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Are catchments leaky?

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Abstract Catchments, generally understood as the drainage areas of low‐order streams, are often regarded as closed hydrologic entities; that is, precipitation (P) minus evapotranspiration (ET) over a catchment equates stream outflow (Q r). Here, we review evidence that catchments can be leaky due to groundwater outflow or inflow across topographic divides, based on catchment mass balance across a continent and several site‐based studies across the globe. It appears that a catchment is more likely to be leaky with the combination of the following factors: small catchment size, positioned at either the high or low end of a steep regional topographic and climatic gradient, underlain by deep permeable substrates that extend beyond the study catchment, and in drier climate or dry seasons and droughts. Catchment leakage has hydrological, geochemical, and ecological implications. Thus, catchments are best framed as semiclosed hydrologic units perched on top of a larger, regional hydrogeological system with no real boundaries regarding the movement of water and solutes. This article is categorized under: Science of Water > Hydrological Processes Water and Life > Nature of Freshwater Ecosystems
(a) Catchment water balance, (b) catchments in a regional context (block diagram from Winter, Harvey, Franke, & Alley, ; USGS 1998), and (c) schematic end members along a regional gradient: Headwater versus lower basin (red), river exporting versus importing (green), groundwater exporting versus importing (blue), with the exchange between surface and subsurface drainage systems facilitated by losing vs. gaining streams (black). R stands for local groundwater recharge
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(a) Number of months per year when evapotranspiration (ET) exceeds P in a 1 x 1o box, (b) annual ET from either the storage of past rain or lateral inflow from neighbor boxes within a month (Reprinted with permission from Kuppel et al. (). Elsevier 2017)
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(a) Maps showing the topography and location of Chaco dry‐land forests, (b) a conceptual model of Andes precipitation, river outflow and seepage into regional aquifers at the foothills, groundwater flow to the east sustaining plants in the valleys and wetlands of the Desaguadero River, (c) 18O signature of plant xylem water from rain in the wet season and groundwater capillary rise in the dry season. (Reprinted with permission from Jobbágy et al. (). Wiley 2011)
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(a) Conceptual model of a karst system including characteristic karst processes (Reprinted with permission from Hartmann, Goldscheider, Wagener, Lange, and Weiler (). Wiley 2014); note that (moving) groundwater divide and the surface drainage and recharge divide (separating autogenic from allogenic recharge) do not always coincide, (b) conceptual model of Jura Mountains karst system showing the recharge points (black dots) conduits (black lines) in the vadose zone, and the conduits in the saturated phreatic zone (red lines, color scaled by drainage area); the catchment area for each spring can be traced back to the recharge points (Reprinted with permission from Malard, Jeannin, Vouillamoz, and Weber (). Springer Nature 2015)
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(a) River basins of Spain showing the Segura basin (https://en.wikipedia.org/wiki/ Geography_of_Spain#/media/File:Spain_River_Basins‐en.png), (b) headwater catchments of the Segura basin, and (c) stream gages and aquifers, showing the lack of overlap between catchment and aquifer boundaries, and the inter‐catchment groundwater flow (black arrows). (Reprinted with permission from Pellicer‐Martínez and Miguel Martínez‐Paz (). Elsevier 2014)
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(a) Topography of the Daly River catchment (Rick van Dam, Australian Natl Ctr for Tropical Wetland Res), (b) springs and carbonate aquifer outcrop, (c) deep regional groundwater flow discharging at springs (b and c Reprinted with permission from Smerdon, Gardner, Harrington, and Tickell (). Elsevier 2012)
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(a) Topographic map of Costa Rica (from Eric Gaba—Wikimedia commons user: Sting), (b) river network and La Selva Biological Station, (c) wetlands (shaded) and sampling sites (Genereux et al., ), (d) the paired catchments, and (e) carbon budget from the paired catchments (Reprinted with permission from Genereux et al. (). Wiley 2013)
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Map of the El Rito and Canjilon headwater catchments. Spring sample points are in pink, and stream sample points in green boxes. White arrow marks the transition from gaining to losing reach of the stream in El Rito. Yellow and blue lines mark cross‐sections shown to the right. (Reprinted with permission from Frisbee et al. (). Wiley 2016)
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Catchment H (zero‐order) and R (first‐order) and observation points (Reprinted with permission from Iwasaki et al. (). Taylor & Francis 2016). In the smaller subcatchment H, deep groundwater loss was 37–45% of annual P, and the DIN export by groundwater is 34–76% of total DIN output
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Groundwater flow under a stream; (a) a cross‐section view showing the simultaneous “Baseflow” into a gaining stream, and the “Underflow” in the valley alluvium, down the valley axis toward the reader, nearly in parallel to, and bypassing, the stream gage, and (b) a plan view showing water table contours and down‐valley groundwater flow slightly bent toward the stream. A stream gage cannot, and is not designed to, measure the underflow. (Reprinted with permission from Winter et al. (). USGS 1998)
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The Amazon Basin (a) showing the regional topography. The study catchment of Neu et al. () is situated in the headwaters of the Xingu River (b) (reused with permission from the publisher, Springer 2011) where groundwater export may be expected. About 88% of catchment outflow is through groundwater, the consideration of which caused a fourfold increase in DOC and 14‐fold increase in DIC export. The map in (a) also shows the Reserva Biológica de Campina, the study catchment of Zanchi et al. ()
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Headwater catchments of Bisley at the Luquillo Critical Zone Observatory (CZO), showing the location of two deep rock cores (B1W1 and B1W2) and two stream gages (Quebrada 1 and 2). The rock cores reveal high‐fracture zones 37 m below the channel bed (B1W1), and 27 m below the ridge line, suggesting the potential for both a significant underflow in the valley and a leakage under and across surface drainage divides. (Reprinted with permission from Buss et al. (). Wiley 2013)
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Location map, monitoring network, and conductivity measurements in valley alluvium (Reprinted with permission from Käser and Hunkeler (). Wiley 2016). Note the presence of high‐permeability zones 10s of meters below the stream beds along the main stem (point a to h)
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(a) The Qr/R ratio over the Cedar River basin, with groundwater equipotential lines and direction of regional groundwater flow (blue arrows), (b) geologic map and cross‐sections showing the upper catchments centered over the carbonate Hollandale Embayment (A–A') collecting recharge over a larger area, and the south‐dipping carbonate bed of Devonian‐Carboniferous age (B–B') diverting groundwater down‐dip and out of the lower catchments. (Reprinted with permission from Schaller and Fan (). Wiley 2009)
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Numerical model simulations to illustrate the influence of climate (a and b), substrate permeability (c), and downgradient sediment thickening (d) on the relative importance of local versus regional flow. (Reprinted with permission from Schaller and Fan (). Wiley 2009)
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(a) Map of Qr/R for the 1,555 catchments/basins (lower‐left inset illustrates how nested basins are presented), (b) Qr/R plotted against catchment area, and (c) Qr/R against annual P. (Reprinted with permission from Schaller and Fan (). Wiley 2009)
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Schematic illustration of how geologic structures can promote and divert groundwater leakage, creating an exporting catchment not predicted by its position on the regional river drainage system. (Reprinted with permission from Schaller and Fan (). Wiley 2009)
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Water and Life > Nature of Freshwater Ecosystems
Science of Water > Hydrological Processes

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