5 Catchment Hydrologic Behaviour
5.1 Catchment hydrologic response and behaviour
Flow represents the catchment integrated response to a rainfall event and to the catchment antecedent conditions (or soil moisture conditions) just prior to that rainfall event (Kirchner, 2009; Pauwels and De Lannoy, 2006). The way in which this response changes over time is known as a river‟s flow (or hydrological) regime.
Examination of flow regimes can provide information about the likely response of a catchment to a rainfall event, susceptibility to low flows and flood flows, monthly, seasonal and annual patterns of flow, impact assessment and whether it is potentially useful (Duncan and Woods, 2004). Climatic and physiographic attributes specific to the catchment are major influences shaping this flow regime, providing an insight to the dominant processes that may control the catchment‟s runoff response.
5.1.1 Effect of climatic attributes on catchment flow regimes
The climatic attributes of a catchment influence the amount of water entering the catchment. Runoff generally follows the seasonal distribution of rainfall unless snow accumulates in large enough quantities to affect runoff regimes (Riggs, 1985). At shorter timescales, catchment-to-catchment variations in precipitation and PE influence the amount of water available at the surface for runoff. Along with topography and other catchment attributes, climatic attributes broadly control the frequency of flood flows and low flows (Snelder and Biggs, 2002).
Rainfall depths depend on the size, location and topographic characteristics of the catchment, the interaction of the catchment with the passage of weather systems over the area and the precipitation-generating mechanisms of these systems. In the Lake Taupo catchment, the temporal and spatial variability of rainfall is considerable. In mountainous areas (i.e., the Kaimanawa Ranges and the volcanoes of the Tongariro National Park), precipitation is generally greater (>2000 mm/a) than in lowland parts of the catchment (i.e. Taupo and Turangi, <1200 mm/a). In
Catchment hydrologic behaviour | 85 addition, at higher elevations lower temperatures result in more of this precipitation falling as snow and lower evapotranspiration rates (Snelder and Biggs, 2002).
5.1.2 Effect of physiographic attributes on catchment flow regimes The transformation from precipitation to streamflow is dependent on many variables (Riggs, 1985). Catchment physical attributes influence the ability of a catchment to store water and the timing and magnitude of the runoff response. The relative importance of each of the physiographic attributes in the estimation of hydrological behaviour varies from catchment to catchment, and is reflected in the hydrographs from which two principal components can be identified – the fastflow response and the baseflow (groundwater) response (Duncan and Woods, 2004). The hydrological response in terms of the timing and volume of the flood flow and baseflow response is strongly influenced by catchment physical properties which control the movement of water once it reaches the ground (Duncan and Woods, 2004; Snelder and Biggs, 2002)
The type of vegetative cover influences water yield, surface interception of rainfall and evaporation rates. Consequently, vegetative cover influences the amount of water available to reach the river and the frequency and duration of low flow periods (Snelder and Biggs, 2002). Over 55 % of the Lake Taupo catchment (by land area) is forested. Forested areas tend to have higher interception rates (Kelliher et al., 1993) and lower annual water yields (Fahey et al., 2004) relative to grassland and pasture.
Conversely, pastoral catchments (19% of catchments land area) generally have higher annual yields and peak flows and can be more responsive (higher quickflows) than comparable forested catchments (Fahey et al., 2004).
Water retention and conductivity characteristics of soils determine soil water storage and movement (Kelliher and Scotter, 1992), acting as a buffer in the hydrological cycle. These characteristics vary depending on the type of soil in a catchment and are important because they affect evaporation, surface runoff, groundwater recharge and stream flow through infiltration, redistribution and drainage processes (Kelliher and Scotter, 1992). Most of the soils around the Lake Taupo catchment derive from the tephra and ashes from the Taupo eruption (Gibbs, 1968; Molloy, 1998; Rijkse, 1987). Pumice soils are most prevalent (Rijkse, 1987).
The generally sandy or gravelly texture of pumice soils means they are porous and well-draining, allowing rapid movement of air and water while still capable of storing large amounts of water for plants (Molloy, 1998).
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The presence of groundwater is related to catchment geology, controlling storage and transmissivity (Snelder and Biggs, 2002) and significantly influences patterns of discharge, variability and flashiness (Rosen and Coshell, 1998). The volcanic ash, pumice and ignimbrite found in the Lake Taupo catchment readily absorb water that falls on them allowing percolation to groundwater which feeds many of the rivers and streams in the catchment (Roper, 2001; White, 2001). Groundwater acts as a buffer between the rainfall event and streamflow response by dampening flood response and reducing overall flow variability. Areas of more permeable lithologies (Oruanui and Taupo ignimbrite, and rhyolitic pyroclastics) generally have lower mean flows and specific discharges (Morgenstern, 2008). These types of catchments tend to have higher and more persistent baseflows, more groundwater storage and respond less rapidly to rainfall (Duncan and Woods, 2004). Schouten et al., (1981) estimate that as much as 95% of annual discharge from the rivers draining the permeable areas of the Lake Taupo could be derived from groundwater. In less permeable areas (Whakamaru ignimbrite) specific discharge and mean flows are comparatively higher. These catchments respond more quickly to rainfall events with steep recessions and lower baseflow volumes (Duncan and Woods, 2004). The baseflow proportion of annual runoff from catchments draining less permeable lithologies is less than 80% (Schouten et al., 1981).
The time it takes for water to reach the river and be transported from the catchment is influenced by geomorphic characteristics including the catchment‟s size, shape, slope and drainage density (Chow, 1964; Post and Jakeman, 1996; Schumm, 1956).
It is widely accepted that streamflow is directly related to the size of the catchment (Beven, 2001; Chow, 1964). In larger catchments, more water can be captured and discharged. In terms of shape, a long narrow basin is more likely to have a more rapid response to rainfall events, shorter recessions and larger peak flows than a catchment which is rounder (Chow, 1964). In a study of 17 small mountain catchments (0.04-0.65 km2), Post and Jakeman (1996) conclude that wider catchments drain more slowly and have longer quickflow recession time constants.
Gently sloping catchments tend to have more subdued and delayed flow response (longer baseflow recessions) than catchments with higher gradients and steeper slopes because it takes longer for precipitation to reach drainage channels.
Catchments with higher drainage densities showed more attenuated quickflow response periods, since water is able to reach drainage channels more quickly. In catchments with lower drainage density, water needs to travel through a larger area of the catchment before it reaches the river, reducing catchment response times.
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5.1.3 Effect of regulation on natural flow regimes
Hydro power schemes and the dams associated with them can alter natural flow regimes by physically blocking the river, storing excess runoff, or releasing water according to human needs. Effectively, hydro power schemes and dams modify the magnitude and timing of catchment runoff (Poff and Hart, 2002). This can result in flood responses downstream of infrastructures being dampened and flood peaks attenuated (Bednarek, 2001). Stored water can be released to augment low flows and can provide more uniform monthly flow distributions (Duncan and Woods, 2004). While the variability of flow over longer periods is often less, over shorter time frames fluctuations can be significant in response to electricity spot prices and electricity demands (Bednarek, 2001; Young et al., 2004).
The TPS is the largest and most complex of hydro schemes in the Lake Taupo catchment. The impact on the flow regime of the Tongariro catchment is significant.
Streamflow during „normal‟ conditions is most affected. Up to 80 m3/s can be diverted from the river for generation at Tokaanu Power Station, between 16 m3/s (minimum flow condition) and 160 m3/s (maximum turbidity level). Stephens (1989) reports the operation of the scheme has resulted in reduced normal flow, less seasonal variations in normal flow, reduced frequency of small flood („freshes‟), truncated recessions and minimum flows soon after rain, higher flows during drought and artificially induced surges (for example, those associated with required recreational releases).
In the Kuratau catchment, the release of water follows the typical diurnal energy demand profile (Figure 5.1) and water is held back in the lake for generation at peak times. Peak demand periods are during early morning and evening; demand is less overnight. Short term flow variability is increased. The Hinemaiaia Power Scheme is largely run-of-river which means it has relatively limited storage volume and short residence time (EPA, 2010). Although there is some fluctuation related to the daily energy demand profile, there is relatively little modification to the unmodified streamflow entering the system above the scheme (refer Figure 2.8).
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Figure 5.1. Typical load profile of daily energy use. Note the two peak demand periods during day and lower demand overnight. Source: Transpower New Zealand Ltd (2010)
5.1.4 Summary
The assessment of the climatic physiographic and hydrologic attributes can provide some insight into the important processes and mechanisms for the rainfall-runoff response in the Lake Taupo catchment. The seasonality of streamflow (including high and low flow regimes) is largely controlled by climatic attributes (Snelder and Biggs, 2002) while physiographic attributes mainly influence the movement of water once it reaches the ground. The type of vegetation affects surface interception of rainfall and evapotranspiration and can consequently affect catchment water yield. Of the water that reaches the ground, the transmissivity of the soils and geology of the groundwater system play a significant role in determining the rate at which water passes through the catchment. Geomorphic attributes are also important factors controlling the speed at which water reaches the river and is transported out of the catchment. The development of hydro power schemes has modified the downstream magnitude and timing of flow. In the following sections, the relationships between these physical attributes and various hydrologic characteristics are investigated for a number of sub-catchments of Lake Taupo.
Further, the effects of hydro power schemes on natural flow regimes are identified.
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