Some Issues of Scale That Challenge Understanding of Evapotranspiration.

Life is dynamic, adaptive, self-organized and dissipative so it tends to manipulate physical processes for its benefit—such is the case with evapotranspiration. Evaporation is a physical process, and transpiration is life’s way of controlling the natural evaporation process. The energy source of all terrestrial life on Earth is the sun, and plants cannot harvest this energy without controlling the water cycle. Thus wherever life flourishes in diverse forms, it tends to dominate the water cycle by optimizing evapotranspiration. The fraction of available energy at the Earth's surface that is partitioned to evapotranspiration depends primarily on soil water storage. The amount of water stored in the soil is determined by the partitioning of rainfall into infiltration and runoff at the soil surface. Our limited understanding of the dynamics of the top centimeter of soil and the role that living systems play in this regard, and our minimal concern for the water that drains below the root system, are having major and under-appreciated impacts on the water stored within the root zone—thus evapotranspiration. Minimal attention to these factors combined with unresolved disciplinary boundary conflicts and over emphasis on the short term (a few seasons or less) are limiting a deeper understanding of the disastrous effect that many modern agricultural practices are having on the sustainability of terrestrial life. This presentation outlines several challenges that we are capable of overcoming. Further, it offers some insights into the magnitude of the effects that we often overlook because of difficulties with estimating their impact.