WATER AND SEDIMENT YIELD IN SMALL FORESTED HEADWATER
CATCHMENTS. THE RIBERA SALADA (SOUTHERN PYRENEES)
Ramon J. Batalla
University of Lleida and Forestry and Technology Centre of Catalonia
rbatalla@macs.udl.cat / ramon.batalla@ctfc.cat
Long-term high resolution series of flow and sediment transport are still scarce in
Mediterranean mountainous catchments. This is important information to assess,
between others, effects of land-use changes on water resources and erosion rates, as
well as for modelling purposes. Flow discharge is measured continuously since 1998 in
two nested catchments (2.65 and 65 km2) of the Ribera Salada, a mountainous
watershed in the Southern Pyrenees representative of the extensive forest land-use in
this upland region. In addition, suspended sediment is manually sampled during low
flows and by means of automatic ISCO samplers during floods. Sediment samples cover
almost the whole range of discharges for the two streams. Catchments are located over
molassic gradually varied detritic substrate and with a dominant Pinus sylvestris cover.
Ribera Salada basin has a mean annual precipitation of 780 mm and drains to the River
Segre in the Rialb Reservoir. Mean discharge of the Cogulers creek is 0.0049 m3/s
(0.0018 m3/s·km2), while mean discharge of the Canalda stream attains 0.150 m3/s
(0.0023 m3/s·km2), both values below the regional averages. Runoff in Canalda is less
variable through time in comparison with Cogulers (CV>100%). Annual runoff
coefficients for the whole period are 8% in Cogulers and 10% in Canalda, also lower
than the regional values. The Canalda stream shows a positive statistically significant
relation between discharge and sediment concentration; in contrast, the relation in
Cogulers is not significant. Suspended sediment concentration in Canalda is strongly
sensitive to discharge, indicating immediate access of flow to sediment stocks and the
unlimited supply of the system; fine particles tend settle in the low gradient channel
during flood recession, hence sediment becomes readily available for the forthcoming
floods; this fact stresses the importance of the riverchannel as sink and source of
sediment in relatively large catchments (<10 km2). In contrast, the high gradient of the
Cogulers creek impedes the sediment to deposit in the channel; sediment yield appears
to be mostly controlled by the contribution from the slopes, with little influence of the
fluvial processes. In this catchment, the relation between discharge and suspended
sediment transport describes a parabolic-like pattern: a) from baseflows to small floods
(<0.6 m3/s) suspended concentrations declines suggesting sediment exhaustion in the
system, and b) transport recovers again during high-intensity storm events when surface
erosion takes place and peak discharges get to exceed 3 m3/s. Sediment yield in both
catchments are of the order of 5 Mg/km2·y, value in the low range of forested
Mediterranean basins. High forest cover and geological structure tend to minimise
runoff and sediment delivery in this type of catchments, exemplifying low-intensity
geomorphic activity in undisturbed and relatively stable Mediterranean landscapes.