Abstract: River Arbúcies biophysical modelling, final report
The Arbúcies river is the smallest of the eight main demonstration catchments in REFRESH. It is a subcatchment of the larger river La Tordera, located near to the coast of Catalunya in N.E. Spain. As the catchment is largely forested, with only small areas of agriculture (6 %), the Arbúcies is not substantially affected by diffuse pollution from arable land. The main source of pollutants (with respect to eutrophication) was believed to be the sewage treatment works (STW) of the town of Arbúcies. This study aimed to calibrate the hydrological and hydrochemical models of PERSiST, INCA-N with PERSiST and INCA-P, to simulate future hydrology and soil- and stream-water nitrogen and phosphorus concentrations under a number of different climate and land use scenarios.
For the climate scenarios, two of the models (KNMI-RACMO2-ECHAM5 and SMHIRCA-BCM) predicted a slightly wetter future with higher average runoff due to increased winter precipitation, whereas the third climate model (HadRM3-HadCM3Q), predicted a much drier future for all seasons. None of the climate scenarios had any great impact on nutrients (NO3 and soluble reactive phosphorus [SRP] concentrations). However, for the drier climate scenario, the streamwater total phosphorus (TP) concentrations increased substantially due to decreased dilution of the STW effluent. Furthermore, for the drier climate scenario, the total nutrient load transported by the river decreased substantially due to decreased diffuse leaching.
The four land use scenarios described a combination of future possible storylines, ranging from market- to environmental-oriented, and global to more local/national regulations. The land use scenarios had an influence on fertiliser applicationation rates, population, STW effluent, abstraction, and, to some extent, atmospheric nitrogen deposition. In the model-based applications, stream water quality was found to be more sensitive to land use changes under the dry climate scenario due to the impact from the STW. Total phosphorus increased more than two-fold for the marked-oriented land use scenarios when combined with the dry climate scenario. This did however not change the ecological status of River Arbúcies, which was classified as “good“ by the river outlet for contemporary conditions, as well as for all simulated future scenarios.
The modelling results also suggested that the STW effluent was very influential on in-stream nitrate concentrations before an improved nitrogen removal procedure was implemented in 2006. After this was installed however, sources from semi-natural areas dominated, namely deposition and soil mineralisation. Hence, possibilities for improving the water quality by local management with respect to nitrogen are very limited. For phosphorus however, the STW effluent was the dominant source for all simulated scenarios. The biological status for macroinvertebrates was also classified as “good“. However, the macroinvertebrates in the larger catchment of La Tordera are often affected by STW effluents, and prolonged periods of lower stream flow during future climate scenarios in combination with an increasing population pressure may put higher demands on STW treatment efficiencies in order to preserve or not to degrade the macroinvertebrate status.