Consideraciones finales

Hydrological pathways or routes that water follows to reach a stream channel have a strong control on the concentrations of organic carbon. In the UK, upland streams are highly flashy in nature, for example, discharge peaks rapidly in response to heavy storm events and falls rapidly once the event

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ceases. Consequently, dramatic changes in water chemistry are generally associated with such periods of high discharges (Cresser and Edwards, 1987; Abesser et al., 2006b, 2006a). Distinct chemical changes are associated by an increase in DOC, and metals like Al, Fe and Mn increase, while pH, conductivity and base cation concentrations decrease (Abesser et al., 2006a; Abesser and Robinson, 2010). These changes are due primarily to changes in hydrological pathways during storm events, for example, despite low DOC concentrations in rainwater (Neal et al., 2005), water draining laterally through the organic rich layers of peat tends to be high in DOC concentrations. Water moves laterally through the upper soil horizons either as unsaturated flow or more typically as shallow perched saturated flow above the main groundwater level (Ward and Robinson, 2000). This flow occurs when the lateral conductivity in the upper horizons of the soil profile is considerably greater than the overall vertical conductivity of the soil profile. Therefore, initial DOC concentrations are high during a storm event as the event water raises the watertable, and the water moves laterally through organic rich upper soil horizons. However, during a prolonged precipitation, dilution could result in lower DOC concentrations. Hence, flowpaths are a function of the intensity and length of the rainfall event, and the time since the previous rainfall (Worrall et al., 2002). This suggests that the rainfall-runoff response time of the soil-stream system and the rainfall event frequency influence the temporal variability in the relationship between precipitation and DOC concentration.

At low flows, stream water mainly comes from lower mineral soils and ground water, which is of relatively deep origin and usually long residence times. As the water moves through deeper horizons, the processes such as mineral weathering, cation exchange, decomposition of organic matter, adsorption, oxidation and reduction alter the water chemistry producing less acidic baseflow chemistry, which is rich in base cations such as Ca, Mg and Si,

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and depleted in metal ions such as Al, Fe, Mn (e.g. Neal et al., 1997). During intense storm events, runoff generation processes and hydrological pathways shift from deeper base-flows to shallow subsurface and surface flows, which increase the stream discharges. At such flows, the runoff is predominantly through upper organic and organo-mineral soil horizons thereby water chemistry tends to be more organic-rich. As the hydrological pathways vary at different flows, a variety of chemical patterns each with a distinct set of chemistry is observed.

In general, during storm events runoff takes three basic flowpaths for stream flow generation: overland flow, shallow sub-surface flow and deep sub- surface or ground water flow. Overland flow is a surface flow that occurs in two ways, either as Hortonion overland flow where the rainfall intensity exceeds the rate of infiltration, and as infiltration excess or saturated overland flow (Ward and Robinson, 2000). Jenkins et al. (1994) opined that Hortonian overland flow is unlikely to be an important hydrological pathway in the case of upland catchments, except in arid areas, alternatively, saturation-excess overland flow, which occurs when the soil infiltration capacity is nearly zero resulting in water moving down slope, is commonly seen in uplands (Becker and McDonnell, 1998). However, Burt et al., (1990) suggested that blanket peat catchments in the UK produce significant ‘Hortonian’ infiltration-excess overland flow since infiltration rates into peat appeared to be low. Holden and Burt (2002) in an experimental study have shown that these low infiltration rates were a result of surface saturation of the peat and found that saturation- excess overland flow can develop even during very low-intensity rainfall. Shallow water table areas adjacent to the stream channels and subsequently the lower valley slopes and hill slopes are the major sources for such flows (Evans

et al., 1999a; Ward and Robinson, 2000). As a hydrological event progresses, saturated areas expand and more areas in the catchment become sources of

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runoff; in contrast, as the event ceases, the sources areas shrink. This is the basis for the popular ‘variable source area concept’ (Hewlett and Hibbert, 1967). When the overall vertical conductivity is overcome by the lateral conductivity, horizontal flow through soil layers as ‘sub-surface flow’ occurs. This lateral movement of water can occur through upper soil horizons or at shallow perched water tables above groundwater (Ward and Robinson, 2000). However, the hydraulic conductivity of peat is unlikely to be uniform in all directions (Surridge et al., 2005), for example, there exists some perched conditions where sub-surface flow may break back through to the surface, termed as ‘return flow’. At low flow conditions, the movement of water is mainly through deeper mineral horizons as ‘groundwater flow’. Another significant subsurface flow path in upland areas is pipe-flow and macro-pore flow (Beven and Germann, 1982; Holden et al., 2001). In forest soils, water moves rapidly through macropores or pipes that include old root channels, cracks or animal burrows (Holden and Burt, 2003). Movement through these macropores and pipes is important in that they are the major source of stream acidity even under baseflow conditions, and sources of DOC and Al under high flows (Jones, 2004). Consequently, stream water chemistry reflects the hydrological pathways followed by the water before reaching the stream channel. A combination of one or more of the above pathways can explain the majority of the variability in stream water chemistry. For example, in a study on radiocarbon ages of DOC exported from peatlands, Schiff et al. (1998) have found that almost all DOC in stream waters is produced in the uppermost layers of peat (Billett et al., 2006), however groundwater is more likely to contain older carbon indicating that the soil water component is the major controlling factor.

There has been increasing interest in investigating the mechanisms responsible for the DOC exports from a wide range of catchments (Hornberger

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Wagner et al., 2008) in order to understand the episodic response of DOC to varying hydrological conditions, and the chemical processes leading to DOC transport. In upland forested catchments of the Colorado Rocky Mountains, Hornberger et al. (1994) found that the temporal variability in DOC concentrations was mainly due to the flushing of the near-surface soil DOC pool. In a similar study in the Trout Beck catchment, Worrall et al. (2002) identified that discharge from a peat system behaves like a three end-member system with between-event water being low in DOC and storm events characterised by two types of water (with varying DOC concentrations) depending on their relative resident times in the soil. Similarly, in a forested catchment in Germany, Hangen et al. (2001) observed a delay in DOC concentrations with respect to peak hydrograph due to the time lag associated with the onset of streamflow and displacement of DOC-rich waters from the topsoil to the stream via macropores. In a study on a forested catchment in New York, Inamdar et al. (2004) ascribed the observed delay in DOC peak to the hydrological connectivity in the catchment and developed conceptual models to explain DOC evolution during storm events.