HESS Opinions: Are soils overrated in hydrology?

Abstract. Traditional "physically-based" hydrological models are based on the assumption that soil is key in determining water's fate. According to these models, soil properties determine water movement in both saturated and unsaturated zones, described by matrix flow formulas known as the Darcy-Richards equations. Soil properties would also determine plant available moisture and thereby control transpiration. These models are data demanding, computationally intensive, parameter rich and, as we shall show, founded on a wrong assumption. Instead, we argue the reverse: it is the movement of the water through a porous medium, creating preferential patterns, that determines soil properties; while water movement is primarily controlled by the ecosystem's reaction to the climatic drivers. According to this assumption, soil properties are a “consequence”, rather than a “cause” of water movement. It is not the soil that is in control of hydrology, it is the ecosystem. An important and favourable consequence of this climate and ecosystem-driven approach is that models developed with this approach do not require soil information, are computationally cheap, and parsimonious. Our assumption is motivated by several arguments. Firstly, in well-developed soils the dominant flow mechanism is preferential, which is not particularly related to soil properties, such as pF curves. Secondly, we observe that it is the ecosystem, rather than the soil, that determines the land-surface water balance and hydrological processes. Top-down analysis by large-sample datasets reveal that soil properties are often a poor predictor of hydrological signatures. Bottom-up models do not directly use field measurements of the soil, but "rebalance" the observed soil texture and translate it to soil structure by vegetation indices. Thus, soil-based models may be appropriate at small spatio-temporal scale in non-vegetated and agricultural environments, but introduce unnecessary complexity and sub-optimal results in catchments with permanent vegetation. Progress in hydrology largely relies on abandoning the compartmentalized approach, putting ecosystem at the centre of hydrology, and moving to a landscape-based modelling approach. This change in perspective is needed to build more realistic and simpler hydrological models that go beyond current stationarity assumptions, but instead regard catchments as the result of ecosystems' coevolution with atmosphere, biosphere, hydrosphere, pedosphere, and lithosphere.