ACTIVIDADES GENERALES PARA LA CONSTRUCCIÓN DE REDES DE ACCESO EN COBRE

Another common assumption underlying IRF is that all runoff from different generation mechanisms is treated equally without any differentiation. This, however, is only valid in a certain sense when single runoff generation mechanism totally dominates in a small catchment. For example, Horton overland flow (infiltration excess) significantly dominates under impermeable soil and intense rainfall, or Dunne overland flow (saturation excess) significantly dominates under permeable soil and long storm duration. In fact, water particles generated from different mechanisms travel through different paths before arriving at the outlet. Runoff generated as Dunne overland flow, for instance, starts moving from saturated riparian areas, goes into channel network, and finally arrives at the outlet. Runoff generated as Horton overland flow starts from unsaturated upper-hillslope areas, goes downward into saturated riparian areas and channel network, and arrives at the outlet. Runoff generated as subsurface storm flow starts from the location of its infiltration within unsaturated area, goes vertically to the saturated layer and laterally downward along hillslope very slowly as subsurface water, comes out as seepage flow at saturated areas, then joins channel network and arrives at the outlet. Based on the above argument, travel time distributions corresponding to different runoff generation mechanisms could be very different. We therefore extend the concept of CIRF to each runoff generation mechanism, i.e., CIRF corresponding to Dunne overland flow, CIRF corresponding to Horton overland flow and CIRF corresponding to subsurface storm flow respectively. The final definition of CIRF is thus given as following: _Ω( /τ,Ω)=

[

]

Where Ω indicates different runoff generation mechanisms. For convenience, we note hHorton , hDunne and hsub as CIRF corresponding to Horton overland flow, Dunne overland flow and subsurface storm flow respectively.

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Figure 5.3 shows typical examples of hHorton, hDunne and hsub generated based on the hypothetical distributed hydrological model introduced previously. According to its definition, CIRF can only be constructed at a specified instantaneous moment. In this work CIRF is always constructed at the moment immediately before rainfall stops. For the purpose of conceptual illustration, in Figure 5.3 the climate, soil and topographic conditions are carefully chosen as the inputs or parameters of the model, so that all three runoff generation mechanisms could occur together during the same storm event. Note that in Figure 5.3 the area under each curve is unit, which is consistent with the definition of probability density function. Here the sample spaces of the probability density functions are the connected areas of Horton overland flow generation, the connected areas of Dunne overland flow generation, and the areas of subsurface runoff generation respectively. The areas of subsurface runoff generation are defined as those unsaturated areas receiving infiltration. In this work, it is assumed that once a particle infiltrated reaches the saturated layer, it will flow all the way to the channel without being trapped by the hollows on the bedrock or taken up by evapotranspiration. This assumption can be relaxed later and the effects of bedrock topography or evapotranspiration on subsurface water movement can be explored in future work.

Figure 5.3 confirms that the travel time distributions corresponding to different runoff generation mechanisms, as manifested by CIRFs, are significantly different from each other. CIRF for Horton overland flow, hHorton, has a lower peak and larger span. CIRF for Dunne overland flow, hDunne, has a higher peak and smaller span. The time-to- peak of h_Horton is also longer than that of h_Dunne. The travel time of each particle is governed by its travel path and travel celerity. The travel path of each particle, from either Horton overland flow or Dunne overland flow, consists of two parts: hillslope part and channel part. The travel celerity across the channel part is much higher than the travel celerity across the hillslope part. The water particles of Horton overland flow start from unsaturated areas which are at some distance from the channel zone. The travel time of these particles across hillslope will be comparable with their travel time in the channel, especially in small catchments. The particles of Dunne overland flow start from saturated areas including riparian zone and channel zone. The travel time of these particles across

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hillslope will be negligible when just a small percentage of catchment area is saturated. If within a small catchment the soil is shallow and permeable and storm duration is long enough, the saturated area expands and the travel time across saturated hillslope area could be increasingly significant.

Different from Horton overland flow and Dunne overland flow, the flow path of a particle from subsurface storm flow essentially consists of four parts: vertical unsaturated subsurface flow part from ground surface to saturated layer, lateral subsurface flow part from the location of its infiltration to the saturated area, overland flow part across saturated area to channel network, and channel flow part through channel network to the outlet. The movement of an infiltrated water particle in unsaturated layer is subject to many physical mechanisms such as recharge, evapotranspiration, lateral unsaturated subsurface flow and so on. The estimation of the travel time within unsaturated layer is beyond this work. For the sake of simplicity, it is assumed that a particle from subsurface arrives at the saturated layer immediately after infiltration, and moves laterally as saturated subsurface flow until it reaches the channel. Hence the flow path of a particle from subsurface storm flow also consists of hillslope part (lateral subsurface flow from the location of infiltration to the channel) and channel part. The celerity of subsurface flow is of magnitudes less than that of overland flow, so the travel time through subsurface flow part is way much longer than the channel part. h_sub is exclusively dominated by hillslope response as shown in Figure 5.3.

Table 5.1 shows the mean hillslope travel times and mean channel travel times for each runoff generation mechanisms averaged through the whole catchment. Note these mean travel times are empirical values obtained from a hypothetical distributed hydrologic model. The climatic conditions, topographic conditions and soil properties used to calculate the values in Table 5.1 are the same as those to generate Figure 5.3.

It is straightforward that the advection effect of catchment on the runoff generated can be effectively captured by CIRF. CIRF also effectively quantifies the dispersion effect of catchment, both geomorphologic and kinematic, because the travel times of water particles are estimated from spatially heterogeneous flow paths and varying local

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velocities. Hydrodynamic dispersion is not captured, but it is much less significant comparing with the others and could be regarded as negligible (Robinson et al., 1995; Botter and Rinaldo, 2003). CIRF incorporates these dispersion effects in a single function to quantify the dynamic behavior of water particle movement at the catchment scale, and thus can be a useful tool to investigate the temporal pattern of catchment hydrological response.