• This study devised a remote sensing-based method to continuously track the density changes in forests canopy within a catchment to elucidate the significance of forest growth in water balance on a catchment scale.
• This study utilized a novel method to assess the canopy evaporation, transpiration and soil evaporation separately by reconstructing the seasonality of forest phenology.
• This study elucidated that transpiration significantly increases and groundwater recharge decreases with forest growth in young or well-managed forests.
• This study pointed out that forest growth should be considered in long-term studies and ignoring the density change underestimates potential hydrological hazards and risks.

　Forest ecosystems are critical for adjusting the dynamic balance of the hydrological cycle. This balance is affected by vegetation community types, phenology and forest density. However, previous long-term and catchment-scale model studies usually have just focused on the changes in forest areas without the above factors. In this study, we designed a methodology and created a credible model using the Soil and Water Assessment Tool (SWAT) that can precisely estimate water balance variations caused by different ecosystem situations during long-term changes in forest density.

　We focused on the Yamato River catchment in Western Japan, which includes three planted forests and one native forest with markedly different vegetation community composition and management policy. In the process, we examined the ratio of coniferous vegetation and broad-leaved vegetation in different forest areas, used remote sensing methods to quantify the maximum and minimum leaf area index (LAI) of each forest region over 40 years, and calibrated the model by comparing the LAI growth curve, evapotranspiration and streamflow with observed data. Moreover, we separated the decadal canopy evaporation, transpiration and soil evaporation from the SWAT output results. We found that (1) forest evapotranspiration has increased in recent decades because of the increase in atmospheric CO2 level and the management practice for economic benefit policies; (2) in young or well-managed forests, the forest water balance may have changed significantly with forest growth.

　For long-term studies, it is necessary to distinguish the growth characteristics of different forests during different periods, and a detailed definition of a mixed forest is required. The forest parameters and growth characteristics are critical for understanding forest ecosystems and cannot be ignored at catchment-scale. (see Fig.2)

Reference
published in Science of the Total Environment (IF=10.753), 809, 151159, 2022, https://doi.org/10.1016/j.scitotenv.2021.151159