Unique Relationship Between Rate and Cumulative Flow: A Property of Infiltration and Evaporation in Soils

Infiltration and evaporation are key components of the Earth hydrological cycle that depend on soil properties and climatic conditions. Here we show that, for both infiltration and evaporation, the water flow rate is a unique function of the cumulative flow in the soil. Hence, for a given soil, infiltration and evaporation curves corresponding to different wetting and drying rates are represented by one curve characterizing the capacity of the system for the relevant process. This insight reveals a natural and powerful property that shapes water interactions between soil and atmosphere. A quantitative expression of the evaporation capacity curve is proposed. A dimensionless representation of this model offers a simple tool to predict evaporation under natural soil and climatic conditions that vary in space and time. The infiltration and evaporation capacity models can be easily embedded in Earth System Models to estimate the hydrological response at various spatial and temporal scales.