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.