Modelling the impact of climate and land cover change on hydrology and water quality in a forest watershed in the Basque Country (Northern Spain)

The separate and combined effects of climate change and forest clearcutting on discharge, suspended particulate matter (SPM) and particulate organic carbon (POC) load at a seasonal and annual scale were evaluated for the Oka catchment. Compared to the baseline scenario (1970-2000), climate change displayed a decrease in annual rainfall (RCP4.5: 27% and RCP8.5: 28%) and an increase in mean temperature (12% for both emission scenarios). Together with the decrease in rainfall, a decrease in discharge and, consequently, in SPM and POC load was also displayed. In RCP4.5, annual discharge, SPM and POC load decreased by 16%, 68% and 38%, respectively. A slightly larger decrease was found in RCP8.5: 21% for discharge, 70% for SPM load and 41% for POC load. Evapotranspiration (ET) increased relative to the baseline, with a change of 15% (RCP4.5) and 16% (RCP8.5). With regard to forest clearcut scenarios, annual discharge ranged between 3% (Scenario 1) and 15% (Scenario 3). At the same time, ET decreased by between 2% (Scenario 1) and 13% (Scenario 3) relative to the baseline (2001-2012). The model predicted a rise in SPM load of between 19% (Scenario 1) and 106% (Scenario 3). The predicted annual POC load ranged between 9% (Scenario 1) and 47% (Scenario 3). The combination of climate change and forest clearcutting scenarios showed a reduction in discharge, SPM and POC load compared to the baseline. Discharge, SPM and POC load decrease ranged between 2-18%, 30-63% and 12-36% in scenarios 8 and 7, respectively.