“Efficiency Space” – A Framework for Evaluating Joint Evaporation and Runoff Behavior


      Dr. Randal Koster, NASA Goddard Space Flight Center


At the land surface, higher soil moisture levels generally lead to both increased evaporation for a given amount of incoming radiation (increased “evaporation efficiency”) and increased runoff for a given amount of precipitation (increased “runoff efficiency”). Evaporation efficiency and runoff efficiency can thus be said to vary with each other, motivating the development of a unique hydroclimatic analysis framework. Using a simple water balance model fitted, in different experiments, with a wide variety of functional forms for evaporation and runoff efficiency, we transform net radiation and precipitation fields into fields of stream flow that can be directly evaluated against observations. The optimal combination of the functional forms – the combination that produces the most skillful stream flow simulations – provides an indication for how evaporation and runoff efficiencies vary with each other in nature, a relationship that can be said to define the overall character of land surface hydrological processes, at least to first order. The inferred optimal relationship is represented herein as a curve in “efficiency space” and should be valuable for the evaluation and development of GCM-based land surface models, which by this measure are often found to be suboptimal.