INDICE DE HIRSCHMAN 19

Several studies have used hydrograph separation techniques such as EMMA to identify the principal sources of stream flow generation (Buttle, 1994; Burns et al., 2001; Soulsby et al., 2003; James and Roulet, 2006). EMMA is a linear mixing approach that uses tracers to identify sources assuming conservative mixing. This technique generally employs isotopes (Buttle, 1994; Brown et al., 1999a; Burns et al., 2001) and chemical tracers (Soulsby et al., 2003; Abesser et al., 2006b; Neill et al., 2011) to solve a mass balance equation in order to determine the proportion of water inputs to stream from each end-member or source. In this study, a chemical tracer method is adopted to investigate the hydrological pathways. Calcium (Ca) and magnesium (Mg) are chosen as tracers based on the following observations: a) they have a relatively strong correlation with flow, b) they are relatively abundant in all subsurface solutions and dominate during low flows (Chapman et al., 1997); and c) there are differences in concentrations between the end-members, as emphasized by Hooper et al.

(1990). Moreover, these cations are largely replaced by metal ions, such as Mn and Al, which constitute the principal exchanged elements, hence, they are good tracers to distinguish groundwater from soil water and rainwater (Joerin

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A two to n-component mixing model (Pinder and Jones, 1969; Robson and Neal, 1990) is used to separate the streamflow components. This approach has been successfully applied to conservative tracers such as alkalinity (Abesser et al., 2006a) and Ca (Gkritzalis-Papadopoulos et al., 2011) where the following mass balance equations were used to define a two-component flow:

(Eq. 6.1)

(Eq. 6.2)

where, Q is discharge (m3_.s-1_{), Ca is calcium concentration and the} subscripts streamwater, groundwater and soilwater refer to those sources; the latter two are the identified end-members. From the two above equations (Eqs. 6.1 & 6.2), the proportion of groundwater can be defined by:

(

)

(Eq. 6.3)

Application of this approach is based on the following assumptions:

1) As a first approximation, it is assumed that the total stream flow at any given time is a mixture of two flow components with distinct chemical signatures (Christophersen et al., 1990).

2) These components are a groundwater end-member that contributes during lowflows or ‘baseflow’ conditions, and a soilwater end- member contributing during peak or storm flow.

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3) Stream water chemistry is controlled by physical mixing rather than by equilibrium chemistry (Hooper, 2003), thus uniquely identifying the proportions of the two end-members at any given time.

4) End-members within each sub-catchment, to a first approximation, are spatially and temporally invariant (Christophersen et al., 1990; Wade et al., 1999).

These assumptions simplify the existing complex hydrological processes in the River Dyke catchment, in order to allow some general understanding of hydrological setting in the catchment. However, a simple two-component model is deemed appropriate, as this number of factors explain the majority of the variance in the chemistry data (Section 6.2), and it is considered adequate for the purpose of this study which attempts to provide a simple representation of the changing contribution of soil water and groundwater sources. This is in agreement with similar studies carried out in other upland catchments (Billett and Cresser, 1992; Giusti and Neal, 1993; Wade et al., 1999; Soulsby et al., 2002), which have demonstrated that the majority of variability in stream water was explained by changes in groundwater and soilwater contributions.

In this study, soilwater is classified as ‘shallow’ soilwater that comes from depths between 0 and 50 cm (acrotelm layer) of soil profile and ‘deep’ soil water, which comes from depths > 50cm (catotelm layer) and extends down to the surface of mineral/bedrock weathering zone (see Figure 3.3). Groundwater is essentially considered as the water coming from mineral zones that contain signatures of weathering derived base cations (for example, Ca and Mg).

During base flow conditions, stream chemistry reflects the effects of bedrock weathering, and thus stream chemistry is marked by the signature of

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deeper (B/C) soil horizons or groundwater sources. Equally, during peak or high flows, stream water chemistry is dominated by the constituents from surface and shallow-surface (O/A) soil horizons. However, studies have demonstrated that even at high flows, groundwater can contribute significant amounts to streamflow (Soulsby et al., 1998; Burns et al., 2001; Abesser et al., 2006b). Hence, even at peak discharges, streamflow is still a mixture of groundwater and soilwater. The proportion of water from these two end- members varies during and after the storm events, which can be estimated by using EMMA.

Hooper et al. (1990) used end member mixing analysis to demonstrate that different flow paths dominate under different antecedent moisture conditions; depending on the discharge history, the signature of end-members may either come from a riparian zone (and/or near stream areas), or from more distant but hydrologically connected parts of the catchment. However, the proportions of different soils immediately adjacent to the streams have much greater impact on the stream water chemistry than those soils at further distances in the catchment (Billett and Cresser, 1992).

Stream chemistry provides information on hydrological pathways when chemically inert solutes that are delivered conservatively through the catchment are used for the hydrograph split (Peters and Ratcliffe, 1998). Calcium is used as a conservative tracer for hydrograph separation (Wade et al., 1999; Joerin et al., 2002; Gkritzalis-Papadopoulos et al., 2011) and its conservative behaviour can be inferred from the matrix plots (Figure 6.1 - Figure 6.3), and confirmed through its simple linear relationship with Mg (another conservative tracer used in this study). These two tracers are characteristic of groundwater (Chapman et al., 1999) and when they are used for hydrograph

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separation, the results explain the geographic source (or hydrological flowpaths) by delineating soilwater and groundwater and their contribution, rather than the residence time of water. This allows us to estimate the proportions of two source components so that runoff responses of individual sub-catchments can be compared.