DEFINING FLOW CONTRIBUTION SUBAREAS USING WATER CHEMISTRY DATA
DATA DISCUSSION
Fig 4 shows the contribution to discharge of each subcatchment as a function of the total discharge at the catchment outlet (gauging station) based on the conductivity balance approach. This distribution is shown for every sampling moment (except for the moment when Qgs = 18000 l/s where it remains in general similar to the high flow period). Extraneous discharge contributions were observed for samples taken at the rising limb of the hydrograph (i.e. number 6 and 8). This phenomenon was also observed by Caissie et al. (1996) and Walling et al. (1975), who explained it, for the catchments they studied, as the result of a flushing out of older groundwater into the stream. The figure reflects that during low waters (Qgs less than 400 l/s), subcatchments 1 and 15 (28 km2) account for about 60% of the total discharge.
During high waters (Qgs >700 l/s), these subcatchments account for only 30% of the total discharge, while subcatchment 2 is the one with the highest contribution of about 35%. Therefore, during high waters, a 65% contribution is coming from a 48 km2 surface out of a total catchment area of 116 km2.
Fig 4: Percentage discharge contribution of each subcatchment (numbers in the legend) to the total discharge at the gauging station. Numbers at the bottom axis refer to the sampling time shown in Fig 2.
Water chemical composition is highly variable within the catchment, in time (Fig 3) and in space. Fig 5 shows a good example of spatial variability of electrical conductivity. Waters in subcatchment 1 show the highest mineralisation (600-1800 µS.cm-1) with calcium sulfate facies due to the presence of diapir.
Waters in subcatchments 15 and 10 (mainly sandstones) show the lowest mineralisation (60-140 µS.cm-1) with calcium bicarbonate facies. The figure also shows the mineralisation variability (shaded zones) at the sampling points as a function of discharge derived from the chemical balance. Zones can be fitted to ellipses in which the slope of the line corresponding to the main axis is a function of the dilution degree in the waters. Lines are defined by the equation y = -a.ln(x) + b. Slope values (a) are shown in the figure.
Except for subcatchment 1, with an important dilution during high water periods, all the others show low dilution. So, in a subcatchment with very high discharge variability (e.g. no. 2), water mineralisation remains within a narrow range, whereas in another catchment with smaller discharge variability (no. 1), mineralisation varies considerably. The subcatchment 15, however, shows low variability in both discharge and mineralisation. Comparing these results with the geological map (Fig 1), the subcatchments with more influence of soilwater-groundwater in the discharge response (smaller value of a; less dilution by piston flow) are related to the sandstones (6, 10, 15) and limestones (2). A smaller influence is seen in subcatchments in lutites (8, 13).
Fig 5: Chemical variability of waters in each subcatchment (1, 2, 4, 6, 8, 10, 13 and 15) according to their discharge variability. The parameter a refers to the slope of the straight lines in the ellipses.
A principal component analysis was conducted on chemical data of the whole catchment (Fig 6). For a better comprehension of chemical variability, Table 1 shows the average values of major elements at the different sampling points. The factorial plane reflects the most information in the analysis, with factor I (61 % of the variance) characterised by SO4=, Cl-, Ca2+, Mg2+, and Na+, showing the high mineralisation of waters from subcatchment 1, and factor II (15 %) characterised by HCO3-. The subcatchments with higher values in factor II are the ones in the Western part of the Altube River (2, 8, 13), while those of the Eastern part (4, 6, 10, 15) show lower values. This differentiation can be related to lithology, as the bedrock material in the West is carbonated and in the East it is well-washed sandy rock.
Referring to samples taken from the channel of the Altube River, the relation of water in the main channel to the water flowing from each subcatchment can be analysed. Two groups of samples can be distinguished:
one group with high values of factor I, related to situations when subcatchment 1 has a larger influence (low waters); the other group with lower values of factor I and higher values of factor II, related to the increasing water contribution of subcatchment 2 during high water periods (Fig 4). Samples taken at the gauging station (point 17) are situated in the bottom-middle part of Fig 6, between subcatchment 1 and waters coming from subcatchments of the Eastern part of the Altube River. This demonstrates once again the importance of the hydrochemical contribution of that part of the catchment.
Fig 6: Factorial plane showing the hydrochemical pattern of the Altube River catchment and subcatchments.
So as to have a clearer view of the evolution of water chemistry in the Altube River, another component analysis was made (Fig 7) using data only from control points along the river (Fig 1). The significance of the factors I (56 %) and II (18 %) is identical to that in Fig 6. Waters from subcatchment 1 show the most variable mineralisation, while at point 17 the chemical variability is much lower due to the influence of the tributaries (4, 10, 15) which have lower and less variable mineralisation (Fig 5).
The chemical behaviour of waters does not depend only on the discharge value, but also on the time when samples were taken. Thus, samples taken when the discharge at the gauging station was 80 l/s and 87 l/s show different evolutions, because the first samples were taken on the rising limb of the hydrograph, while the second samples were taken at the peak. The influence of the hydrological context can also be observed by comparing evolutions at discharge values of 445 l/s and 571 l/s; for the first value, the hydrograph was at its maximum point and for the second the water discharge was decreasing but within a period of very high waters. Thus, different chemical evolutions of water flowing along the river at different moments of the hydrograph have been distinguished. The evolutions at the top of the figure (Qgs = 571 and 640 l/s), almost parallel to the factor II axis, occurred during high water when subcatchment 2 had the largest contribution. The chemical evolutions in the middle part of the figure (Qgs = 170, 445, 87, 205 and 1174 l/s), almost 45º from both axes, occurred at a maximum point of discharge. The evolution with Qgs = 80 l/s occurred shortly after the maximum, and the evolution with a discharge value of 25 l/s was during a very calm moment of the hydrograph after a long period without precipitation.
Fig 7: Chemical evolution along sampling points in the Altube River (Fig 1) for different discharge values (referred by lines) and different hydrological situations (described by schematic hydrographs). Factorial plane. Discharges refer to data at the gauging station at the time of sampling. Symbols refer to different sampling points along the Altube River.
Table 1: Average concentration and standard deviation of sulfate (mg/l) (83 values) and electrical conductivity (µScm-1) (170 values) in the subcatchment control points.
Subcatchment 1 2 4 6 8 10 13 15 17(the outlet)
SO42- Average 702 63 51 24 51 18 55 16 293
St. deviation 167 22 21 12 23 4 8 8 124
Cond. Average 1416 464 162 465 278 93 463 134 608
St. deviation 314 68 49 112 107 38 75 55 201
Finally, we tried to broadly separate the annual hydrograph at the gauging station (point 17) according to the sources of waters within the catchment (Fig 2), using the discharge percent contribution shown in Fig 4. Four subareas are considered as sources: the most important subcatchments (1, 2 and 15) and the remaining catchments together. The influence of the three important subcatchments is clearly observed.
CONCLUSIONS
The chemical composition of running water has been used to study the different streamflow sources in the Altube catchment. The most influential subcatchments for different hydrological situations were detected. During low waters, 24% of the catchment accounts for 60% of the total discharge, whereas during high water periods, 41% of the basin accounts for the 65% of the total discharge. Discharge at each point deduced from the chemical mass balance allows us to establish a general relation between discharge and electrical conductivity. Conductivity is therefore used as an easy field tool to estimate water discharge.
A principal component analysis shows that lithological signals in waters can help us to differentiate waters coming from subcatchment 1 (diapir), from the West (limestones) and from the East (sandstones) of the Altube catchment. Moreover, the dilution degree (variability of the mineralisation of water with discharge) observed in each subcatchment stream is related to the influence of soilwater-groundwater on discharge response and therefore to its regulation capacity. The principal component analysis also allows us to differentiate the hydrochemical evolution along the Altube river depending on the hydrological situation (Fig 7). It is possible to use these preliminary results as a validation tool for rainfall-runoff models.
The next step is to apply water chemistry studies within each subcatchment in order to understand the chemical signature of landuse influence and water pathways.
ACKNOWLEDGMENTS
The authors thank the Basque Meteorological Service of the Basque Government for supporting this investigation. Dr. W. Chełmicki and Dr. L. Holko are thanked for the helpful comments on the manuscript.
REFERENCES
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