How efficient are one-dimensional models to reproduce the hydrodynamic behavior of structured soils subjected to multi-step outflow experiments?

SummaryIn this paper we investigate numerically the three following questions: (1) Do MSO data show the impact of soil structure on water flow? (2) If so, to what extent are the common one-dimensional hydraulic models able to apprehend any preferential flow features? and (3) What are the predictive capabilities of these models parameterized with MSO data? A Bayesian framework was used to infer the hydraulic models under virtual MSO conditions, for soil samples with different levels of heterogeneity. This, by coupling the HYDRUS-1D model with the DiffeRential Evolution Adaptive Metropolis algorithm (DREAM). Regarding questions 1 and 2, our findings indicate that (i) large outflow observed during the first steps of MSO may express the behavior of a real macropore, or of structural heterogeneity inside the soil core; (ii) this behavior cannot be characterized with the MV (Mualem, 1976 and van Genuchten, 1980) and DR (Durner, 1994) models whereas mobile–immobile (Philip, 1968 and van Genuchten and Wierenga, 1976) and dual-permeability (Gerke and van Genuchten, 1993a and Gerke and van Genuchten, 1993b) preferential flow models can provide excellent fits depending on the soil architecture type; (iii) in the presence of macropores, the DUAL model performed excellently despite frequent convergence problems of the HYDRUS-1D code. Furthermore, neglecting the first MSO steps can result in a perfect match of the soil matrix behavior by the MV model. Regarding question 3, a virtual infiltration front experiment reveals that predictive capabilities of the MIM model parameterized with MSO are not satisfactorily. This indicates that the MIM model underlying concept induce excellent MSO fits for wrong reasons. Similar findings hold for the DUAL model and soil architectures other than macroporous. For a macroporous soil, i.e., the conceptual structure for which the DUAL model was designed, the latter model parameterized with MSO data can provide consistent results under infiltration conditions. This, however, should be verified with real soils. Lastly, neglecting the first MSO steps to calibrate the MV model may induce, in the presence of macropores, significant errors when predicting the matrix behavior under infiltration. This because of the macropore-matrix water transfer.