Synthetic CIRF

5.3.3.1 Construction of synthetic CIRF

The hydrological response at the outlet of a catchment is governed not only by dynamics of water particles movement through the catchment, but also by the runoff composition from different runoff generation mechanisms, i.e., the relative dominance of Horton overland flow, Dunne overland flow, or subsurface storm runoff. Both of water particle movement and runoff composition are controlled by climate, soil and topographic conditions. In order to examine the impact of climate, soil and topography on the temporal pattern of catchment hydrological response in a comprehensive way, we construct synthetic CIRF. The synthetic CIRF is just a composite of CIRFs corresponding to three runoff generation mechanisms weighted with the volumetric proportions of each runoff within the total runoff generated, and can be mathematically expressed as:

) ( ) ( ) ( ) ( h t Q Q Q Q t h Q Q Q Q t h Q Q Q Q t h Sub S D H S Dunne S D H D Horton S D H H + + + + + + + + = (5.7)

Where QH , QD and Q are the volumes of runoff generated from Horton S mechanism, Dunne mechanism and subsurface storm flow mechanism respectively.

The resulted synthetic CIRFs are shown in Figure 5.4.Most of CIRFs show two waves among which the higher wave is due to overland flow, and the lower wave is due to subsurface storm flow. Under imperious deep soil and short rainfall storm duration, nonetheless, it is possible that the whole catchment keeps unsaturated during the event

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and subsurface flow is not feasible. In this case, only Horton overland flow occurs, and thus the synthetic CIRF only has one wave.

5.3.3.2 Synthetic CIRF under various conditions

The climate, soil and topographic conditions used in this subsection are also carefully selected so that they can represent the typical natural conditions, and their impacts on synthetic CIRF can be illustrated as clearly as possible. Note the total area under each synthetic CIRF curve is equal to one. The volumetric runoff compositions corresponding to these combinations of conditions are shown in table 5.2. In a synthetic CIRF, if there is more surface overland flow, i.e., the proportion of Horton overland flow and Dunne overland flow is higher, the first wave due to overland flow will be higher or wider than that in another synthetic CIRF where there is less surface overland flow, and the second wave due to subsurface flow will be lower or narrower. Vice versa.

Figure 5.4(a) shows the impact of climate, in which we set the mean slope E(S)=0.2830, hydraulic conductivity Ks=5.0*10-6m/s and soil depth=4.0m. The soil is permeable enough, i.e., infiltration capacity is always larger than the rainfall intensity, so that there is no Horton overland runoff in any one of the three cases in Figure 5.4(a). Given the same landscape properties, the saturated hisllslope area is larger under humid climate than that under arid climate, so that the hillslope response is more significant under humid climate. Therefore, under humid climate, the first wave is apparently wider than that under more arid climate and the peak is lower too. Li et al. (2009) found that the ratio of Dunne overland flow volume over subsurface flow volume is governed by the landscape properties only, so this ratio remains roughly the same for all three cases in Figure 5.4(a), as shown in Table 5.2. This is also manifested by the fact that the area under the first wave is about the same for all three cases in Figure 5.4(a). The travel time of subsurface flow is mostly controlled by subsurface flow velocity and travel distance. Subsurface flow velocity is governed by soil properties and topography, which are the same for the three cases in Figure 5.4(a). The travel distance values of subsurface flow, defined as the flow distance from the location of infiltration downslope to the channel, are not significantly different among the three cases in Figure 5.4(a).

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Figure 5.4(b) shows the impact of topography, in which we set hydraulic conductivity Ks=5.0*10-6m/s, average soil depth=4.0m, and climate is humid. Similar to Figure 5.4(a), only Dunne overland flow and subsurface flow are feasible in the three cases in Figure 5.4(b). Under the same climate, if the topography is steeper, subsurface water flows downwards along the hillslope at a larger velocity, and more soil water exfiltrates out of the ground surface at saturated areas and becomes subsurface storm flow. Less soil water leads to less saturated area, and finally results in less Dunne overland runoff. Therefore with the catchment mean slope increasing, the generated volume of subsurface flow is increasing and that of Dunne overland flow in decreasing, as shown in Table 5.2. This is also illustrated by Figure 5.4(b) that the first wave (corresponding to Dunne overland flow) under steeper topography is lower, and the second wave (corresponding to subsurface flow) is higher. Moreover, under steeper topography, the overland flow velocity, subsurface flow velocity and channel velocity are all higher than those under flat topography, so in Figure 5.4(b) the time-to-peak values, of both the first wave and the second wave, are smaller under steeper topography.

The impact of hydraulic conductivity is shown in Figure 5.4(c). Note in this work the soil hydraulic property is assumed isotropic, i.e., the vertical hydraulic conductivity and longitudinal hydraulic conductivity are the same. Under the same climate, i.e., the same rainfall intensity and storm duration, lower hydraulic conductivity leads to lower infiltration capacity, and less rainfall infiltrates into the soil and more remains on the surface as infiltration excess (Horton overland flow). In the first case in Figure 5.4(c), the hydraulic conductivity is very low comparing with the rainfall intensity, and only a small part of rainfall infiltrates into the impermeable soil. Most of the catchment area keeps unsaturated and Horton overland flow totally dominates. Subsurface flow is negligible, so there is only one wave in the synthetic CIRF corresponding to the first case in Figure 5.4(c). In the second case, the hydraulic conductivity is moderately high, and most of the rainfall infiltrates into the soil. Dunne overland flow apparently dominates over Horton overland flow, and subsurface flow is minor. In the third case, the hydraulic conductivity is high enough that all rainfall infiltrates into the soil, i.e., no infiltration excess. Subsurface flow is more significant than that in the second case as shown by the higher

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second wave. The time-to-peak of the second wave in the third case is also earlier than that in the second wave since subsurface flow velocity is higher with large hydraulic conductivity.

The impact of soil depth is shown in Figure 5.4(d). Soil depth, as shown in Figure 5.4(d) and table 5.2, affects the temporal pattern of hydrologic response only indirectly through its effect on runoff composition. In Figure 5.4(d) the hydraulic conductivity is high enough for all the three cases so that no Horton overland flow occurs, and we only focus on Dunne overland flow and subsurface flow. Shallower soil depth implies less soil storage capacity, and it is easier to fill it up. That is, shallower soil column is more easily to get saturated, and thus is more in favor of Dunne overland flow. This is shown by the increasing peak value of the first wave with decreasing soil depth in Figure 5.4(d). Subsurface flow occurs all the time, in both the storm duration and between-storm period, and the subsurface flow generated during the between-storm period is of greater importance, since the storm duration is usually much shorter than the between-storm duration. Shallow soil, nonetheless, also makes it more easily for soil moisture to be evaporated out during between-storm period, and thus less subsurface flow occurs. This is confirmed in Figure 5.4(d) by the decreasing of the peak value of the second wave with decreasing soil depth. Soil storage capacity doesn’t have discernable effect on the time- to-peak values.