Effects of surface seal
Soil surface seal can be formed due to reasons like surface crusting, salinization, or reduced tilth. The thickness of the seal is typically 5 mm, as is specified for C2. Formation of soil surface seal may cause lower porosity, reduced infiltration capacity, and decreased irrigation efficiency (Souza et al., 2014; Hardie and Almajmaie, 2019). To investigate the effects of the seal on water infiltration and soil water distribution, four additional simulations had been run with much lower permeabilities for the topmost 5 mm-layer compared to that for C0. The simulation runs with various low permeabilities cause convergence failure at different times. Since the permeability of the surface seal is the only parameter subject to change, it can be responsible for the convergence failure. Therefore, the infiltrability (i.e. infiltration capacity) of the soil surface can be simply determined by such a method known as ‘try-and-error’. Accurate quantification of infiltration is essential for irrigation system design, irrigation scheduling and irrigation systems optimization and management.
Fig. 10 illustrates the infiltrabiliy as a function of the permeability of the seal. The infiltrability for C0 (i.e. without seal) is 1.05×10-3 kg/s, which is 51% greater than the actual infiltration rate applied for this case. As is shown in Fig. 10, the infiltrability drops exponentially or polynomially with the decrease in permeability of the surface. The infiltrability for a permeability of 5 mD, which is a typical order of magnitude for developed seals of soil surface according to McIntyre (1958), is about 97.1% lower than that for C0. This result is comparable to 93.0% as has been proposed by Hardie and Almajmaie (2019). However, it is far higher than 40~50% that have been reported by previous studies (e.g., Eisenhauer, 1984; Bu et al., 2014). It should be noted that to what extent the soil surface seal affects infiltration, largely depends on how large the permeability of the superficial soil will be altered due to the formation of the seal, and the rate of rainfall or irrigation. It is straightforward that low-permeability soils are expected to have smaller changes in infiltrability once they have seals on the surface. The relatively high permeability of loamy sand before surface sealing in this study is thought to be the primary reason for the tremendous reduction in the infiltration. Furthermore, overestimation of porosity of the seal due to sampling difficulty is a normal reason for underestimation of infiltration reduction based on sampling tests (Hardie and Almajmaie, 2019). Last but not least, numerical simulation can hardly describe the integrity of the seal, which however is probably quite common for actual cases. When the permeability of the superficial horizon becomes lower than roughly 500 mD, the infiltrability of the soil tends to decrease slowly, and when the permeability is below 100 mD, the infiltrability becomes marginal.
Fig. 11 assists in a further step to identify the impacts of soil surface seal on infiltration. It can be seen that the infiltration through the tillage layer is strongly controlled by the surface seal. The ‘1410 mD’ in the legend corresponds to the permeability of the surface layer for C0, as is illustrated in the figure, reduced permeabilities for the additional cases render remarkably lower VWCs along the depth, except for the case ‘775.5 mD’ in the early times. This can be an effect analogous to soil water redistribution. Reduced permeability caused weak infiltration and dominant redistribution of the soil water. Nevertheless, if the permeability of the surface seal is extremely low, the soil can be pretty dry, as is demonstrated in Fig. 11. The presence of soil surface seal hinders infiltration and greatly reduces saturation of soil, which will further lower hydraulic conductivity as low as 1/2000 folds, as have been proposed by some scholars (e.g. McIntyre, 1958).
For our simulation cases in Fig. 11, introduction of a surface seal with permeability of 140 mD causes change in VWC from 0.21 to lower than 0.15 for the loamy sand under irrigation.
Figure 10. The infiltrabilities of soil for various cases of permeability of the surface crust considered.
Figure 11. Comparison of vertical profiles of VWC between cases with differing permeabilities of soil surface seal at various times.