CONCURRENT CONTINUOUS MONITORING VERSUS SPOT GAUGINGS
Leong D1 & Waldron R2
1
Tonkin & Taylor Ltd
2
Hawkes Bay Regional Council
Background
Spot flow gauging involves the measurement of the discharge at a given location at a point in
time. It is a field technique commonly used by hydrologists to fill knowledge gaps, to assist in
such endeavours as:
- Understanding patterns of flow losses and gains along a river, including the effect of
surface and ground water abstractions
- Assessing compliance against management plans and consent conditions
- Developing low flow estimates for ungauged locations and understanding catchmentwide low flow behavior, e.g. for water allocation
- Developing synthetic time-series records for ungauged locations via transposition from
gauged catchments
- Tracking the propagation of flow events in a river system for various investigations, e.g.
water quality, ecology, etc.
It is a cost-effective method and sound results for most applications can be obtained under
certain conditions, predominantly stable low flow recessions. As an alternative, contemporaneous
continuous monitoring is sometimes used, but remains uncommon because temporary monitoring
sites involve considerably more effort to establish and maintain. However, the information able to
be gathered from such sites provides an additional dimension to the understanding of streamflow
behaviour, and gives superior outcomes under all circumstances compared with spot gaugings.
Aims
In this presentation, we show that for certain applications, spot gaugings alone do not provide
sufficient information. Indeed, simple flow correlations developed from concurrent gaugings, if not
treated with due care, can lead to misjudgment of the predicted streamflow behaviour under
particular managed flow regimes. To illustrate this point, we refer to recent field investigations
completed for the Waipawa and Tukituki Rivers in Central Hawkes Bay aimed at understanding
and, ultimately, modelling the pattern of streamflow losses and gains in the middle reaches of
these rivers, which are potentially affected by the operation of the proposed Ruataniwha Water
Storage Scheme.
Method
The Hawkes Bay Regional Council has a very extensive database of historical flow gaugings for
over 500 locations in the wider Tukituki catchment dating from the 1950’s and earlier. The
majority of gaugings have been conducted in a steady low flow setting and represent quasiequilibrium conditions. Whilst concurrent gaugings have been very useful for developing flow
correlations, mistiming issues, especially under higher flows, introduce significant uncertainty to
the correlation of medium to high flows. These correlations also cannot generally be used to
predict the flow losses and gains under transient conditions, such as from a managed flow
release from the proposed dam, or even in a natural freshet/flood in the river.
Losses and gains are expressions of river-aquifer interaction. This is normally included in
comprehensive regional groundwater models through a dynamic link at the wetted perimeter of
the channel. However, where there is a low permeability zone underlying the river bed (and/or the
river is perched above the regional groundwater table) the shallow aquifer adjacent to the river
bed can be regarded independent of the deeper regional groundwater. In such situations the
problem may be simplified to one of river interaction with dynamic bank storage.
Results
On this basis, a conceptual model has been specifically developed and calibrated using
monitoring data to represent the dynamic flow exchange/conveyance processes in the Waipawa
and Tukituki Rivers. Figure 1 is a diagrammatic representation of the modelled flow processes.
Figure 2 shows the simulated flow response at the downstream end of the Tukituki losing reach
compared with the recorded flow for that location (i.e. Tukituki upstream of Tukipo confluence).
Figure 1 Conceptual model of losing reach including river flow–gravel storage interaction
Figure 2 Calibration of the conceptual model of the Tukituki losing reach, from Tukituki at SH50 to upstream
of Tukipo confluence
The findings from these investigations underline the importance of a targeted field monitoring
programme involving continuous streamflow monitoring, and the value of the data collected for
understanding transient flow behavior and for the development of a calibrated conceptual model
that is able to simulate the dynamic flow behavior across the losing reaches of these rivers.