HYDROLOGICAL SYSTEMS MODELLING RESEARCH PROGRAMME: HYDROLOGICAL PROCESSES
Phase II Quantification of Hillslope, Riparian and Wetland Processes
'A Field Study of Two and Three Dimensional Processes in Hillslope Hydrology for Better Management of Wetlands and Riparian Zones'
'Experimental and Laboratory Measurements of Soil Hydraulic Properties'
1061 & 1086/1/04
The study of hillslope flow path mechanisms is a vital component of hydrology for a number of reasons. Mechanisms of rapid and slow responses at the hillslope scale affect the magnitude of storm flow peaks at the small catchment scale (< 10 km2) and contribute to runoff hydrographs at the large (10 - 100 km2) catchment scale. Mechanisms of water storage during wet periods and subsequent release from hillslopes during dry periods affect the sustainability of small catchment practices and can also have a significant control on low-flow rates at the large catchment scale. Mechanisms of lateral flow, accumulation, redistribution and residence times of water on the hillslope will influence the nature and location of land use practices. Conversely, changes in land use practices may affect all these hillslope flow path mechanisms and therefore the perturbations in flow generation mechanisms due to land use changes are important to define.
Detailed observations of surface and subsurface water dynamics have been made at three sites with distinctive geologies and climate. Analyses from measurements made in these catchments are used, firstly, to describe the streamflow generation mechanisms. Highlighting the significant features of these mechanisms at each site allows for some generalisation and a first insight into the classification of critical hillslope criteria required for estimating streamflow responses at catchment scales.
The three research catchments are described and hillslope, wetland and riparian zone streamflow generation processes are then deduced from the hydrometric observations of the dynamics of soil water, groundwater and streamflow response to rainfall and evaporation. The typical streamflow generation processes in the Weatherley research catchment are summarized in Table ES1. The streamflow generation mechanisms were then grouped into zones of similar behaviour, Table ES2.
Table ES1. Summary of streamflow generation mechanisms and their occurrence, Weatherley research catchment.
|A||Rapid lateral flow near the surface due to macro-pore conductance. Local perched water table of short duration. Matric pressure head discontinuity with deeper perched water table, D.||In upper slope segments in downstream catchment during high intensity events and some low intensity events with large volumes (>30 mm).|
|B||Accumulation at the toe of the slope segment with emergence and flow over bedrock.||In upper slope segments in downstream catchment.|
|C||Slow percolation to water tables perched on bedrock.||In all slope segments for most events except low intensity and volume.|
|D||Water tables perched on bedrock and in bedrock hollows.||Disconnected from soil water in upper slopes of eastern side of downstream catchment, but connected in lower slopes and in upstream catchment during moderate to intense events.|
|E||Seepage of groundwater through fractured bedrock.||Assumed to occur in all slope segments.|
|F||Rapid macro-pore, lateral flow in flatter marsh slopes and infiltration to marsh ground water.||Vertical recharge is more rapid than lateral movement in lower slopes of downstream catchment and in upstream catchment, except when groundwater rises into macro-pore layers.|
|G||Marsh ground water level fluctuation||Rapid for most events in lower downstream catchment. Slower, but connected in upper catchment.|
|H||Exfiltration, surface runoff and macro-pore discharge to stream||In downstream catchment. Exfiltration not observed in upstream catchment.|
|I||Groundwater discharge into stream||Occurs in upstream and downstream catchments. Some near stream groundwater ridging during intense events.|
|J||Unsaturated redistribution of soil water to bedrock. No groundwater on soil/bedrock interface.||In upper parts of western slope. Generates slowly to soil/bedrock water table downslope.|
Table ES2. Summary of flow generation zones, Weatherley research catchment.
|1||Upper slopes of eastern half of the catchment, where delivery of water in a disconnected near surface macro-pore zone delivers water to bedrock outcrop at the toe of the slope. Soil matric pressure is not continuous between responses in near-surface layers and deeper layers near bedrock. Surface water runoff generation for no more than 20 to 30m upslope contributes to flow at the toe. Slow deep groundwater from soil/bedrock interface recharges to toe and to lower slopes and bedrock. All water from this zone is delivered to Zone 2.|
|2||Recharge from upslope zone and infiltrating water raise groundwater levels at seepage lines and wetland areas. Some flow in near-surface macro-pores. However, it is normally associated with the resident groundwater rising into the macro-pore layers, particularly adjacent to the stream and seepage lines leading to the stream. Some groundwater ridging near the stream yields increased hydraulic gradients for short periods during moderate to intense events.|
|3||Near stream surface and near-surface water runoff, dominated by groundwater intersecting rapid delivery macro-pore layers.|
|4||Some flow in near-surface macro-pore layers, but mostly due to intersection of soil/bedrock perched water. There is generally soil matric pressure continuity between upper and lower layers. Near the stream, water is delivered through groundwater rising into macro-pore layers.|
|5||No perched water tables are evident, even during intense events. Little macro-pore discharge in near-surface layers, even during intense events. Significant wetting to deep horizons with slow delivery of unsaturated water to lower slopes.|
Three dominant streamflow generation mechanisms, (overland flow, near surface macro-pore flow and groundwater flow), are assumed to contribute to the stream and local seepage zones linked to the stream during an event. These streamflow generation mechanisms were quantified using simple, physically based techniques, applied to the measured soil water dynamics and runoff data of an event in April 2001. In addition to the physically based techniques, simple unit response functions, comprising an advection-dispersion model (ADM), were selected to represent the response of the three streamflow generation mechanisms. These were used in convolution integrals to translate the excitation function (excess rainfall or percolation dynamics) into simulated responses per unit length of the stream.
The length of stream was divided into 100m reaches and the overland flow, macro-pore flow and groundwater flow ADM's were applied to both sides of the stream and linked seepage zones and routed to the weirs in the upper catchment (UC)and lower catchment (LC) separately. The simulations show a remarkably good fit to the data, although the macro-pore response appears to over predict the recession limb. In both the upper and lower catchment, the near-surface discharge yields significantly more water than the surface runoff. In the upper catchment, the macro-pore layers yield approximately 70% of the discharge while in the lower catchment this source yields some 92% of the total flow generated.
Two additional events were analysed, the first with a low antecedent water content and the second with a high antecedent water content. Although the depth of rainfall was similar for both events, the corresponding runoff responses differed dramatically. The methodology is robust, provided the excess rainfall is properly quantified using the physically based Green-Ampt runoff/infiltration generation algorithm. The analyses show that the antecedent water content and rainfall intensity control the streamflow generation response, particularly as they influence the near surface macro-pore response.
The methodology is offered, not as an alternative technique for catchment runoff simulation, but as a technique for identifying and quantifying the volumes, residence times and transfer rates of sources and streamflow generation mechanisms in order to parameterise or refine algorithms, already inherent in most physically based catchment scale simulation models. The convolution integral methodologies are particularly appropriate for defining sources, residence times and fluxes during rainfall events as well as during low flows.
It is concluded that there is a need for a range of hilsllopes, wetlands and riparian zones to be monitored so that generalisations of behaviour can be made and built into deterministically based models. It is also evident that hydromteric observations alone are not sufficient to properly quantify streamflow generation sources and rates of transfer. It is recommended that the combination of natural isotopes or chemical species tracers sampling together with hydrometric observations of surface and subsurface phenomena must be used as widely as possible to narrow the error in describing volumes, residence times and transfer rates of the sources of streamflow.