Characterizing landscape influences on hydrological flow pathways in a peri-urban Mediterranean catchment

Stable isotopes are an important tool to describe the movement of water through the hydrosphere. They are used as tracers to characterize hydrograph properties. In field studies, stable isotope analyses using hourly and bulk end-member calculations can be used to estimate baseflow/precipitation contributions and to relate hydrograph response to land cover across a watershed. To enable proper planning of paved surface expansions and the nature of storm drainage systems, advanced understanding of the influences of spatial land-use patterns on Mediterranean streamflow regimes are needed to support water management in peri-urban catchments. This study focuses on Ribeira dos Covões, a small peri-urban catchment (around 6 km²), located in central continental Portugal. The catchment is composed of sandstone in the west portion (56%) and limestone in the east portion (41%), with some alluvial deposits (3%) in the main valleys. Flow and precipitation data were collected every five minutes during storms for several years. In 2018, sampling campaigns also included the collection of pre-event, event, and post-event water stable isotopes in different seasons of the year for streamflow at four sites, representing distinct land coverage and lithological landscape combinations. Preliminary results using precipitation and baseflow fraction calculations based on oxygen-18 measurements show that the catchment outlet provides a 49% baseflow contribution (old water fraction) at the beginning of the dry season and 36% in the wet season. An 85% baseflow contribution was estimated for Quinta (mainly forest area in sandstone) during the dry season, and 64-74% for Espírito Santo (largely urban in sandstone) during the wet season. The baseflow contribution at Porto Bordalo (urban area in limestone) is not significant because the flow is controlled by precipitation. Further investigation will involve connecting the results of most recent stable isotope data analyses to the approaches that were used in the past (e.g. separation of baseflow based on low-pass digital filters). Such connection will clarify streamflow response from distinct peri-urban pattern and lithological landscape combinations and their contributions to catchment runoff, aiming to explore the similarities and differences among these methods and quantify the effects of hydrological regime and land use changing patterns over time.