River and stream water quality and ecology - Greater Wellington ...

River and stream water quality and ecology in the Wellington region: State and trends
summer and can support high periphyton growth despite relatively low nutrient
concentrations. While reduced flows in these rivers largely reflects the less
frequent summer rainfall and naturally lower groundwater inputs associated
with the eastern Wairarapa hill country, in some other streams, water
abstraction may be a contributing factor. As noted in Section 3.4.2, there has
been a considerable increase in consented water abstraction in the Wellington
region in recent years, for both urban water supply and irrigation in rural areas
(principally for dairying in the Wairarapa). Despite a cap being in place on
new surface water takes from some small streams in the central Wairarapa
Valley, such as the Parkvale and Mangatarere streams, Hughes and Gyopari
(2011) demonstrated that the flow in these streams is depleted by the
abstraction of hydraulically connected groundwater. Further much of this
groundwater has not to date been considered as part of the consented surface
water allocation. A new framework for conjunctive management of surface and
groundwater in the Wairarapa Valley has recently been recommended to
address this issue (see Keenan et al. 2012).
The benefits of riparian vegetation for stream health by way of increasing
stream shade and reducing water temperatures and periphyton growth are well
known (eg, Parkyn 2004) and have been demonstrated to be particularly
effective in small rivers and streams. For example, Perrie (2008) reported a
reduction in maximum water temperatures of several degrees for the Enaki
Stream in the Mangatarere catchment near Carterton (Figure 8.5), despite the
presence of relatively young riparian plantings. Monitoring at RSoE sites has
shown that high levels of shade can limit periphyton growth even at sites with
elevated nutrient concentrations (eg, Parkvale Stream tributary at Lowes
Reserve and Whareroa Stream at Waterfall Road), thereby potentially
mitigating some of the effects of nutrient enrichment associated with upstream
land use activities. There are also a number of other potential benefits from
riparian vegetation, including interception and attenuation of nutrients,
sediment and pathogens, improved bank stability, and improved habitat diversity
and complexity through leaf litter and woody debris inputs (eg, Parkyn 2004).
Water temperature (º C)
25
20
15
Upstream
Downstream
0
16/02/2006 18/02/2006 20/02/2006 22/02/2006
(Source: After Perrie 2008)
Figure 8.5: Daily variation in water temperatures upstream and downstream of a
planted reach of the Enaki Stream (near Carterton) for a week during February 2006
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