Abstract
Water storage and flow in shallow subsurface drives runoff generation, vegetation water use and nutrient cycling. Modelling these processes under non-steady state conditions is challenging, particularly in regions like the subtropics that experience extreme wet and dry periods. At the catchment-scale, physically-based equations (e.g., Richards equation) are impractical due to their complexity, while conceptual models typically rely on steady state assumptions not found in daily hydrological dynamics. We addressed this by developing a simple modelling framework for shallow subsurface water dynamics based on physical relationships and a proxy parameter for the fluxes induced by non-unit hydraulic gradients. We demonstrate its applicability for six generic soil textures and for an Acrisol in subtropical China. Results showed that our new approach represents top soil daily fluxes and storage better than, and as fast as, standard conceptual approaches. Moreover, it was less complex and up to two orders of magnitude faster than simulating Richards equation, making it easy to include in existing hydrological models.
Highlights
The top few centimetres of soils experience large fluxes associated with cycles in wetting and drying and bioturbation from plant roots and fauna
Since the Subsurface Modelling Framework (SSMF) was developed with the idea to offer a simple and flexible alternative to the complex solving of Richards equation, we evaluated the performance of the SSMF against Richards equation solutions for a range of soil textures
Similar to the HYDRUS simulations, the SSMF mostly fell within the range of the measured θd (Figure 4b), and the normalised root mean square deviation (NRMSD) between the SSMF values and the data was 0.07, slightly lower than for HYDRUS
Summary
The top few centimetres of soils experience large fluxes associated with cycles in wetting and drying and bioturbation from plant roots and fauna. These fluxes, combined with rainfall, impact and compaction can drastically change pore structure over time [1,2]. The complexity and dynamics of the pore structure of shallow soils [4] are not adequately considered in existing hydrological models, which could lead to errors in predictions. The potential impact of surface pore structure dynamics on hydrological processes is significant in agricultural regions, where tillage, traffic, carbon depletion and periods of bare soil exacerbate temporal variability. Pore structure dynamics include changes to soil bulk density, pore size distribution and the connectivity of pores [1], but temporal changes in these properties and their impact on hydraulic conductivity are often ignored in hydrological modelling [3]
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