HYDROLOGICAL PROCESSESINVITED COMMENTARY

Hydrol. Process. 19, 2087–2089 (2005)Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/hyp.5904

A third paradox in catchment hydrology andbiogeochemistry: decoupling in the riparian zone

T. P. Burt*Department of Geography,University of Durham, Durham, UK

*Correspondence to:T. P. Burt, Department ofGeography, University of Durham,Durham DH1 3HP, UK.E-mail: t.p.burt@durham.ac.uk

If the rapid mobilization of large amounts of ‘old’ water (of variablechemistry) during times of storm runoff does indeed remain para-doxical (Kirchner, 2003), then a satisfactory mechanistic explanationseems likely to demand a distributed perspective. The variable chem-istry of stream water can, of course, be successfully modelled usingend-member mixing models as a combination of various sourcewaters, but such an approach is essentially lumped and the influ-ence of spatial pattern on processes of interest ignored. Pilgrim et al.(1979) showed some time ago that the chemistry of individual storescan vary over time; this may not just be a question of residencetime, but also of changing conditions along a flow path.

Falkenmark and Chapman (1989) distinguished between ‘verti-cal’ hydrological processes, like rainfall and evaporation, and the‘horizontal’ elements of runoff production. In ecology, a similar dis-tinction may be drawn: Reynolds and Wu (1999) note that, whereasthe study of patch dynamics emphasizes vertical fluxes, the land-scape perspective is more horizontal. This approach promotes anordered, pattern-oriented, or geographical analysis, emphasizing thespatial distribution of (and interactions among) ecological entities sothat, rather than viewing a landscape as simply a random mosaic ofpatches, the emphasis is on organization and linkage, with hydrolog-ical flow paths providing the functional structure. Put another way,the landscape system is more than just the sum of the individualpatches. The words ‘order’, ‘organization’ and ‘linkage’ suggest afunctional entity: the key factor is the degree of connectivity betweenthe landscape elements.

Reference here to ecological systems is deliberate: the chemistry ofstream water, of course, reflects the biogeochemical processes oper-ating within the catchment. However, the previous paragraph makesit clear that there is more to understanding the changing composi-tion of stream water than just chemistry and biology. The functionalbehaviour of a catchment system depends on the hydrological pro-cesses operating (surely a worthy topic for discussion in this journalmore than any other!). Having just emphasized functional linkage,it might seem self-contradictory to argue that ‘desirable’ catchmentbehaviour (i.e. pollution mitigation) can apparently arise from lackof connection, rather than the converse. The point is that the chem-ical composition of flow lines can be reset within the near-streamzone. Thus, the riparian zone is perhaps the most important element

Received 15 March 2005

Copyright 2005 John Wiley & Sons, Ltd. 2087 Accepted 11 April 2005

T. P. BURT

of the hydrological landscape given that it candecouple the linkage between the major landscapeelements, hillslope and channel. In a sense, there-fore, the riparian zone must, paradoxically, actboth as a conduit and a barrier, linking the ter-restrial and aquatic environments, but also actingas a barrier between them.

Bishop et al. (2004) argue that, since the chem-istry of water moving downslope is determinedat any given point by soil chemistry, the partic-ular role of the riparian soil is to set the streamwater chemistry, since this is the last soil in contactwith the water before it becomes runoff. Whilstemphasizing the critical role of the riparian zone,this approach nevertheless seems to perpetuatethe lumped approach (stream water as a mixtureof different source waters) and ignores the influ-ence of flow pathways, and the evolution of waterchemistry as water progresses through the ripar-ian zone. Evolution of water chemistry dependson the residence time of water within the ripar-ian zone, which in turn is controlled by topogra-phy and sedimentary architecture. For example,Welsch et al. (2001) studied topographic controls

on the chemistry of subsurface flow. They foundthat, although there was more potential for den-itrification in the wetter areas where nitrate-richwater converged, the throughput of water over-whelmed any denitrification effect; average resi-dence time in the riparian zone was low, prevent-ing anaerobic conditions and significant denitrifi-cation. A similar condition was described by Burtand Arkell (1987). At the other end of the spec-trum, where water has a long residence time in theriparian zone, denitrification will be very effective,but the throughput of water is too small to influ-ence river water nitrate levels significantly (e.g.Burt et al., 1999). The optimal condition wouldseem to be one of intermediate permeability, whereresidence times are sufficient to allow significantamounts of both throughflow and denitrification.To operate effectively, the riparian ‘buffer zone’,therefore, requires a balance between the functionsof conduit and barrier; this is not just a ques-tion of interaction with different soil conditions,but depends on flow path length and residencetime within a distinctive biogeochemical environ-ment.

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Figure 1. Evidence of denitrification along a flowpath at the NICOLAS field site in Brittany, France. Note that the reduction in nitrateconcentration is accompanied by the rise of 15N in groundwater, indicating differential uptake of 14N by denitrifying bacteria. Such

fractionation is not evident in nitrogen assimilation by vegetation. Data kindly supplied by Jean-Christophe Clement

Copyright 2005 John Wiley & Sons, Ltd. 2088 Hydrol. Process. 19, 2087–2089 (2005)

INVITED COMMENTARY

Buttle (1994) reviewed the processes responsi-ble for conveying pre-event water rapidly to thechannel during storm events: groundwater ridg-ing, translatory flow, macropore flow, saturation-excess overland flow, kinematic waves and releaseof water from surface storage. Not all processesoccur in all catchments, of course; but, whicheverpredominate, the near-stream saturated zones areinevitably the centre of attention (Cirmo andMcDonnell, 1997). The key to understanding thespatial pattern of flow-path chemistry lies in anintegrated experimental design. This must achievea three-dimensional picture of the shallow ground-water system (e.g. Bates et al., 2000; Jung et al.,2004) and allow water sampling along flow lines.Ideally, a wide range of tracers should be mea-sured, in order to define the changing nature offlow-path chemistry; use of coupled isotopes seemsto offer much potential in this regard (e.g. Clementet al., 2003a,b; Figure 1). Such experiments willnot only provide the means of defining relevantprocess mechanisms within the riparian zone, butalso, in time, of calibrating appropriate models.

Extrapolation beyond individual case studiesdemands the development of distributed, physi-cally based models that will successfully couplehydrology and biogeochemistry. And yet, by defi-nition, hydrology and biogeochemistry are decou-pled within the riparian buffer zone. This apparentcontradiction is addressed via knowledge of thespatial pattern of hydrological processes, i.e. theflow paths operating within the catchment system.

References

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Bishop K, Seibert J, Kohler S, Laudon H. 2004. Resolving thedouble paradox of rapidly mobilized old water with highlyvariable responses in runoff chemistry. Hydrological Processes18: 185–189.

Burt TP, Arkell BP. 1987. Temporal and spatial patterns ofnitrate losses from an agricultural catchment. Soil Use andManagement 3: 138–143.

Burt TP, Matchett LS, Goulding KWT, Webster CP, HaycockNE. 1999. Denitrification in riparian buffer zones: the role offloodplain sediments. Hydrological Processes 13: 1451–1463.

Buttle JM. 1994. Isotope hydrograph separations and rapiddelivery of pre-event water from drainage basins. Progress inPhysical Geography 18: 16–41.

Clement JC, Aquilina L, Bour O, Plaine K, Burt TP, Pinay G.2003a. Hydrological flowpaths and nitrate removal rates withina riparian floodplain along a fourth-order stream in Brittany(France). Hydrological Processes 17: 1177–1195.

Clement JC, Holmes RM, Peterson BJ, Pinay G. 2003b. Isotopicinvestigation of denitrification in a riparian ecosystem in westernFrance. Journal of Applied Ecology 40: 1035–1048.

Cirmo CP, McDonnell JJ. 1997. Linking the hydrologic andbiogeochemical controls of nitrogen transport in near-streamzones of temperate-forested catchments: a review. Journal ofHydrology 199: 88–120.

Falkenmark M, Chapman T (eds). 1989. Comparative Hydrology .UNESCO: Paris.

Jung M, Burt TP, Bates PD. 2004. Towards a conceptual modelof water table response. Water Resources Research 40: W12 409.DOI: 10·1029/2003WR002619.

Kirchner JW. 2003. A double paradox in catchment hydrologyand geochemistry. Hydrological Processes 17: 871–874.

Pilgrim DH, Huff DD, Steele TD. 1979. Use of specific conduc-tance and contact time relations for separating flow componentsin storm runoff. Water Resources Research 15: 329–339.

Reynolds JF, Wu J. 1999. Do landscape structural and functionalunits exist? In Integrating Hydrology, Ecosystem Dynamics andBiogeochemistry in Complex Landscapes , Tenhunen JD, Kabat P(eds). Wiley: Chichester; 273–296.

Welsch DL, Kroll CN, McDonnell JJ, Burns DA. 2001. Topo-graphic controls on the chemistry of subsurface stormflow.Hydrological Processes 15: 1925–1938.

Copyright 2005 John Wiley & Sons, Ltd. 2089 Hydrol. Process. 19, 2087–2089 (2005)