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.
Bibliographical noteFunding: This study was part of the UK–China Red Soils CZO project, funded by the National Environment Research Council (grant NE/N007611/1) and the National Sciences Foundation of China (NSFC: 41571130051, 41571130053, 41371235).
Acknowledgments: Special thanks go to the staff of the Ecological Experimental Station of Red Soil, of the Institute of Soil Sci. of CAS, who provided the detailed meteorological data. We would like to thank three anonymous reviewers for providing constructive criticism on an earlier version of this manuscript.
Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4441/11/8/1725/s1:
S1. HYDRUS 2D set up and calibration. We provide in the Supplementary Material the Matlab code of the developed model (ssmf.m), its subdaily routine (ssmf_subdaily.m) and the pedotransfer equations for different soil textures (pedotransfer_func.m). This is original content, developed by Lucile Verrot, made freely available as part as this study, and developed in Matlab 2014a.
- hydrological modelling
- soil water content
- soil water fluxes
- vadose zone
- non-unit hydraulic gradient
- transient state