Time series analysis of the long-term hydrologic impacts of afforestation in the Águeda watershed of north-central Portugal

The north-central region of Portugal has under- gone significant land cover change since the early 1900s, with large-scale replacement of natural vegetation types with plantation forests. This transition consisted of an initial con- version primarily to Pinus pinaster, followed by a secondary transition to Eucalyptus globulus. This land cover change is likely to have altered the hydrologic functioning of this re- gion; however, these potential impacts are not fully under- stood. To contribute to a better understanding of the poten- tial hydrologic impacts of this land cover change, this study examines the temporal trends in 75 years of data from the Agueda watershed (part of the Vouga Basin) over the period of 1936-2010. A number of hydrometeorological variables were analyzed using a combined Thiel-Sen/Mann-Kendall trend-testing approach, to assess the magnitude and signifi- cance of patterns in the observed data. These trend tests indi- cated that there have been no significant reductions in stream- flow over either the entire test period, or during sub-record periods, despite the large-scale afforestation which has oc- curred. This lack of change in streamflow is attributed to the specific characteristics of the watershed and land cover change. By contrast, a number of significant trends were found for baseflow index, with positive trends in the early data record (primarily during Pinus pinaster afforestation), followed by negative trends later in the data record (primar- ily during Eucalyptus globulus afforestation). These trends are attributed to land use and vegetation impacts on stream- flow generating processes, both due to species differences and to alterations in soil properties (i.e., infiltration capac- ity, soil water repellency). These results highlight the impor- tance of considering both vegetation types/dynamics and wa- tershed characteristic when assessing hydrologic impacts, in particular with respect to soil properties.