in a Mediterranean mountainous experimental catchment (Vallcebre, Catalonia)
J. Latron, F. Gallart, C. Salvany
Institute of Earth Sciences ‘Jaume Almera’, CSIC, Sold i Sabaris s/n., 08028 Barcelona, Spain.
1 Introduction
The study of catchment runoff generation mechanisms notably started in the early 1960s when several authors (Cappus, 1960; Hewlett and Hibbert, 1967) showed with field data that infiltration excess overland flow identified by Horton (1933) was not in many cases the leading mechanism but was only a particular case of streamflow generation. Many field studies under temperate climates have since confirmed the predominance of other runoff-generating processes, mainly subsurface flow and saturation excess overland flow.
However, as Dunne (1978) suggested, all these mechanisms are complementary rather than contradictory, and dry areas like Mediterranean regions provide frequent examples of the occurrence of all the above mentioned mechanisms due to both high rainfall intensities and varying antecedent conditions.
Former studies on the interaction between soil moisture, groundwater and streamflow response in the study area show the role of infiltration excess runoff in dry periods (Latron and Gallart, 1995; Latron et al., 1997) and stressed the main role of saturation excess runoff generation mechanisms (Llorens and Gallart, 1992; Latron et al., 1997), the role of terraced topography on the generation of saturated areas (Gallart et al., 1994), and the delay of the saturation and drying of semi-permanent saturated areas when compared to the mean soil water reserve (Rabada et al., 1993, Gallart et al., 1997).
The present work tries to exploit the extensive piezometric network installed in 1995 in order to describe the dynamics of the phreatic system and to discuss some non-linearities observed especially during the wetting-up transitions, which remain a challenge for hy- drological modelling in seasonal climates (Piiiol et al., 1997).
2 The study area
The Cal Rod6 catchment (4.17 km2) is located in the southern margins of the Pyrenees, at altitudes between 1100 and 1600 m a. s. 1. Bedrock is dominated by red smectite- rich mudstones locally prone to badlands formation and massive limestone beds. The
climate is mountain Mediterranean with a mean annual temperature of about 9°C and a mean annual rainfall of about 850 mm. Water deficit occurs usually in summer, but can be advanced or delayed depending on the usual climatic variability of Mediterranean climates. Most of the hillslopes (35 % of the catchment surface area) were terraced in the past for cultivation and are now used for cattle stockbreeding or forestry. Forest (Pinus sylvestris) covers nowadays 60 % of the catchment. Soils are loamy and well structured, with a thickness that varies up to 3 m because of the terraced micro-topography. The present general arrangement of the instruments in the Cal Rod6 catchments (Fig. 1) consists of 4 runoff gauging points, 7 rainfall recorders, 1 weather station, 3 tensiometric profiles, 9 TDR (time domain reflectometry) soil moisture measurement points, 3 weekly- measured wells, and 4 continuous recording piezometers.
+ gauging rtition l minfall recorder
0 TDR probes + recording pieromcter
$ well
Cal Rod6
Figure 1: General arrangement of the measuring network.
3 Soil moisture and phreatic levels
Soil moisture values measured down to 80 cm show a wide range (0.15 to 0.60 cm3/cm3), are temporally well correlated and are spatially controlled mainly by vegetation cover and position in the hillslope; forested or upslope areas being drier than grassed or downslope ones. The points with less variation correspond to the deeper situations in the profiles and downslope locations where saturation is frequent. Position in the hillslope, by means of the topographic index (Beven and Kirkby, 1979), vegetation cover and soil depth have been
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used to estimate catchment water reserves from soil water content point measurements in order to validate internal state variables of hydrological models.
Free shallow aquifers can be found in these soils over clayey bedrock even on steep hillslopes because of the low hydraulic conductivities. Phreatic level variations show in general a greater inertia than soil moisture values, thus being better correlated with deeper points in the profiles and downslope situations. Nevertheless, continuous recording piezometers demonstrated that phreatic levels react usually very quickly to rainfall events of sufficient magnitude to produce relevant runoff. This highly dynamic behaviour was observed before and justified the use of recording piezometers (Latron et al., 1997). The rapid response is a rule except when the rainfall event occurs after a long drought period that depleted the phreatic level (Fig. 2). In these dry antecedent conditions, the reaction of the phreatic level to rainfall can be delayed by 120 hours.
_ _ 4.Nov.97 ELNov-97 12-Now97 16-No+97 20.Nov.97
Figure 2: Streamflow and water table response to the first rainfall event after a dry period (Can Vila catchment). Compare the water table response to rainfall for the different events.
4 Runoff events
A close relationship between phreatic levels and runoff coefficients has been demonstrated before for these catchments (Latron et al., 1997). Nevertheless, the continuous record of phreatic levels allows better analysis that makes possible the identification of anomalies of this general rule. In a very simplistic manner, runoff from a catchment driven by subsurface flow can be approximated by some negative exponential function of water reserve (Beven and Kirkby, 1979). The plot on Fig. 3 shows the log-linear relationship between phreatic levels and runoff volumes at daily scale. Although with scatter, the general trend of the points fits an exponential function if a few outliers are excluded.
These outliers coincide with events over ‘dry’ antecedent conditions prior to rainfall and correspond to days with runoff volumes much higher than those that could be predicted by the phreatic level, and thus by saturation mechanisms based models. For these events the low soil moisture contents and the deep location of phreatic levels strongly reduce subsurface and saturation excess contributions to streamflow.
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100.0
10.0
1 .o
0.1
6.Nov.97 e
7.
-1 60 -120 -80 -40 0
water table depth (cm)
Figure 3: Relationship between daily runoff (Can Vila catchment) and water table depth.
Days corresponding to Fig. 2 are indicated.
5 Conclusions
The hydrological functioning of the Cal Rod6 catchments is usually controlled by sub- surface flow and saturation excess overland flow, as it corresponds to humid temperate environments. Nevertheless, the continuity of subsurface water transfer is interrupted al- most every year, after a water deficit period, when the Mediterranean character of the climate becomes evident and stress infiltration excess runoff generation mechanisms. Be- tween these two kinds of behaviour there is a threshold, subject to hysteresis delays, that represents a progressive substitution of the processes.
The analysis of the mechanisms active during the dryer and transition periods is not a merely academic exercise, as it can provide a bridge to understand the behaviour of catch- ments in rather dry areas, or to allow the simulation of the likely hydrological consequences of a drying climate.
Acknowledgements
This work has been carried out within the VAHMPIRE Project, supported by EC Environ- ment and Climate Research Programme (ENV4-CT95-0134).
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