Evapotranspiration (ET), the process by which plants release water vapor into the air, is often less than it could be. Understanding when and why this happens is important for the exchange of gases between the Earth's surface and atmosphere, and for predicting how ecosystems change. However, it's challenging to predict ET because it's affected by many factors, including soil moisture and air dryness. In a conifer forest in Colorado's Rocky Mountains, we studied how ET changes with changes in soil moisture and air dryness. We used data from sensors and satellite estimates to measure ET, soil moisture, and water content in the air. We found that when soil moisture decreased, ET also decreased. When air dryness increased, ET initially increased, but then decreased beyond a certain threshold. We also found that soil moisture and air dryness interact. When soil moisture was high, air dryness had a bigger impact on ET. When soil moisture was low, it became the limiting factor, and air dryness had less of an impact.This study improves our understanding of ET and can help predict how forests respond to changes in climate.