The response variable must be measured on two occasions at a minimum and the measurements must be directly comparable over time; additionally, the explanatory variables must exhibit variation across time (Allison, 2009). In contrast to multiple logistic regression that measures a probability response for change from one land cover to another, fixed effects regression generates an estimate of the relative effect of an explanatory variable over time, thus explicitly estimating how changes in the explanatory variables affect the response variable temporally (Wooldridge, 2009); doing so eliminates time-constant effects, both observed and unobserved. When two time periods are used, the technique is referred to as first differencing (Wooldridge, 2009).

Here we utilize the changes in wetland area as a way to evaluate hydrologic connectivity as affected by the glaciers themselves, and by other hydrologic and climatologic variables. Like many glaciated mountains nowadays, ice cover in the Cordillera Blanca is decreasing (Kaser, 1999, Rabatel et al., 2013, Thompson et al., 2006). Permanent ice cover declined from 723 km2 in 1970 (Kaser, Ames, & Zamora, 1990) to 482 km2 by 2010 (Burns & Nolin, 2014). Ice loss in the high Andes is associated with multiple social and biophysical consequences including changing water supplies for human uses as well as ecosystem integrity (Bradley, Vuille, Diaz, & Vergara, 2006), a coupled relationship that has been observed globally (Viviroli, Dürr, Messerli, Meybeck, & Weingartner, 2007). The effects on stream discharge are often region-specific based on climatological differences. There is, however, a widely accepted conceptual model that broadly describes the impact of glacier recession on stream discharge. Introduced by Braun, Weber, and Schulz (2000), the model has been reproduced in a number of studies with each contributing slight variations (Hock et al., 2005, Jansson et al., 2003, Moore et al., 2009, Nolin et al., 2010, Rees and Collins, 2006). More recently conceptualized as “peak water” by Carey et al. (2014), the model characterizes the impact of glacier retreat on surface hydrology by an initial increase in annual stream discharge during the early stages of deglaciation, followed by a decline in stream discharge as the glacier volume becomes too small to sustain flows. In addition to change in mean annual discharge, the hydrological transformation is characterized by an increase in discharge variability (Collins, 2008).

Recently, Baraer et al. (2012) have explored the peak water concept for the specific case of the Cordillera Blanca. Their study suggests that most of the streams in the Cordillera Blanca have undergone a decline in annual and dry season stream discharge since the 1980s that is related to glacier retreat. In addition to decreasing mean annual discharge, an increase in the annual variability of flows was observed. The researchers predicted that when glaciers lose their hydrological influence, yearly stream discharge should stabilize at a slightly lower level than earlier and dry season discharge is anticipated to be lower. Baraer et al. (2012) corroborated conclusions from previous studies focusing on the Cordillera Blanca: Juen, Kaser, and Georges (2007) and Vuille, Kaser, and Juen (2008) showed that reductions in glacier area may result in no major net change in annual total runoff, but seasonal variation should increase. In 2005, Pouyaud et al. (2005) projected a similar stream discharge evolution, but in a different timeframe.

Much of the high mountain landscapes within the Cordillera Blanca are within the boundary of Huascarán National Park where valley peatlands and wet meadows (together referred to herein as wetlands) form patches that are changing spatially. A case study from one valley, Quilcayhuanca, evaluated changes from 2000 to 2011. Over the 11-year interval, wetlands in Quilcayhuanca became smaller and more fragmented (Bury et al., 2013). Other research performed at the scale of the park (all valleys inside the park) found that wetland spatiotemporal change trends are nonlinear with wetlands increasing in area from 1987 to 1999, then decreasing in area from 1999 to 2010 (Polk, 2016). Relevant to this study is that in 1987, wetland occupied 12,402 ha and by 1995, wetland had increased to 13,858 ha, an 11.7% increase over 8 years (Polk, 2016). The wetlands receive input from groundwater, precipitation, and surface water flows, while they also contribute to aquifers and streamflow (Gordon et al., 2015, Maldonado Fonkén, 2015). Outflows from peatlands are seasonally variable, increasing during rainy periods and decreasing in the dry season when evapotranspiration is higher and inputs are lower (Rydin & Jeglum, 2013).

We explored the possible statistical relationships between area of wetlands and shifting hydrologic processes by testing seven hypotheses (Table 1). Because wetlands are spatially proximal to glaciers in the study area (Fig. 1), change in glacier area should be associated with change in wetland area (H1). In most Cordillera Blanca watersheds, proglacial lakes dammed by moraines (or engineered impoundments) store glacier meltwater. Ice loss is associated with increasing lake surface area and volume (Emmer, Vilímek, Klimeš, & Cochachin, 2014), so wetlands could be changing as a statistical function of both glaciers and lakes (H2). Baraer et al. (2012) showed that the Santa River discharge is decreasing while variability is increasing as glaciers recede, so changes in discharge and glacier area could be used jointly to estimate wetland area change (H3). By combining those three hypotheses, changes in glacier area, lake area, and discharge can be used as explanatory variables (H4). Some wetlands in the Peruvian Andes are thought to be groundwater-supported fens (e.g. Cooper et al., 2010), but in the study area wetlands may be maintained only by precipitation (bogs). As such, H5 introduces a lagged precipitation variable in place of lake area to address this possibility, and maintains the relationship between glacier area and discharge; a lagged variable is used because some water held in wetlands may be stored from prior rainy seasons, although the residence time is unknown (Gordon et al., 2015). In H6, we include four variables, changes in glacier area, discharge, lake area, and precipitation, to estimate wetland change. Finally, it may be that changing discharge and precipitation may alone predict wetland change (H7), as glaciers recede, assuming that the influence of meltwater will diminish and precipitation will be the key driver of streamflow (Baraer et al., 2012, Mark and Seltzer, 2003).

Section snippets

Study area

The Cordillera Blanca mountain range is located between 8.5 - 10° S and 77° 00′ - 77° 50′ W and is oriented on a northwest-southeast axis. It is comprised of a series of parallel valleys, each its own watershed in the Santa River basin (Fig. 2). The scale of analysis was the watershed level and we evaluated seven watersheds within the Santa River basin. All sample watersheds are located on the west side of the Cordillera Blanca and drain into the Santa River, which flows to the Pacific Ocean.

Results

Wetland change could be best predicted from changes in glacier area and mean annual discharge, although prior precipitation was also statistically significant (Table 4, Table 5). First, the bivariate regressions were computed to test H1 and to determine whether or not any of the explanatory variables might individually explain change in wetland area. One variable, prior precipitation (PrePrior) was highly statistically significant (p < 0.001, two-tailed) and the within R2 was 0.5571 (Table 4).

Discussion

Connectivity between land change and hydrologic processes are not well understood in tropical mountain regions, but the methods and results presented here show that a spatially explicit econometric analysis demonstrates a promising approach that can be further developed and used by land change scientists and land managers preparing for change associated with glacier recession. We are aware of only one other study located in the tropics that investigated the connections between landscape and

Conclusion

Decadal evaluations of other similar high mountain ecosystems show that wetland area may increase (H. Li et al., 2014, Postigo et al., 2008), decrease (Bury et al., 2013; Z. W.; Li et al., 2015, Nie and Li, 2011), exhibit stasis (Gao, Li, & Brierley, 2012), or both increase and decrease (Yao, Zhao, Gao, Sun, & Li, 2011). More could be done to evaluate effects of hydrologic connectivity (DeFries and Eshleman, 2004, Wohl et al., 2012), especially in light of the possible nonlinear relationships

Acknowledgments

This work was funded by the National Science Foundation under the Coupled Natural and Human Systems Program (“Hydrologic Transformation and Human Resilience to Climate Change in the Peruvian Andes” Award Number 1010381); the National Science Foundation under the Doctoral Dissertation Research Improvement Grant (“Glacier Recession and Implications for High-Elevation Peatlands” Award Number 1333141); the University of Texas Department of Geography and the Graduate School; and the American

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