Elusive Elements of Evaporation and Runoff Behavior Hidden Within Traditional Hydrological Measurements

Soil moisture exerts key controls on surface hydrological fluxes; in many climates, higher soil moisture levels 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, and this provides the motivation for a unique hydroclimatic analysis. 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 evaporation and runoff that can be compared to observations-based estimates. This allows for the determination of the optimal combination of the functional forms and thus for an estimation of how evaporation and runoff efficiencies vary with each other in nature – something that cannot be determined directly at the large scale with traditional measurements. The inferred optimal combination serves as a valuable guide for the development and evaluation of GCM-based land surface models, which by this measure are often found to be suboptimal. The framework also offers some tantalizing sidelights; for example, using the framework, one can infer surprisingly reasonable estimates of rooting depth from knowledge of precipitation, net radiation, and streamflow alone.