Evapotranspiration and Land Surface Process Responses to Afforestation in Western Taiwan: a Comparison between Dry and Wet Weather Conditions

An afforestation project was initiated in the western plain of Taiwan to convert abandoned farming lands into forests to improve the ecological and environmental conditions. This study was conducted to understand the potential impacts of this land cover change on evapotranspiration (ET) and other land surface processes and the differences in the impacts under clear and rainy weather conditions. Numerical simulations with the land surface covered with crops and forests were conducted using an atmosphere-land coupled model during a summer monsoon season. Sensitivity experiments were conducted to understand the possible impacts of the convection and atmospheric planetary boundary layer parameterization schemes used in this study on the simulation results. The results of the entire simulation period indicate that, although the maximum solar radiation is increased by 30 W m-2 from about 800 W m-2 due to smaller albedo after afforestation, the maximum ET near noon time is decreased by about 3 mm h-1 from about 17 mm h-1 on clear days, mainly due to increased stem area index and reduced wind speed within the forest canopies. Meanwhile, the maximum sensible heat flux is increased by about 100 W m-2 from 200 W m-2 due to larger total vegetation area (the sum of leaf and stem area index) in forests. Similar responses in ET and other land surface processes to the afforestation are obtained for clear weather conditions. In contrast, ET is increased and sensible heat flux is decreased under rainy conditions. The changes in ET are more important on clear days than on rainy days in determining the responses over the entire simulation period. The sensitivity experiments confirmed the simulated responses of ET and other land surface processes. The results suggest that afforestation would modulate hydrological cycles by reducing ET on clear days and elevating ET on rainy days, thereby reducing the risks of hydrological extremes.