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The Galápagos archipelago: a natural laboratory to examine sharp hydroclimatic, geologic and anthropogenic gradients

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Poor understanding of the water cycle in tropical ecosystems has the potential to exacerbate water shortages and water crises in the region. We suggest that the Galápagos Islands provide an excellent proxy to regions across the tropics as a result of sharp hydroclimatic, anthropogenic, and pedohydrologic gradients across the archipelago. Hydroclimatic and pedohydrologic gradients are found across different elevations on single islands, as well as across the archipelago, whereas anthropogenic gradients reflect land use and land cover change across islands as population and growth in tourism have affected individual islands differently. This article highlights specific opportunities to further examine our understanding of the interactions between water and critical zone processes in tropical ecosystems, making connections between the Galápagos archipelago and much of the understudied tropics. The Galápagos archipelago offers a natural laboratory through which we can examine current threats to freshwater security as well as the dynamics of coupled natural and human systems. WIREs Water 2016, 3:587–600. doi: 10.1002/wat2.1145

Conceptual models of the hydrogeology of Santa Cruz and San Cristóbal, the most highly populated islands, based on SkyTEM surveys, geochemical validation, and our interpretation of work by Violette et al.
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Map of climate zones on Galápagos (after Huttel) illustrating how climate zones are distributed based on elevation above sea level and wind direction. Note that several volcanoes on Isabela and the primary vent on Fernandina are calderas, and thus, have different climates because of an orographic effect, where the high walls of the caldera shield the inside of the caldera. Older islands with weathered topography no longer have distinctive calderas, so the orographic effect is felt across the entire island.
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Conceptual diagrams illustrating our hypotheses regarding elevation‐controlled gradients across the Galápagos. (a) With an increase in elevation, the importance of fog‐water input into the water budget will increase, with a large proportion of annual water input in the highlands coming from occult precipitation. At the dry coastal elevations which receive water from convective rainfall, the importance of fog water as an input into the groundwater system decreases to zero. (b) As a result of land use change on the archipelago, we suggest that invasive (i.e., ‘transformer’) species in the Galápagos have affected the highland areas more than they have the lowlands, largely due to greater water availability with higher elevations. (c) The increase in the amount of water inputs to the system results in soils with a lower hydraulic conductivity and higher soil‐moisture retention due to high concentrations of hydrated clays at high elevations, and high hydraulic conductivity and low soil‐moisture retention at low elevations due to the limited amount of water received along the coast, the nature of the precipitation at low elevations, and the structural differences in the soil.
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Preliminary calculation of the normalized difference built‐up index (NDBI) from Landsat imagery captured in 2002 and 2014 on San Cristóbal Island. Red indicates built‐up/barren areas in 2002 and 2014, respectively, and green in the 2014 image indicates revegetated areas in the 12‐year span. Note that cinder cones and younger lava flows exist in the NW portion of the island that has a notable barren appearance in both images. Circles indicate principal areas of change of barren/built‐up land area over the intervening 12 years—such changes in NDBI are a result of increased infrastructure development and beach erosion.
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Thirteen‐year (1995‐2008) averaged monthly means for δ18O in precipitation. Data in this original figure are derived from the Global Network of Isotopes in Precipitation dataset at a single coastal station located ~200 M.A.S.L., Santa Cruz Island, Galápagos. A seasonal shift in the δ18O is hypothesized to represent the different types of precipitation contributing to the ecosystem. Between December and May of non‐El Niño years, the islands are dominated by convective rainfall events; between June and November, the lowlands are extremely dry and the highlands experience a nearly constant fog. Note that the ranges of δ18O values in the precipitation are smaller for fog inputs than for the convective rainfall inputs.
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Science of Water > Stocks and Flows of Water
Water and Life > Nature of Freshwater Ecosystems

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