Memo - Kinloch Community ...
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Memo
File No: 47 03 10
Date: 14 September 2021
To: Brent Sinclair
From: Bevan Jenkins
Subject: Hydrology review for Whangamatā stream, Kinloch - Taupō
Summary
1. A recent decline in the flow of the Whangamatā Stream, Kinloch – Taupō has been noted by
members of the community.
2. Waikato Regional Council has undertaken a review of hydrology, climate and land cover data
to help identify the potential causes of the low flow conditions.
3. Most (70-90%) of the water in Whangamatā Stream comes from springs which are sourced
from groundwater. That groundwater is recharged by rainfall throughout the catchment.
4. Flow records show that the Whangamatā Stream has experienced multiple periods of low
flows (e.g. 1987, 1995, 2004, 2017, 2021).
5. Analysis of aquifer recharge patterns indicates these periods of low flow correspond to periods
of reduced annual rainfall and subsequent recharge.
6. Analysis of groundwater patterns show that they reflect the recharge patterns, and these
groundwater patterns are very similar to the streamflow.
7. Assessment of available water use and land cover data show that there have been only minor
changes in these factors, suggesting they are not a major driver of observed low flows.
Introduction
The recent decline in the flow of the Whangamatā stream, Kinloch – Taupō has been noted as a matter
of some concern by members of the community. Residents observed reduced flows and drying of
some sections, and this was most pronounced 26 August 2021 with rain producing some increase in
flow by 29 August. Some of these observation points are mapped in Figure 1. Complete drying was
observed in the mid-sections of the stream, about 1 km upstream of Whangamata Rd (pers. comm.
Deb Borlase). Flows continued from springs located at the headwaters although at about half their
normal rate (pers. comm. Ian Britten). Some sections continued to flow at a trickle, but no longer
supported fish. For example, a pool was devoid of fish that normally supported 20 trout (pers. comm.
Deb Morlase). Trout have persisted closer to the headwater springs. People have noted low flows at
least since early June 2021.
An initial review of the hydrology was undertaken in an attempt to identify possible causes of the
decline. The intent of this memo is to analyse the stream flow record for the Whangamatā and place
it in hydrological context.
This memo draws on and extends the work carried out by Dr Ed Brown (Brown 2003) in response to
similar concerns about declining stream flows at that time. It is noted that the work of Dr Brown uses
a different location for streamflow, and therefore the presented streamflow graph is not the same.
Historically there have been measurements undertaken at several locations. However, currently WRC
only undertakes routine flow measurements at “Whangamatā Stm (Kinloch) at Whangamatā Rd
(1300_1)” (used in this analysis – see Figure 2 for location).
Doc # 21186610
Figure 1. Observed drying and reduced flows are mapped based on conversations with local residents. Thanks
to Deb Borlase for her observations and compiling reports from neighbours. Thanks also to Ian
Britten for observations of springs where flow arises for the Whangamata Stream (pers. comm.
3 September 2021).
Doc 21186610 2
Figure 2. Map of Whangamatā catchment. Stream flow measurement location (blue) and groundwater
measurement locations (orange). Catchment generated from River Environment Classification
version 2.5 (REC2.5).
Stream flow
The streamflow records used in this analysis are spot measurements, that is measurements taken at a
discrete point in time rather than a continuous flow record. The current monitoring regime has made
an average of 8 flow measurements per year over the last 10 years.
Figure 3. Whangamatā Stm (Kinloch) at Whangamatā Rd (1300_1)* streamflow.
*This is a different site than that used by Brown (2003), as regular measurements are no longer made at that
site.
Doc 21186610 3Most of the water in Whangamatā Stream comes from springs which are sourced from groundwater.
Schouten et al (1981) reports baseflow as a percentage of total flow ranges between 70 to 90 per cent,
with the remainder from overland flow during large rainfall events for streams in the Taupō catchment.
Given the discrete springs in the Whangamatā catchment, it is likely that the baseflow is towards the
higher end of this range indicating that groundwater inputs via springs dominates the total flow.
Figure 3 shows the recent decline in the Whangamatā Stream flow since higher flows were measured
in 2019. Similar declines have occurred in the past, with low flows measured in 1987, 1995, 2004, and
2017.
Rainfall
The Virtual Climate Station Network (VCSN) precipitation and potential evapotranspiration products
(Virtual Climate Station data and products | NIWA) were used for this analysis, with the single station
or node that is located in the Whangamatā catchment analysed.
There is a strong relationship between deviations from mean annual rainfall pattern and the measured
flows in the stream as shown in Figure 4. That is, a period of lower-than-average rainfall results in a
reduction in aquifer recharge, a drop in streamflow and vice versa.
Figure 4. Whangamatā Stm (Kinloch) at Whangamatā Rd (1300_1) streamflow (trace) and annual rainfall
(VCSN) deviation (bar) from the period (1975 - 2020) mean.
To place the rainfall patterns in a longer-term historical context, across the region there has been a
decline in annual rainfall post 1980, compared to the previous three decades. This is shown for a long-
term record at Hamilton in Figure 5, where only 11 of the last 40 years have experienced above average
rainfall. In addition, at Hamilton, 2020 was the driest year since records began in 1907 (n = 107).
Doc 21186610 4Figure 5. Annual deviation in precipitation for Hamilton.
Accounting for evapotranspiration
A simple approach was adopted to estimate recharge as rainfall that has fallen minus the water that
leaves the catchment via evapotranspiration. The rainfall and potential evapotranspiration (PET) from
the VCSN were used. Potential evapotranspiration will be a higher value than actual
evapotranspiration because it is not limited by water availability. In addition, this method will also
overestimate recharge because it includes the proportion of water that runoff directly (overland flow).
However, as a first approximation, it is useful to determine any patterns over time.
Figure 6. Whangamatā Stm (Kinloch) as Whangamatā Rd (1300_1) streamflow and 24 month rolling average
estimated recharge (VCSN).
The PET for each month was subtracted from the monthly rainfall. Next a rolling 24-month average
was applied to the resulting estimate of recharge as shown in Figure 6. There is a clear relationship
between the rainfall minus PET and the streamflow, particularly over the last decade. The choice of a
Doc 21186610 5rolling 24-month mean is arbitrary and was chosen to dampen the signal in a similar manner to a
groundwater reservoir.
Groundwater
The location of the two groundwater bores that WRC currently monitors are shown in Figure 2. Stream
flow shows a strong relationship to both groundwater level timeseries in Figure 7 and Figure 8. This
indicates that the groundwater is driving a lot of the variation that occurs in the streamflow.
Figure 7. Whangamatā Stm (Kinloch) at Whangamatā Rd (1300_1) (blue) and groundwater level for Bores
(Waikato Region) at Hayman Phil & Julia (Rowlands) (72_356) (maroon).
Figure 8. Whangamatā Stm (Kinloch) at Whangamatā Rd (1300_1) (blue) and groundwater level for Bore
(Taupo) at Ramsey (68_301) (maroon).
Doc 21186610 6Landcover
The catchment landcover was queried for the 4 available periods (1996, 2001, 2008, 2012, and 2018)
from the Land Cover DataBase version 5 (LCDB5). The catchment used in this analysis is the
Whangamatā Stream where it enters Lake Taupō, which enables an update of the earlier Brown (2003)
analysis. There is a slight difference in catchment area (3110.5 ha versus 3139 ha) between Brown
(2003) and this analysis. Note as the Whangamatā Stream at Whangamatā road site is located further
upstream, this analysis includes landcover that occurs downstream of the measuring location.
The catchment landcover has been relatively stable over the analysis period (1996 - 2018), with a slight
increase in exotic forest in the early part of the record as shown in Figure 9 and Table 1. The alteration
from exotic grassland to exotic forest is likely to have had an impact of increasing evapotranspiration
rates. However, pasture remains the dominant landcover in the Whangamatā Stream catchment.
There will also have been transient impacts such as that associated with forestry harvesting and
planting of replacement trees. There has been an increase in built-up area, in general terms this leads
to a reduction in infiltration and an increase in ‘flashiness’ in the affected area. However, the impact
of this will also be determined by the stormwater system (e.g. soakage pits would increase recharge).
Figure 9. Landcover class area (ha) in the Whangamatā catchment for 1996, 2001, 2008, 2012, and 2018 from
LandCover Database 5 (LCDB5).
Table 1. Landcover class area (ha) in the Whangamatā catchment for 1996, 2001, 2008, 2012, and 2018 from
LandCover Database 5 (LCDB5) (Accessed from WRC GIS system).
Year
LCDB Class name
1996 2001 2008 2012 2018
Broadleaved Indigenous Hardwoods 78.7 78.7 86.7 86.7 86.7
Built-up Area (settlement) 7.1 28.2 50.9 50.9 50.9
Deciduous Hardwoods 3.6 3.6 3.6 3.6 3.6
Exotic Forest 644.3 706.1 542.4 528.6 585.6
Forest - Harvested - - 138.9 118.7 49.6
Gorse and/or Broom - - 16.8 28.0 28.0
High Producing Exotic Grassland 2283.1 2200.2 2171.4 2191.4 2192.7
Indigenous Forest 81.1 81.1 81.1 81.1 78.9
Low Producing Grassland - - - 2.8 15.7
Urban Parkland/Open Space 12.6 12.6 18.8 18.8 18.8
Doc 21186610 7Water Use
There has been a small reduction in the volume of water consented to be taken over the period 2002
to 2011 (Table 2). There are currently three groundwater takes in the catchment. The previously
consented surface water take (29.8 m3/d) was not replaced upon expiry (D. Jones 2021 pers. comms.).
There has been very little change in the estimated amount of water allocated between 2002 and 2021.
However, over this period the streamflow has reached the maximum flow recorded and also has
experienced periods of decline.
Table 2. Water use in the Whangamatā catchment in 1996/97, 2002 and 2021 (using data from Brown (2003)
and D. Jones (2021 pers. comms.). *Note the change in method for permitted takes means a
comparison is not possible between the years.
1996/97 2002 2021
Count Volume Count Volume Count Volume
(m3d-1) (m3d-1) (m3d-1)
Groundwater 0 0 4 374.8 3 371.5
takes
Surface 0 0 1 29.8 0 0
water takes
Permitted 121 1815.0 177 2655.0 - -
takes - Brown
(2003)
Permitted - - - - - 408.8
takes - WAC
Total 1815.0* 3059.6* 780.3
Rather than try to replicate the method from Brown (2003) for the estimate of permitted take in the
catchment, Waikato Regional Council’s Water Allocation Calculator (WAC) tool was used to estimate
the permitted volume. This is the method used in the allocation of water throughout the region. If all
permitted takes are utilised the reduction in stream flow at the bottom of the catchment will be 4.7
litres per second (Koh (2021 Pers. Comms.). If all authorised water is taken (permitted and consented)
then the reduction in streamflow at the bottom of the catchment will be 6.8 litres per second. The
magnitude of the reduction in stream flow that has been experienced is far greater than this amount.
Summary
There are a range of possible causes for the decline in the Whangamatā streamflow:
• The natural variation in precipitation, drives recharge rates to the aquifer, which in turn
supplies the spring discharge and streamflow. There have been a number of periods previously
when the Whangamatā Stream experienced low flows, in particular, 1987, 1995, 2004, 2017,
and the current 2021 (ongoing). The majority of the pattern in declining and increasing
streamflow seems to be largely controlled by the patterns of rainfall and evapotranspiration.
• There has been an increase of water use from the mid 1990’s to the early 2000’s.
Subsequently, the consented volume has remained relatively constant for the next 20 years,
with a very slight reduction in volume allocated. Over this period of constant allocation, the
streamflow experienced the maximum flow recorded and periods of decline and therefore
doesn’t match the pattern of use. Additionally, the magnitude of the reduction in stream flow
measured is far greater that the volume of water use. However, any consumptive water use is
likely to have greater effects during times of low flows.
• There is likely to have been a small increase in evapotranspiration from changing landcover
but this change occurred earlier in the record and is also likely to have had only a very minor
effect. In addition, there will have been an impact from increase in built-up area, with the
Doc 21186610 8significance of this dependant on the extent of rainwater infiltration reduction from paved
surfaces.
References
Brown, E. 2003. Memo. Decline in Whangamatā stream flow, Kinloch. Discover ID = 808788
Jones, D. 2021. Pers. comms. “RE: Whangamatā Stream Kinloch Taupo”. Discover ID = 21494905
Schouten, C. J.; Terzaghi, W.; Gordon, Y. 1981: Summaries of water quality and mass transport data for
the Lake Taupo catchment, New Zealand. Ministry of Works and Development, Wellington.
167 p.
Vant, B. 2021. Pers. comms. Whangamatā Stream flows, added to DM641577 (1993564). Discover ID
= 21659572
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