RESULTS AND DISCUSSION Hydrological features

The event on 22 July 1995 occurred during low water flow conditions, discharge at the outlet (RS) was only 3.5 l.s^-1. Rainfall started at 14:30 hours and lasted until 21:00 hours. This event was a typical intensive

(up to 60 mm.h^-1) summer rainstorm with three short interruptions. The total amount of the four showers was 30 mm (Fig 2). The corresponding hydrograph is characterized by three discharge peaks (40, 34 and 17 l.s^-1 at the outlet) in response to the successive rainfall events (18, 7.4, 3.3 and 1.5 mm) (Fig 2). The rapid response (5 minutes) of the stream and the tributaries to the rainfall suggests that the contributing areas are close to the brook and especially to the outlet.

Water table measurements show a general water level increase between 15 and 20:00 hours, with a 0.4 to 0.5 m value (Fig 3). The piezometers located in the upper part of the slope (E, for example) indicate a maximum water level at 15:30 hours, and then a plateau or a decrease is observed. Whereas the water level of the downslope piezometer A only begins to increase at 19:00 hours and reaches a maximum at 22:00 hours, which could indicate a downward wave propagation on the slope.

Fig 2: Hyetograph and hydrograph of the stream at the outlet (RS) and of the total volume of the flow, while it represents only 32 % of the catchment area.

Isotopic features

The ¹⁸O content of streamwater during baseflow conditions (-9.3 ‰) is very similar to the spring value (-9.35 ‰) suggesting that stream baseflow is composed exclusively of groundwater (Ladouche, 1997).

Consequently, groundwater flow controls baseflow and its isotopic composition can be used to characterise the pre-event component. The isotopic rainwater composition varies between -3 ‰ and -6 ‰ but most

The isotope hydrograph separation has been performed using the event weighted isotopic signature of each shower (McDonnell et al., 1990) and with a constant pre-event signature (-9.3 ‰, stream baseflow value).

The hydrograph separation (Fig 5) at the outlet indicates that, the event contribution can reach a maximum

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value of 40 to 45 % at the two main discharge peaks (40 and 34 l.s^-1). The same magnitude is observed at sites BH and RUZS. At the site RAZS, the rain contribution is only 33 % of the discharge. For the whole rainfall event, the new water contribution is 22 % at the outlet and is relatively important in comparison with a 10 % value obtained for a 40 mm rainfall event (Ladouche et al., 2001).

0 hydrograph separation using ¹⁸O and variation of their contribution to streamflow at the outlet.

The analyses of samples of porous cups (site SC) show that there is a layering of the isotopic composition of the water (Fig 6). In the upper layers (0 to 40 cm), the signal is enriched (-4.5 to -7 ‰ value) indicating a mixing of waters (pre-event and rain water) and reflecting an important contribution of the event rain water, whereas in the deeper layers (65-90 cm) the signal is impoverished (-9 ‰) indicating a minor influence of the rain. A comparison with streamflow indicates that, during peak flow the variation of the signal at station RS is strongly influenced by that of station RUZS or the upper layer signal of the saturated area. However, during the recession period the deeper layers of this saturated area dominate.

Fig 6: Variations of δ¹⁸O in the porous cups (Site SC) at various depths (25-90 cm).

Fig 7: Variations of DOC, silica and discharge at the outlet.

Geochemical features

During the storm event, the rainwater is acidic (mean pH of 4.5), with low concentrations of major elements (Total Dissolved Salts = 5.6 mg.l^-1) (Idir, 1998). Ammonium, calcium, nitrate and sulphate are dominant, whereas dissolved silica and DOC are negligible, as already observed (Probst et al., 1990; Ladouche et al., 2001). The concentrations of almost all the elements decrease during the first two showers on 22 July and increase temporarily during the third shower. Streamwater pH is circumneutral (pH 6.0-6.5), indicating an important buffering capacity of the catchment, and calcium and sulphate are the dominant ions.

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According to their chemical behaviour in relation to the discharge variations at the outlet (RS), two groups of chemical parameters can be identified at the upper subcatchment site (RAZS) and at the BH tributary:

the elements which are diluted with increasing discharge (SO₄^2-, Na⁺ and H₄SiO₄) and the elements which are concentrated with increasing discharge (DOC, NO₃^-, Cl^-, K⁺, Ca²⁺, Mg²⁺ and alkalinity) (Fig 7).

In streamwater, the rapid increase of Ca, K and DOC concentrations could be due to canopy and soil surface leaching by the first rain drops, whereas the high concentrations of sulphate or silica in streamwater are diluted by the low rain content.

In order to identify the contributing sources to the chemical composition of streamwater, end-member mixing diagrams (Christophersen et al., 1990) have been performed for major elements (Idir, 1998).

Among analysed parameters, the linear mixing diagram between DOC and silica (Fig 8) shows that streamwater at the outlet (RS) can be explained mainly by two obvious end-members: the component with high DOC and low silica concentrations characterizes the upper horizons of the saturated area (1: Q sat), whereas the component with high silica and low DOC content represents the deeper layers of the hillslopes (2: Q hill) as already observed for another event (Ladouche et al., 2001).

Fig 8: Mixing diagram between DOC and silica for the sampling sites;

1: upper layers of the saturated area;

2: deep layers of the hillslopes.

Fig 9: Two-component (deep layers of hill slopes: Qhill; surface layers of the saturated area: Q sat) hydrograph separation using DOC.

A chemical hydrograph separation has been performed at the outlet using silica and DOC. The results of the separation performed using DOC with constant end-member values (11 and 1.5 mg.l^-1 values respectively) are presented in Fig 9. For the whole event, the surface waters draining Q hill represent 55 % of the total volume while the remaining 45 % is attributed to Q sat. Nevertheless, the contribution of Q sat can represent 62 % and 68 % respectively of the discharge during the two main peak flows. The results of the separation using silica are similar, even if the Qhill contribution (64 %) is slightly higher than that obtained with DOC.