Effects of replacing natural grassland to Eucalyptus plantations on actual evapotranspiration using remote sensing data in Uruguay

Abstract

Uruguay has increased its Eucalyptus plantation area since the Forest Law No. 15,939 promulgation in 1987. This study investigates the impacts of replacing natural grasslands with Eucalyptus plantations in Paysandú, Uruguay, focusing on actual evapotranspiration (ET) using remote sensing techniques. Hence, soil moisture (SM) maps with high spatial resolutions were obtained using the multiple linear regression methodology with Landsat 8 optical and thermal data. These SM maps were integrated into the Walker et al. (2019) algorithm to estimate ET over two paired watersheds for the 2013–2023 period. The resulting SM demonstrated strong correlations with in situ measurements with determination coefficients (R²) higher than 0.88 and Root Mean Square Errors (RMSE) of about 0.03 m³ /m³ . The ET estimates captured seasonal dynamics, showing higher rates in forested areas, mainly during the spring-summer growing time. The analysis revealed that afforestation may significantly alter the water balance, increasing ET in the Eucalyptus-dominated watershed compared to the grassland site. The proposed methodology, coded in Google Earth Engine, simplifies the integration of different datasets to monitor ET with high spatial resolution, offering a valuable tool for land use planning and forest management, especially in poorly instrumented regions under land cover changes, such as grassland afforestation.