The Castelhanos Stream, Portugal

The Castelhanos stream (c. 11 km length) is a tributary of Grândola stream (c. 30 km length) which flows to the river Sado. Castelhanos stream runs at an altitude between 220 and 130 m and is located at approx. 20 km from the Atlantic Ocean (Fig. 1). Part of Castelhanos stream is included in Herdade da Ribeira Abaixo, a state-own propriety (c. 221 ha). This propriety is the field station of Lisbon University since 1993 and is also a LTsER station.

Figure 1. Characteristics of the study area in Portugal. From top-left and clock-wise: location of the study area in western Europe, Portugal, local hydrological system of Castelhanos stream and normal climatogram for nearby climate station, Sines.

The Castelhanos stream is a temporary Mediterranean stream, located in an area with an average annual precipitation of 533 mm and average annual temperature of 16.4 ºC (Sines, 1990-2020). Castelhanos stream discharge to Grândola stream to the north. The mean annual discharge of Grândola stream if about 50 dm3 s-1, with very large differences between wet and dry years, with its aquifer being recharged only by rainfall. 

The geologic setting of the study area is mostly formed by schists and sandstone (greywackes) belonging to the Paleozoic metasedimentary flysch series of the geologic South-Portuguese Zone. Weathered materials distribute according to geologic and topographical gradients, where more thin materials predominate in the bottom of hills and in less deep valleys, contrasting with coarse materials that tend to produce colluvial deposits in most of the deep valleys. Weathered elements accumulate in longer talwegs reaches forming narrow alluvial flats. The formation of these deposits has been attributed to the Pliocene and Pleistocene wettest periods. 

More recently, streams have been carving and eroding these flatlands, promoting the formation of narrow and shallow stream channels that we observe today. A sequence of stream and small ponds is a distinctive feature of these systems contributing for the hydraulics of flowing waters and alluvial groundwater storage. In the ponds, near surface water table may be preserved until mid-summer depending on the distance to water channel and sedimentary setting.  As a consequence the hydrological conditions at the study site show a drying phase (from end spring till the end of summer), with streams and ponds intermittently or completely dry during the dry season.

Most of Castelhanos stream basin is occupied by montado, an agro-silvo-pastoral ecosystem characterized by the presence of Mediterranean evergreen oaks (Quercus suber, Quercus rotundifolia), and a diverse shrubby understory vegetation, dominated by Cistus, Cytisus, Genista and Erica species. Along the stream riparian vegetation is well structured, with deep-rooted trees (Alnus glutinosa, Quercus faginea sub. broteroi) and dense herbaceous layer, mostly graminoids. The small and temporary ponds are the habitat for several helophyte species and submerged macrophytes.

Figure 2. From top-left and clock-wise: Montado understory vegetation, stream and ponds macrophytes diversity.

Most areas are grazed by sheep or cattle, and cork harvest is an important economic activity. The Herdade da Ribeira Abaixo is managed by the University of Lisbon, aiming at the nature conservation, science and teaching, with minimal agro-silvo-pastoril activities, while its neighbouring areas are more intensively managed. Within the area multiple small streams drain to the Castelhanos stream, and these present little hydrological modification by human activities.

In recent years tree mortality as become a concern for management, being associated with the combined effects of climate change and disease. Climate change has also been causing a shortening of the period with surface water availability, impacting aquatic communities. To adapt, the Herdade da Ribeira Abaixo to these challenges, several ponds were built adjacent to the streams (Fig. 3) as a nature-based solution with the goal of supporting biodiversity by increasing surface water availability for plants and animals, while promoting local water infiltration. These ponds and nearby streams are being studied within RECHARGE project to understand their relevance for biodiversity support and provision of ecosystem services.

Figure 3. Streams and built ponds. From top-left and clockwise: natural stream; pond in early summer, spring and early spring.