# Simulation of the Infiltration of Fractured Rock in the Unsaturated Zone
## 1 Introduction  
The thicker the vadose zone, the more important are water infiltration dynamics for contaminant ingress into the saturated zone. In brittle sedimentary rocks or volcanic sequences, fractures have a major impact on this flow (e.g., [PERSON] [1], [PERSON] and [PERSON] [2]). It follows that the accurate prediction of unsaturated-zone fracture flow, and infiltration rate and depth during individual rain fall events is key for developing contamination management and treatment strategies ([PERSON] [3], [PERSON] et al. [4]). The high permeability and porosity contrast between fractures and matrix results in complex flow patterns (e.g., [PERSON] [5], [PERSON] et al. [6]). Although, typically, fractures contribute only a sub-percent fraction to the porosity of fractured porous rock (e.g., [PERSON] [7]), their permeability usually is many orders of magnitude larger than that of the rock matrix, dominating transport behaviour (e.g., [PERSON] [1]). A tiny fraction of the overall fracture population tends to carry most of the flow ([PERSON] et al. [8, 9]), and this flow tends to occur along preferential paths in the fractured rock mass ([PERSON] et al. [10], [PERSON] et al. [11], [PERSON] et al. [12]). This impact of fracture-related heterogeneity on flow is the subject of a vast literature ([PERSON] [13], [PERSON] and [PERSON] [14], [PERSON] [15], [PERSON] et al. [16], [PERSON] [17], [PERSON] and [PERSON] [18], [PERSON] et al. [19]). In fractured porous media, the rock matrix normally has a much higher matrix potential (capillary pressure) than the fractures. Thus, upon fracture infiltration from the earth's surface, water is sucked into the rock matrix. This process is known as spontaneous imbibition. Examining borehole data from Yucca Mountain, Nevada, USA, [PERSON] [20] asserted that capillary forces immobilise infiltrating water in the rock matrix even if the fractured rock is highly saturated. On the basis of the results of dual continuum fracture and matrix simulations, [PERSON] and [PERSON] [21] inferred that for fracture flow to occur, the rock matrix must be saturated. Their results seemed to indicate that for most parts of the vadose zone at Yucca Mountain, the impact of the fractures on infiltration could be neglected. However, [PERSON] et al. [22], [PERSON] [23], [PERSON] and [PERSON] [24] challenged this on the basis of field observations, arguing that infiltrating rain water rapidly flows down fractures and open channels with little or no lateral diversion. In their conceptual model, water only saturates the immediate vicinity of fracture walls and therefore keeps flowing downward. Following their reasoning, deep infiltration would not be limited to the scenario of a fully saturated rock matrix, and, due to the low fracture volume, even for a low precipitation rate, rain water may transgress a thick vadose zone, reaching the water table. Experiments conducted by [PERSON] and [PERSON] [25], [PERSON] et al. [26], [PERSON] et al. [27] show that flow rate influences fracture flow in more intricate ways. Water can either imbibe the matrix, such that the wetted halo keeps up with the saturation front in the fracture(s) or water may shoot far ahead through interconnected fractures. Fingering within the fracture plane may also contribute to the fast movement of the water front in the fractures ([PERSON] [28], [PERSON] [29]). Due to the highly nonlinear process interactions, underpinning this behaviour, predicting the velocity and penetration depth of infiltration fronts in the fractured vadose zone is challenging. Modelling this rich set of potential behaviours by numeric simulation without oversimplification was the objective giving rise to this paper. We present a new hybrid model that is validated with physical experiments. Beyond this numeric simulation research and software engineering, we apply the new scheme to a few idealised models of fractured rock to obtain first insights on the systematics of fracture infiltration in the vadose zone.