Evidence of optimal water use by vegetation across a range of North American ecosystems

We present empirical evidence for a relationship between the modal ( most frequent) soil moisture level and the soil moisture level at which maximum evapotranspiration occurs for twenty-four flux tower sites in North America. We considered correlations and linear regressions between these two variables at annual, seasonal, bimonthly and monthly time scales for unimodal distributions of soil moisture, and found significant relationships between these two soil moisture variables at all time scales. Correlation was stronger during the summer than the winter, suggesting stronger coupling during the growing season. This coupling of modal soil moisture and soil moisture of maximum evapotranspiration suggests that vegetation may be optimizing productivity with respect to water use across different systems.