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The subsurface flow was mentioned above in the part of saturation excess overland flow.

So, what is subsurface flow? Subsurface flow comes from the subsurface hydrologic processes, which are very complicated and unable to observe directly. The subsurface hydrologic processes are driven by many factors that influence the paths and rates of water movement. Therefore, subsurface flow is the major source of uncertainty in hydrologic models (Beven, 2012).

Subsurface flow occurs when water moves down a hillslope through soil layers or permeable bedrock to contribute to the runoff (Figure 1.3c-g). This process requires that the hydraulic lateral conductivity of the environment is larger than the vertical conductivity.

Besides, subsurface flow is favored by the presence of impermeable shallow soil layers.

Subsurface flow can play the key role of flood runoff in humid environments and steep terrain with conductive soils (Anderson and Burt, 1990). We can see the subsurface flow in most upland terrain, and it may be dominant in humid regions with vegetal covering and well-drained soils. Meanwhile, this process can occur only under certain extreme conditions in lowland regions and in drier climates, for example, under high rainfall and high antecedent soil moisture.

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Figure 1.3: (Rinderer et al., 2012) Different types of surface and subsurface runoff processes. (a) Overland flow is generated when the infiltration capacity of the soil layers at or near the surface is exceeded; (b) Overland flow can also be generated when the storage capacity of the soil layers at or near the surface is exceeded, (c) Lateral subsurface runoff increases when ground water rises into more transmissive soil layers, (d) Groundwater table tends to be low and soil water can percolate deep down into the soil or bedrock when the soil has coarse and highly permeable texture, (e) The flow rates toward the nearest stream channel can be high and subsurface runoff is generated when there are horizontal macropores, (f) A perched water table will form leading to lateral subsurface flow within the saturated soil layer when there exists water restricting layer in the soil profile, (g) lateral subsurface flow can also occur when there are water-restricting layers at the soil-bedrock interface.

The subsurface flows can accompany these mechanisms:

Piston effect: The piston effect assumes that water that falls on a slope is transmitted downstream with a quasi-instantaneous pressure wave. It may cause a sudden exfiltration on the watershed.

Gravitational subsurface flow in macropores

Subsurface flow may be carried through macro pores which lead the water to unsaturated areas. Macropores are pores in which the capillarity phenomena do not exist. We can distinguish four types of macropores:

 Natural macropores: these macropores appear due to high initial hydraulic conductivity.

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 Pores which formation is the result of micro soil fauna: these pores normally locate in the superior soil layer (0-100 cm) with a dimension of 1-50 mm.

 Pores which formation is due to vegetation roots. These pores normally become free when the plants die. Therefore, the structure of the macropores network of this type will depend on both the type of vegetation and its growing state.

 Cracks.

Figure 1.4: Pathways followed by a subsurface runoff on hillslopes (Kirkby, 1978)

Detailed cross-section through a hillslope that exposes in the pathways infiltrated water may follow as described in Figure 1.4. Infiltrated water may flow through the structural voids in the soil matrix, which are either small or large (macropores), including the opened passage in the soil caused by decaying roots and animals. Among these passages, subsurface

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flow is favored by macropores. The different permeability of the horizontal soil matrix may lead to the build-up of a saturated wedge above a soil horizon interface. Water can either flow laterally from these saturated wedges through the soil matrix or enter the macropores in the soil, before going to the stream. Both processes mentioned above result in the subsurface flow called interflow.

Pipe flow

Another mechanism of subsurface flow is pipe flow. Pipe flow leads water to an unsaturated environment. This mechanism is similar to macro-pore to some extent, however, pipes are considered to be larger and more connective than macropores, as they can form a continuous network in the soil.

Transmissivity feedback

Subsurface flow can also occur by a mechanism called transmissivity feedback (Weiler and McDonnell, 2004). This phenomenon happens when water infiltrates rapidly along preferential pathways. This leads to the rapid rising of groundwater, reaching the highly permeable soil layers or macro-pore networks. As a consequent, water is then transmitted downslope (Figure 1.5).

Figure 1.5: (Tarboton, 2003) Schematic illustration of the macropore network being activated due to the rise in groundwater resulting in rapid lateral flow

Rapid lateral flow at the soil-bedrock interface

Another mechanism of subsurface flow, called lateral flow at the soil-bedrock interface (Weiler and McDonnell, 2004), occurs in regions with steep terrain, low permeability

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bedrock and thin soil cover (Figure 1.6). In these regions, water can move rapidly through the thin soil layer and perch at the soil-bedrock interface. The addition of only a small amount of rainfall is enough to produce saturation at the soil-bedrock or soil-impeding layer interface because moisture content near the bedrock interface is often close to saturated.

Figure 1.6: Rapid lateral flow at the soil-bedrock interface (Tarboton, 2003) Subsurface stormflow by groundwater ridging

Figure 1.7: Groundwater ridging subsurface stormflow processes in an area of high infiltration (Tarboton, 2003). The shaded areas represent graphs of soil moisture at the base, middle and near the top of the hillslope (a) before the onset of rainfall; (b) as an initial response to rainfall;

and (c) after continuing rainfall.

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The processes involved in the generation of subsurface stormflow by groundwater ridging are illustrated in Figure 1.7. Before the onset of rainfall, the water table slopes slightly towards the channel to maintain the base flow of the stream (Figure 1.7a). When the rain starts, initially the water input leads the water table to rise near the stream but keep further upslope unchanged (Figure 1.7b). This leads to the increase of hydraulic gradient between groundwater and stream, resulting in the subsurface flow into the stream. As the rain continues, the water table rises to the surface over the lower part of the hillslope and the saturated area is expanding uphill. The water arises from this saturated area and runs downslope called return flow (Figure 1.7c). In this case, there is also direct precipitation onto the saturated zone which forms saturation excess runoff.