Quantifying the Independent and Interactive Effects of Environmental Drivers on Dry-Day Evapotranspiration Between Two Slope Positions in a Larch Forest

Differences in environmental conditions due to slope topography result in differences in evapotranspiration along slopes, but it is unclear how changes in environmental conditions affect the variations in evapotranspiration along slopes. Therefore, we monitored dry-day evapotranspiration (ETd), solar radiation, vapor pressure deficit (VPD), and soil moisture downslope and upslope on a larch plantation hillslope from July to September 2023 to reveal the mechanisms driving ETd variations. The results revealed that the difference in ETd values between the downslope and upslope positions varied by month, with comparable ETd values at both positions in July and higher ETd values at the downslope position than at the upslope position in August and September. An ETd model combining the effects of solar radiation, VPD, and soil water content was developed, which explained 68% of the variation in ETd. The contributions of solar radiation, VPD, soil moisture, and their interactions to ETd varied across slope positions, and ETd was limited mainly by solar radiation downslope and by soil moisture upslope. Our study improves the understanding of the mechanisms governing the variations in evapotranspiration along slopes, and provides a new methodology for quantifying the effects of environmental differences between slope positions on evapotranspiration.