Evapotranspiration
The importance of evapotranspiration (ET) in sustaining the global- and continental-scale hydrologic cycle and replenishing the world's freshwater resources has been recognized for thousands of years. State-of-the-art climate models and even the old nineteenth century theories based on the Clausius-Clapeyron equation indicate that global ET should increase in a warmer climate resulting in an acceleration of the hydrologic cycle. However, attempts to reconstruct continental-scale ET over the past century are clouded with uncertainties in both magnitude and direction of long-term trends. Long-term proxies of ET, such as pan evaporation and the imbalance between rainfall and runoff in major river basins, do not agree about whether continental-scale ET is increasing or decreasing. ET is affected by a combination of factors, including warmer temperatures, decreased bulk canopy conductance associated with rising CO 2 concentrations, and large-scale land-use change. Attempts to resolve the uncertainty in ET trends are challenged by the difficulty in integrating microscale processes, such as water transport through soil pores and plant xylem, into a framework that can describe regional- and continental-scale patterns of ET. Novel theoretical 'tactics' are clearly needed to further the development of constitutive laws describing ET, perhaps moving beyond the nineteenth century laws such as the Clausius-Clapeyron, and to describe how to scale ET to regional and continental fluxes.
