Horizontal soil water potential heterogeneity: simplifying approaches for crop water dynamics models

摘要

Soil water potential (SWP) is known to affect plant water status, and even though observations demonstrate that SWP distribution around roots may limit plant water avail-ability, its horizontal heterogeneity within the root zone is of-ten neglected in hydrological models. As motive, using a hor-izontal discretisation signi?cantly larger than one centimetre is often essential for computing time considerations, espe-cially for large-scale hydrodynamics models. In this paper, we simulate soil and root system hydrodynamics at the cen-timetre scale and evaluate approaches to upscale variables and parameters related to root water uptake (RWU) for two crop systems: a densely seeded crop with an average uniform distribution of roots in the horizontal direction (winter wheat) and a wide-row crop with lateral variations in root density (maize). In a ?rst approach, the upscaled water potential at soil–root interfaces was assumed to equal the bulk SWP of the upscaled soil element. Using this assumption, the 3-D high-resolution model could be accurately upscaled to a 2-D model for maize and a 1-D model for wheat. The accuracy of the upscaled models generally increased with soil hydraulic conductivity, lateral homogeneity of root distribution, and low transpiration rate. The link between horizontal upscaling and an implicit assumption on soil water redistribution was demonstrated in quantitative terms, and explained upscaling accuracy. In a second approach, the soil–root interface water potential was estimated by using a constant rate analytical so-lution of the axisymmetric soil water ?ow towards individual roots. In addition to the theoretical model properties, effec-tive properties were tested in order to account for unful?lled assumptions of the analytical solution: non-uniform lateral root distributions and transient RWU rates. Signi?cant im-provements were however only noticed for winter wheat, for which the ?rst approach was already satisfying. This study con?rms that the use of 1-D spatial discretisation to repre-sent soil–plant water dynamics is a worthy choice for densely seeded crops. For wide-row crops, e.g. maize, further theo-retical developments that better account for horizontal SWP heterogeneity might be needed in order to properly predict soil–plant hydrodynamics in 1-D.