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How landscape organization and scale shape catchment hydrology and biogeochemistry: insights from a long‐term catchment study

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Catchment science plays a critical role in the protection of water resources in the face of ongoing changes in climate, long‐range transport of air pollutants, and land use. Addressing these challenges, however, requires improved understanding of how, when, and where changes in water quantity and quality occur within river networks. To reach these goals, we must recognize how different catchment features are organized to regulate surface chemistry at multiple scales, from processes controlling headwaters, to the downstream mixing of water from multiple landscape sources and deep aquifers. Here we synthesize 30‐years of hydrological and biogeochemical research from the Krycklan catchment study (KCS) in northern Sweden to demonstrate the benefits of coupling long‐term monitoring with multi‐scale research to advance our understanding of catchment functioning across space and time. We show that the regulation of hydrological and biogeochemical patterns in the KCS can be decomposed into four, hierarchically structured landscape features that include: (1) transmissivity and reactivity of dominant source layers within riparian soils, (2) spatial arrangement of groundwater input zones that govern water and solute fluxes at reach‐ to segment‐scales, (3) landscape scale heterogeneity (forests, mires, and lakes) that generates unique biogeochemical signals downstream, and (4) broad‐scale mixing of surface streams with deep groundwater contributions. While this set of features are perhaps specific to the study region, analogous hierarchical controls are likely to be widespread. Resolving these scale dependent processes is important for predicting how, when, and where different environmental changes may influence patterns of surface water chemistry within river networks. WIREs Water 2018, 5:e1265. doi: 10.1002/wat2.1265 This article is categorized under: Science of Water > Hydrological Processes Science of Water > Water and Environmental Change Science of Water > Methods
The Krycklan catchment study (KCS) showing key land cover and drainage gradients. Stream research has been conducted at the KCS since 1980. Since 2002, the catchment has been fully instrumented and has supported a large number of studies focused on hydrology, terrestrial and aquatic biogeochemistry, ecology, and land‐use. The KCS infrastructure contains 16 continuously monitored streams (ranging in size from 3.4 to 6790 ha), long‐term climate (since 1980) and land‐use experiments (>10 years), a groundwater network program, and a 150 m ICOS (integrated carbon observatory system) tower (http://www.icossweden.se/) for carbon exchange. The red circular symbols are the monitoring sites and the background color denotes dominant land cover types. The KCS outlet is at C16 in the south‐eastern part of the catchment.
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Different landscape features control hydrology and biogeochemistry in KCS and occupy different space and time dimensions, spanning daily to decadal time scales and square centimeters to hundreds of km2 in area. The dominant source layer is an episodically activated vertical layer in the riparian zone that is affected by sub‐daily processes during snowmelt, but also by seasonal processes that regulate decomposition and transport rates. The groundwater input zones extend from a few square meters to 100 m2 scale and are temporally regulated at event to seasonal time scales, for example by the extent of soil frost. Landscape scale heterogeneity acts at the sub‐catchment scale, ranging from a few hectares to some km2, and is primarily affected by seasonal to annual variability. The deep groundwater contribution operates effectively at the largest temporal and spatial dimensions considered here, primarily beyond inter‐annual periods and 10 km2 scales.
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Hierarchically structured landscape features regulating the spatial variability in hydrology and biogeochemistry. These features include: (a) The dominant source layer (DSL) that regulating the vertical distribution of hydrological pathways in near‐stream environments; (b) Topographically controlled groundwater input zones that contribute disproportionally large inputs of groundwater along stream segments at discrete locations; (c) Landscape scale heterogeneity caused by forest, wetland, and lakes that regulate the response across small catchments; and (d) Longitudinal heterogeneity controlled by a shift in the relative importance of deep groundwater contributions (dark blue arrows), which increase with catchment size.
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One example of long‐term variability and trend for DOC based on the Krycklan C7 stream, from 1986 to present. Data show indications of a long‐term monotonic trend since measurements begun, cyclic pattern driven by wet/dry and warm/cold periods, as well as a step change associated with increased minimum concentrations occurring around 2008.
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30‐year trend for three climate related variables: (a) average autumn (September–October) temperature, (b) number of winter days (November–April) with temperature above 0°C, and (c) late winter (average during March) discharge.
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Hypothetical downstream propagation of DOC following observed upstream contributing concentrations (black lines), as well as four synthetically derived future scenarios (red lines). The synthetic scenarios are based on observed effects of potential climate change drivers on stream DOC and the subsequent downstream mixing of waters from forest‐ and wetland‐dominated catchments during snow melt (see ref 43). These include, Scenario 1: Earlier spring, where the only change is that the snow melt season begins 2 weeks earlier; Scenario 2: Earlier spring combined with no soil frost that would alter the hydrological pathways in the wetland while have minimal impact in the forested soils; Scenario 3: Earlier spring combined increased snow accumulation during winter which would provide a larger peak DOC in the forested catchment and less decline in the wetland, and Scenario 4: Earlier spring combined with less snow that would result in a reduced DOC peak from the forested stream and a similar to normal peak in the wetland.
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Science of Water > Methods
Science of Water > Hydrological Processes
Science of Water > Water and Environmental Change

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