Assessing effects of afforestation on streamflow in Uruguay: From small to large basins

Although there is a general understanding of how afforestation impact on streamflow, there is also requirement for additional empirical data both on a small and large basins scale, primarily due to the significant variability found in global data sets. A multi‐method approach is proposed to understand the impact of plantation forest cover changes on stream flow, aiming to address individual approach weaknesses and enable cross‐validation. Focusing on Uruguay as a case study, the data based analysis indicated a significant decrease in streamflow within four highly afforested large basins of the north region, occurring approximately when the afforestation reached 15% of the total basin area. Also three of those basins showed a significant decreasing trend in streamflow after the change‐point. A stronger link between afforestation increase and lower runoff‐rainfall ratio was found in autumn‐winter season compared to spring–summer, due to higher soil water availability. The model residual approach effectively isolated land use and land cover effects in large basins with gradual afforestation, utilizing long data series. Consistently, statistically significant trends in the residual series indicated decreased streamflow in the after‐afforestation period in the four large basins. Finally a clear difference in the magnitude of the change emerged between the large and small basins, highlighting the influence of rainfall heterogeneity, landscape control and forest management on the scaling behaviour of streamflow and runoff ratio.