Search

With major alterations in the regimes of snow-fed mid-latitude rivers projected for the end of this century, the output from hydroelectric river run-off schemes will potentially be affected to a similar extent. While resulting changes in the annual and seasonal power output have been investigated for a number of catchments, more transparency is needed in terms of dominant mechanisms driving these changes and model component-specific uncertainty. In this study an ensemble of 32 hydrological simulations is used to investigate the hydroelectric power potential of the Clutha River (Southern Alps, New Zealand) under 21st century climate change. The ensemble encompasses two emission scenarios, four General Circulation Models, two bias correction methods and two snow models. The fully distributed hydrological model WaSiM is used to model both the main natural processes and major forms of water management in the catchment. The catchment’s largest hydroelectric scheme is modelled by an external component. In the 2090s the results show substantial increases in the output for winter (18%) and spring (7%), followed by reductions in summer (-19%) and autumn (-4%). A net increase in annual streamflow does not lead to a corresponding increase in annual output, which is attributed to excess water being spilled during high flow events. The driving controls behind this are identified as more winter precipitation, a reduction in the solid fraction of precipitation and an increase in extreme precipitation events. The relatively large variation in the projected seasonal output is found to be primarily caused by the General Circulation Model and the emission scenario, while bias correction and the snow model made a smaller contribution to the overall uncertainty. [ABSTRACT FROM AUTHOR]

The Clutha River is New Zealand's largest river, both in terms of discharge and catchment size. It has a large seasonal flow variability with large spring and summer flows and very low flows in winter. The river is important for its contribution to energy production from two large hydro dams, irrigation for agriculture and horticulture, and tourist activities. The Clutha River flows are modified by discretionary releases of water from the only storage lake in the catchment, at Hawea. The mountainous South Island topography to the west means that a large part of the catchment is in a rain shadow and is regularly subject to long periods without any significant precipitation, similar to many other rain shadow catchments globally. While floods can be mitigated against, droughts are insidious and generally well established before being identified. A hydrological drought is defined as a lack of water in the hydrological system, identified by abnormally low streamflow in rivers and abnormally low levels in lakes and reservoirs. A drought index is proposed for the upper and mid-Clutha catchment that identifies the onset of hydrological drought conditions, measures the severity of the drought, and signals the end of the drought. This index has the advantage over other hydrological indices in that it uses a daily threshold rather than a monthly or seasonal one and indicates the start of a drought as soon as it occurs. [ABSTRACT FROM AUTHOR]

The article reports on a study done on the effect of natural flow variation on the population of juvenile brown trout. A tributary of Motueka river in New Zealand was used for taking the sample for over a period of 5.5 years. It was observed that the density of trout was adversely affected by a flood, however, a low flow period has no effect on the population. A detailed analysis of the study and its results are presented.

A streamflow recession formula for natural basins is derived by nonlinear hydrologic routing of inflow from channel bed and bank storage through a stream channel storage. The formula gives basin outflow as a two-parameter, inverse square function of time and is valid when the ratio of inflow to outflow is approximately constant. It provides a good fit to a master baseflow recession curve from each of 10 basins of differing areas in the greywacke geologic terrain of Canterbury, New Zealand. These basins have no significant lakes or springs and the formula applies to that section of a master recession curve between median flow and minimum recorded 7-day low flow, for the period January to March in a flow record. The master recession curve in an ungauged basin within the same terrain may be predicted if the volume of water to be discharged during this recession can be estimated. This volume is taken to be the product of total stream length, average cross-sectional area of channel bed and bank storage and storage porosity. It can be calculated if there is a reference basin available, similar in character to the ungauged basin, with a measured master recession curve. While further testing of the formula is needed, particularly in other geologic terrains, its application is relatively simple and it should be useful at basin scale for predicting master recession curves in basins where little is known about water storage behaviour. [ABSTRACT FROM AUTHOR]

After the SWAT (Soil and Water Assessment Tool) model was calibrated and validated to historic flow records for the current land use conditions, two additional land cover scenarios (a prehistoric land cover and a potential maximum plantation pine cover) were used to evaluate the impacts of land cover change on total water yields, groundwater flow, and quick flow in the Motueka River catchment, New Zealand. Low-flow characteristics and their potential impacts on availability for water abstraction and for support of in-stream habitat values were focused on. The results showed that the annual total water yields, quick flow and baseflow decreased moderately in the two scenarios when compared with the current actual land use. The annual water balance for the pine potential land cover scenario did not differ substantially from the prehistoric scenario for the catchment as whole. However, there were more notable differences among individual tributary catchments, which could be attributed to the relative area of land cover altered and location of those catchments. Simulated low flows for the prehistoric and potential pine land cover scenarios were both significantly lower than the low flows for the current land use. In summary, under the current land use conditions, both annual water yield and low flow are higher than was the case before human intervention in the area or in a maximum commercial reforestation scenario. [ABSTRACT FROM AUTHOR]

1. Demand for water is increasing and water managers need to know how much they can remove from a stream before there are significant detrimental effects on its biological integrity. Flow reduction alters a number of habitat variables known to be important to aquatic invertebrates such as depth, velocity, temperature and fine sediment accumulation. Some taxa may attempt to use instream refugia to mitigate the effects of flow reduction. 2. We experimentally manipulated flows by constructing weirs and diversions in three small New Zealand streams. Discharge was reduced by 88–96%. We tested the hypothesis that macroinvertebrates would use pools and the hyporheic zone as refugia during short-term (1-month) periods of reduced flow. 3. We sampled hyporheic invertebrates with colonization chambers and pool invertebrates with kick nets within a before-after, control-impact (BACI) experimental design. A suite of physicochemical parameters was measured concurrently including surface and hyporheic temperatures. 4. Flow reduction significantly decreased velocity (60–69%) in all streams. Depth (18–61%) and wetted width (24–31%) tended to decrease but these changes were not always significant. Sediment cover increased the most in farmland streams (10–80%). Apart from decreasing temperature range (18–26%), flow reduction had little impact on the surface water temperatures. 5. Flow reduction had no impact on the abundance of common pool macroinvertebrates or on the abundance, vertical distribution or community composition of hyporheic macroinvertebrates. 6. Our results suggest that aquatic macroinvertebrates are resistant to short-term, severe flow reduction as long as some water remains. [ABSTRACT FROM AUTHOR]

The key elements of the flow regime of the Waipara River on the east coast of New Zealand, that affected fish abundances were the timing of floods and the magnitude and duration of low flows. Generally, fish abundances were highest in early summer, and lowest at the beginning of winter. Spring floods opened the river mouth, allowing recruitment of diadromous fish species, and non-diadromous fish species spawned after the floods in spring or early summer. Reductions in fish abundances over summer and autumn were consistent with the magnitude and duration of low flows, with significant reductions in the year of lowest flow and little change in abundance in the year when low flows were highest. Variations in fish abundances during periods of low flow were consistent with the amount of instream habitat available, such that abundances of species with high velocity preferences decreased during periods of low flow, whereas abundances of species with low velocity preferences increased. [ABSTRACT FROM AUTHOR]