Effects of preferential flow
Heterogeneity of land soils with respect to porosity and permeability
can arise from a bunch of pedologic features or anthropologic
activities, including soil fractures, macropores formed by rotten roots
and/or earthworms, surface crust, soil tillage, and among others (Gerke
and Genuchten, 1993; Cuthbert et al., 2013). In dry environment, soil
fractures are particularly common in numerous farmlands as soil shrinks
due to water loss. These fractures and/or macropores form hydraulic
conduits for preferential flow, which is very common in unsaturated
soils. Presence of preferential paths in soil generally compromises
irrigation efficiency, due to the ‘bypass flow’ in them without
necessity to fill deficit in topsoil matrix (Cuthbert et al., 2013).
Despite the known effects of soil heterogeneity on soil water
distribution and groundwater, soil preferential paths are rarely
accounted for irrigation evaluations.
Fig. 13 and Fig. 14 present vertical plumes of VWC for C4 under
irrigation and in the intermittence of irrigation, respectively. The
configuration of the fractures in the soil has been presented in Fig. 1.
The porosity and permeability of the fractures is assumed to be 0.9 and
100 Darcy, respectively. Ideally, the infiltrated water travels downward
and saturates the soil successively in a known ‘tipping bucket’ fashion
(Emerman, 1995). This means that excess water flows to deeper layers
when the soil moisture level reaches saturation. The presence of soil
fractures for C4 leads to complex distribution of VWC, which is
characterized by apparent preferential flows along the fractures in the
soil, as illustrated in Fig. 13 and Fig. 14 as well. VWCs that is as
high as 0.491 are observed in the fracture network under condition of
irrigation. The values, however, change to 0.457 when there is free of
irrigation, due to water redistribution. It should be noted that
relatively high VWCs are also observed in some of the matrix pores close
to the fractures. This is due to the matrix imbibition of fracture flow
under the function of the capillary pressure gradient between the matrix
pores and fracture apertures.
The VWC plumes have more irregular forms due to uneven wet fronts
induced by preferential flows. The wet front for C4 penetrates to the
depth of 1.35 m at the 160 hr and 1.89 m at the 720 hr respectively,
which is 115.9% and 53.88% deeper than that for C0, only due to the
presence of preferential flow through the fractures. The infiltrated
water travels as fast as 0.2 m per day through preferential pathways in
our model. It should be noted that the configuration of fractures or
cracks are expected to have substantial effects on the local water
movement in soil. This topic, however, is beyond the scope of the
present study.
To further look into the effects of preferential flow on SWC, in Fig. 15
the profiles of VWC are plotted for C0 and C4 at the four selected
times. The soil with fractures has relatively higher VWC at lower depths
in the early time (Fig. 15a, b), due to deeper wet fronts formed by
faster movement of preferential flow. However, since we only calculate
the VWCs for the BOI, the soil within depth of 1.0 m becomes drier at
the later times for C4 than that for C0 (Fig. 15c, d), because a portion
of water has traveled down to below the lower boundary of the BOI (i.e.
1.0 m).
Figure 13. Vertical plumes of VWC for the case C4 at various times under
irrigation.
Figure 14. Vertical plumes of VWC for the case C4 at various times
during the irrigation intermittences.
Figure 15. Vertical profiles of VWC for C4 in comparison with C0 at
various times.