# Simulation of the Infiltration of Fractured Rock in the Unsaturated Zone
## 5 Discussion  
The simulation results confirm that aperture is of central importance for the transport velocity in fractures embedded in a permeable rock matrix ([PERSON] and [PERSON] (2017); [PERSON] and [PERSON] (2018); [PERSON] et al. (2018)). Our work highlights that the velocity of the wetting front during gravity-driven flow in small-aperture fractures strongly depends on the magnitude of capillary-driven fracture-matrix transfer. Our study corroborates [PERSON] et al. (2018) finding that the matrix hydraulic conductivity and initial moisture content are most influential for this exchange. However, this control becomes less and less important with increasing fracture aperture. Above a certain threshold aperture flow velocity matches the analytic solution for gravity-dominated flow in an impermeable matrix.  
While only narrow \(\leq\)1-mm apertures were investigated herein, we observed that for the heavy to violent rain that typifies thunderstorms, gravitational forces began to dominate over viscous in the one-mm aperture fractures underneath the saturated sandy soil layer. This seems to indicate that fracture-matrix transfer will have little effect on the wetting front velocity in cm-aperture fractures. This is the scope of ongoing work on the larger scale which will be the subject of a further publication.  
[PERSON] and [PERSON] (2018) argued that the rock matrix needs to be fully water saturated before there is fluid flow in the fractures. This does not apply for larger fractures. Earlier on, [PERSON] and [PERSON] (2018) had already proposed that it only takes a wet fracture wall for water to infiltrate a fracture. The experimental work on film flow by [PERSON] and [PERSON] (2018) and the numeric simulations presented herein confirm this suggestion: While water spontaneously imbibes the rock surrounding the fracture, this infiltration is restricted to a relatively small fracture selvage and once this wet zone is established, the wetting front in the fracture advances freely. Consistent with the capillary interface model, spontaneous imbibition occurs rapidly directly adjacent to the fracture because of the sharp initial capillary pressure gradient, but the exchange rate quickly drops asympotically as the selvage widens. Transfer is marginal when the rock matrix already is at a saturation that reflects the plateau of the capillary pressure curve at the time when fracture infiltration occurs, and it ceases completely as the matrix approaches saturation.  
Aperture variations induce unique flow patterns within the fracture(s). If they are slight (<10% of the overall aperture), sheet flow remains prominent, in agreement with the conclusion of [PERSON] et al. (2018). When they are larger, fingering destabilises the wetting  
Figure 21: Water velocity (blue arrows) and air velocity (green arrows) and wetting-front within the fracture of models TUFF3D and TUFF3D-VA. (**a**) water velocity, constant aperture, (**b**) air velocity, constant aperture, (**c**) water velocity, varied aperture and (**d**) air velocity, varied aperture.  
front and water flow in the individual fingers is accelerated ([PERSON] [29], [PERSON] and [PERSON] [83], [PERSON] and [PERSON] [84], [PERSON] et al. [85]).