DECLARATION 2- PUBLICATIONS
3.3 Connectivity Concept
3.3.3 Controls and thresholds
Bracken and Croke (2007) proposed a framework of hydrological connectivity that included the five major components that control hydrological connectivity: climate, hillslope runoff potential, landscape position, delivery pathway and lateral buffering (Figure 3.4). Within each of these components there are a number of factors that may influence the extent to which a catchment may be regarded as connected.
Figure 3.4: The components that control catchment connectivity (Bracken and Croke, 2007).
Climate: climate is a key control on the pattern and distribution of runoff within a catchment, specifically the runoff regime as determined largely by the nature and distribution of rainfall.
The response to rainfall and hydrological connectivity is dynamic and will change depending on the nature of rainfall input, antecedent conditions and catchment characteristics.
Hillslope runoff potential: the hillslope is the major landscape unit and is the scale at which most research on runoff generation takes place. There are many factors that influence hillslope runoff, including crusting and surface roughness, heterogeneity within the soil, the
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impact of variable density and type of vegetation, changing catchment morphometry, transmission losses in tributaries and main channels, and the impact of land use.
Delivery pathway: each runoff source has its own specific delivery pattern that is dependent upon its landscape position within the catchment and, in many instances, the management practices employed. The delivery of this water downslope involves flow pathways of variable width, depth and velocity. Dominant controls on the type of runoff pathway include such factors as topography, especially the effects of steepest slope, convergent hillslopes and hillslope hollows. Increasingly the effects of anthropogenic structures are emphasized.
Slope length influences connectivity at the hillslope and catchment scales, and is relatively unimportant at the plot scale. On longer slopes it is more likely that the slope will cease to generate runoff before runoff reaches the slope base or channel. The relationship between slope length, rainfall duration and intensity that produces connected flow at the outlet is complex. At the hillslope scale, a range of investigations have proposed a decrease in runoff per unit area with increasing slope length due to increased opportunity for infiltration (Van de Giesen et al., 2000).
Landscape position: landscape position reflects the relationship between runoff source and distance to the outlet-hillslope or catchment. Intuitively, the probability of hydrological connectivity will be enhanced if the transport distance for water is short relative to the effective contributing area (Bracken and Croke, 2007). In its simplest sense, this can be expressed as distance to stream or outlet.
Lateral buffering: lateral buffering defines lateral connectivity in ecological studies, or the nature of flood inundation between a channel and the adjacent floodplain (Pringle, 2001). It has been recognized as a fundamental control on nutrient and organic matter transfer between the main channel and adjacent areas of the floodplain. Hydrological connectivity will be significantly influenced by the degree to which (a) hillslopes are physically connected to channels and (b) the degree to which lateral buffering acts to limit runoff and sediment delivery to the channel.
Bracken and Croke (2007) proposed thresholds called “volume to breakthrough to quantify changing connectivity between different environments and catchments”. This defined the accumulated runoff volume per unit width to be applied to a point before flow appears at a
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downslope point. This approach is one possible concept of runoff generation and flood production that moves beyond the traditional view that runoff is generated by either the variable source area (VSA) or Hortonian infiltration excess. The framework is best viewed as a structure for exploring potential gaps in the process understanding and, importantly, data necessary to quantify connectivity.
A study done by Ocampo et al. (2006) in Susannah Brook Catchment, Australia showed that
“upland and riparian zones responded to rainfall events almost independently and differently”.
The riparian zone responded faster to rainfall events due to its high antecedent wetness and shallow soils. The upland zone, due to the drier antecedent wetness and deep soils, experienced a significant delay in the generation of a saturated zone. The shallow groundwater systems along the hillslope enabled down-slope transport of fresh water and NO3
that had previously accumulated in the upland zone because of the direct hydrological connection between the two zones. Related to this connectivity was a sharp increase in hydraulic gradient that drove shallow subsurface flow in the stream. These results are vital for the modelling of runoff generation and nutrient export at the catchment scale.
Many hydrological models presume that the groundwater table is connected all the way up the hillslope and that the hydraulic gradient is the same as the local gradient of the land surface.
This assumption is not correct; to adequately model the development and persistence of the shallow groundwater system we need to explicitly track the time varying hydraulic gradient.
A consistent model should show how the hydrological connectivity is established and how it changes in time. It must not only acknowledge the presence of the upland and riparian zones as sources and sinks of NO3 respectively, but illustrate the key role of antecedent conditions and the thresholds needed to exceed before hydraulic connection can be established.
Detty and McGuire (2010) employed a spatially distributed instrument network designed to represent several topographically defined landform features of a glaciated till mantled catchment and monitored shallow water tables and soil volumetric water content for three seasons. The research was intended to investigate how, when, and where shallow water tables develop and describe the resulting hydrologic connectivity between the various components of the catchment. The hydrologic connectivity between riparian and hillslope areas displayed a strong seasonal signature. The results suggested that much of the catchment was hydrologically disconnected from the channel network during the growing season, while most
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of the catchment was continuously connected to the channel network during the dormant season. The largest events in the dormant season allowed shallow transient water tables to develop even at the driest sites and hence nearly the entire catchment could be briefly connected to the stream channel during these events. The seasonal variations in hydrologic connectivity reflected the effects of climate and evapotranspiration on soil moisture storages and shallow groundwater development. These results have implications in modelling sub- surface stormflow, runoff generation and the seasonal or event-based transport of solutes from uplands to streams.
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CHAPTER FOUR
4 METHODOLOGY
Chapter 4 reports on the methodologies used to achieve the objectives of the study. Initially a nested catchment monitoring layout is specified which takes into consideration scale issues from local, field to catchment levels. Materials and methods used to aid in collection of observed data at plot, field and catchment scales are also given. The various laboratory procedures and analysis that were used to sample nutrients, sediments and isotopes is elaborated. The development of a modified ACRU-NPS model is presented with the envisioned incorporation of the connectivity concept into the model. This makes it possible to study hydrological connectivity between land segments and the linked control structures (in this case buffers, wetlands and dams). This approach takes into account the runoff, NO3, P and SS exchanges between the land segments and river channel together with their fate on entering and leaving buffers, wetlands and dams.