Appropriate Simulation of Vertical Soil Water Fluxes and Soil Moisture in Land Surface Schemes with Implications for Runoff Generation

Accurate simulation of the partitioning of net radiation into latent and sensible heat fluxes between the land surface and atmosphere requires estimation of root zone soil moisture. Most codes that simulate these fluxes for land surface/atmosphere coupling use approximate methods that are computationally robust, efficient and parameter parsimonious to simulate soil moisture dynamics in the shallow subsurface within a few meters of the land surface. In contrast, accurate simulation of the partitioning of water from precipitation, throughfall, stemflow, snowmelt, or combination of these by the land surface depends on detailed understanding of the vertical distribution of soil hydraulic properties and the soil moisture profile, using a suitable numerical solver. In the case of soils where the permeability greatly exceeds the most common rainfall rates, infiltration excess runoff generation is infrequent and can be ignored. However, in arid and semi-arid regions with low permeability soils, or in areas of disturbed soils such as conventionally tilled agricultural fields, heavily grazed pastures, urban areas, infiltration excess runoff frequently occurs, particularly under the action of convective rainfall. In this case, some variation of the 1-D Richardson-Richards Equation (RRE) is the only rigorous way to compute the partitioning of water into soil moisture or runoff. General, robust, and accurate solution of the RRE is impeded by a number of challenges, which has led to the development of a large number of approximate solutions. While many of these approximations are verifiable in terms of simulating bulk soil moisture fluxes, they are falsifiable with respect to accurate simulation of runoff generation because the conditions that produce surface runoff demand computationally expensive solutions that are not guaranteed to converge, rendering their use in earth systems models or large-scale hydrological codes impractical. The result is land-atmosphere interaction codes that are quite good at heat flux partitioning at the land surface and are fast, efficient, and reliable, employ assumptions that render them incapable of accurately predicting surface runoff. This necessitates application of further conceptualizations of runoff generation that are generally poor substitutes for the physics of appropriate RRE coupled solutions. This presentation discusses the implications, and reviews current literature that offer promising ways forward.