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Posts Tagged Groundwater

Phreatogammarus fragilis: The fragile well shrimp Waiology Dec 06

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By Daniel Collins

Phreatogammarus fragilis is an endemic New Zealand crustacean that lives in aquifers. It is an amphipod (a relative of the sand hopper), and is one of the largest (commonly up to 25 mm excl. antennae) and strongest swimming of NZ’s stygofaunal* crustaceans. Because it is so rarely observed, it does not have a common name; the best translations are ‘fragile well shrimp’ or ‘fragile groundwater lobster’, ‘fragile’ probably because its appendages broke off when early specimens were being identified and preserved.

The individual below is a 12 mm-long female with a brood pouch beneath the abdomen. It is white and translucent because there is no point in investing in pigments if it’s too dark to see or if there’s no risk of sunburn. This individual was caught in a trap in a 6 m-deep well beside the Selwyn River in Canterbury by Nelson Bousted and identified by Graham Fenwick. It was photographed live in water in a custom-built aquarium with several off-camera flashes.

Photo credit: Nelson Boustead

Waiology will have some more in-depth science of stygofauna in a future post.

* Stygofauna: animals that live in groundwaters, named after the river in Greek mythology, the Styx, which separated the Earth from the Underworld.

Dr Daniel Collins is a hydrologist and water resources scientist at NIWA.

Map: Projected effects of climate change on New Zealand freshwaters Waiology Nov 27

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By Daniel Collins

Maps are helpful tools in communicating and understanding the potential implications of climate change. We have national maps of projected changes in temperature that show faster warming in the north, and in precipitation that show more rain in the south and west and less in the north and east. We also have national maps of projected changes in drought, that show much of the country is likely to experience more severe droughts.

Now, I am able to give you a map of the potential freshwater changes across New Zealand. This includes changes in snow, ice, river flow, groundwater, aquatic ecology, geomorphology, and water use/management.

This is an important step in synthesising and understanding climate change impacts, drawn from existing case studies across the country. Projections are pin-pointed on the map below; in some cases they are more national in scope (e.g., salinisation of coastal groundwater).

This illustrates quite a complex picture. Retreating snow and ice. More flow in Alps-fed rivers, less flow in others. Higher lake levels and lower lake levels. More water demand from both agriculture and city. Higher erosion as well as channel aggradation. Higher lake nutrient levels and more frequent algal blooms.

There is a lot we know but also a lot we don’t know. As yet, we cannot provide a complete national assessment for river flows, nor for groundwater recharge. And very little research has connected the dots between climate change and aquatic ecology. But as new studies are carried out this map will be expanded and the gaps filled in.

In the near future I will describe the projected changes in more detail, so stay tuned.

Dr Daniel Collins is a hydrologist and water resources scientist at NIWA.

Canterbury Lysimeter Network: Measuring the hydrologic inputs to aquifers Waiology Jan 30

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By MS Srinivasan

Irrigated agriculture is growing in Canterbury. This growth has resulted in a greater rush for accessing water resources — surface and ground waters – across the region. Since these water resources are finite, limits on their takes are imposed to conserve them and make them available for other uses. However, setting limits on groundwater has remained a challenge. There are more unknowns than knowns. Does groundwater recharge occur uniformly over a year? Does irrigation add to recharge? How much groundwater could be allocated for irrigation sustainably?

To answer these questions, we need to take a closer look at how much water is draining through the ground and recharging the aquifers, as Paul White discussed two weeks ago. In Canterbury, where there is substantial abstraction from groundwater and where groundwater-fed streams and rivers are vitally important, a new initiative has begun to provide some much-needed data.

The Canterbury Lysimeter Network (CLN) is a scientific endeavour to identify how much land-based recharge, from rainfall and irrigation, happens across the Canterbury Plains at daily, monthly, seasonal, and annual time scales.

The CLN was originally an Environment Canterbury initiative, with NIWA, Aqualinc Research and HydroServices brought in to make it happen. Currently, the CLN has four sites spread across the Canterbury Plains, representing a variety of conditions — foothill vs coastal, high vs low rainfall, well-draining vs poorly-draining soils. All sites are under irrigation.

Each site has a set of three drainage lysimeters, and each lysimeter is 700 mm deep and 500 mm in diameter. A drainage lysimeter is a large cylinder, buried vertically in the soil. These lysimeters measure how much water drains through a soil column. The volume of water draining through a lysimeter is a direct expression of recharge of groundwater. The lysimeters are buried with their top at the soil surface. Water that infiltrates through the lysimeters is collected and measured at the bottom.

CLNSites

CLNlysimeters

Using these continuously recording lysimeters, we can find out how much water drains towards aquifers and when. Monthly data from one of the lysimeter sites, below, shows that groundwater recharge is largely occurring in autumn and winter, outside the irrigation season, even if more water falls during the summer.

CLNmonthlydepths

If we look more closely, we can see that it’s the longer or more intense storms that tend to generate large groundwater recharge.

CLNrechargeevents

What’s next? Our data collection has just started. What we have shown here are preliminary results. We are looking to continue this network over the next few years to quantify precipitation and irrigation recharge over time. Data from this network will also be useful in verifying models of recharge and soil moisture developed by NIWA and others, and allowing better management and allocation of groundwater in Canterbury.

Kinky relationships among Canterbury’s springs Waiology Jan 23

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By Daniel Collins

As Ross mentioned some time ago, one of the frontiers of hydrological research at present is the interface between surface water and groundwater. On the one hand, we need to understand how aquifers are recharged from the surface; on the other, how aquifers in turn discharge water back to the surface. This is important to water resource managers so that they can determine how water use at one location may affect water availability and aquatic ecosystems elsewhere.

One question, particularly relevant to both Canterbury and Hawke’s Bay, is how fluctuations in groundwater levels affect spring flow near the coast. This is actually part of a larger research programme we have looking at the environmental effects of water use.

As part of this research I have been examining streamflow and groundwater data around Lake Ellesmere/Te Waihora, Canterbury. Environment Canterbury has an extensive network of groundwater monitoring wells and streamflow monitoring sites in the area. The wells measure what is known as the piezometric head of an aquifer, derived from the Greek piezein meaning ‘to press or squeeze’, and which refers to the height that water would rise to if subject to atmospheric pressure. For the top-most aquifer, this height basically means the water table (see this earlier post on groundwater anatomy for more).

IrwellSWGWBy comparing piezometric and streamflow data, we can develop a picture of how fluctuations in groundwater translate into fluctuations in streamflow. In the figure below I’ve chosen just one well (L36/0141) and one stream (Irwell River). The first thing you see is a roughly linear increase in flow with piezometric height, at least on the right. Linear relationships like this are typical of water flowing through a saturated porous medium (see Darcy’s law for more). But where the flow drops to around zero, at a piezometric height of about 68 m above mean sea level, there is a hint that this line bends — a kink in the groundwater-streamflow relationship. In fact, it has to bend because you can’t go lower than zero flow.

This kink indicates a threshold groundwater level that divides streamflow behaviour in two. Above the threshold, streamflow increases roughly in step with groundwater level; below the threshold, there is basically no flow at all, no matter how low the groundwater drops. So if the level in well L36/0141 drops below about 68 m AMSL, Irwell River is likely to go dry.

While in some instances this drying is entirely natural, changes in groundwater recharge (say, due to climate change) and abstraction for irrigation could shift the hydrological regime of the stream to become more ephemeral. It could also mean less water flows into Lake Ellesmere. This is all very important when allocating water while avoiding undue water resource or environmental impacts.

Threshold behaviour like this is actually quite common in hydrology, and can be traced back to the underlying physics of water movement. Having a physical explanation of the interaction between aquifers and streams allows us to make more robust predictions of spring flow, and of the ecological and water resource implications that follow.

Rainfall recharge to groundwater Waiology Jan 17

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Guest post by Paul White, Senior Groundwater Scientist at GNS Science.

Groundwaters are very important water resources in many New Zealand regions — important because they are used for water supplies (urban and rural) and because they supply flow to many springs, streams, rivers and wetlands. The two major inflows to groundwater are from rainfall and from surface water.

We need to know the rates of recharge to groundwater so we can manage groundwater use. For example, groundwater use must be significantly less than groundwater recharge to ensure that groundwater wells and springs do not go dry.

Groundwater recharge from rainfall is the subject of this post which will cover some concepts, how it is estimated, measured, uncertainty and some relevant New Zealand water management polices.

Groundwater recharge from rainfall occurs as rainfall trickles through the soil into aquifers. However, only a portion of all rainfall actually reaches aquifers as recharge. This is because some rainfall evaporates from the ground, and some rainfall is transpired by plants back into the atmosphere – processes termed evapotranspiration. Groundwater recharge first reaches a shallow aquifer (termed the water table or unconfined aquifer). Then the recharge may discharge from the unconfined aquifer to surface waters or to deeper, confined aquifers. Flow paths in aquifers are typically understood using models of groundwater flow systems including geological models and groundwater flow models.

It is common for rainfall recharge to be the largest source of groundwater recharge. In those circumstances sustainable groundwater allocation policies should ensure that allocation is less than recharge and that actual use is less than allocation, i.e.

R > A > U

where
R = rainfall recharge estimate provided by science, e.g. projects to characterise rainfall recharge and uncertainty undertaken in the Waterscape research programme;
A = allocation of groundwater, which is a policy decision by the groundwater management authority;
U = use of groundwater.

Estimates and measurements of rainfall recharge are very useful for the development of groundwater allocation policies. Regional councils are responsible for policy decisions on groundwater allocation. Central government also has an input to decision-making. For example a National Environmental Standard, proposed by Ministry for the Environment (2008), recommends a default (in lieu of regional policies) maximum groundwater allocation as 35% of groundwater recharge.

Estimates of groundwater recharge from rainfall are often made using computer models that typically consider rainfall, evapotranspiration and soil properties. Measurements of groundwater recharge can be made with lysimeters — this is typically a tube sunk into the ground that encases a soil column. Water flow from the base of the lysimeter column is measured over time. Models of rainfall recharge at the local scale are typically tested against measurements of groundwater recharge at the local scale. Quantifying rainfall recharge at the regional scale involves the use of models and up-scaled measurements from lysimeters.

Rainfall recharge measurements demonstrate significant inflows of rainfall to groundwater. For example, the Canterbury lysimeters measured groundwater recharge in the range 26% (Lincoln) to 37% (Winchmore) of rainfall in the period 1999 — 2000 (White et al. 2003). These results were used to estimate regional rainfall recharge to groundwater in the area between the Waimakariri River and Rakaia River in the range 19.2 to 23.9 m3/s providing a useful indication of groundwater sustainability in comparison with annualised groundwater allocation (approximately 42.6 m3/s) and estimated groundwater use (6.8 m3/s) in the period.

Environment Canterbury established groundwater allocation zones in 2004 and adopted allocation limits based on estimates of land surface recharge (rainfall plus irrigation). Zones in which total allocation exceeded those limits were designated as being fully allocated and referred to as ‘red zones’. The Rakaia-Selwyn groundwater allocation zone has been classified as a ‘red zone’ from the initial adoption of this management policy and Environment Canterbury has recommended decline of further groundwater allocation (e.g. in the Rakaia-Selwyn hearings), reviewed groundwater consents, and placed annual volume limits on groundwater pumping.

This post shows some of the applications rainfall recharge measurements to groundwater resource management and groundwater resource characterisation.

References:

Ministry for the Environment 2008. Proposed National Environmental Standard on Ecological Flows and Water Levels. Discussion Document. 61p.

White, P.A., Hong, Y-S., Murray, D., Scott, D.M. Thorpe, H.R. 2003. Evaluation of regional models of rainfall recharge to groundwater by comparison with lysimeter measurements, Canterbury, New Zealand. Journal of Hydrology (NZ) 42(1), 39-64.

Related Waiology posts:

The low down on groundwater
The importance of groundwater
Where does NZ take its water from?

Where does NZ take its water from? Waiology Nov 28

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By Daniel Collins

I mentioned previously how much water we are allowed to use in NZ. The amount varies markedly from region to region, and is growing over time, with Canterbury and Otago accounting for well over half of the consumptive takes (excluding the Manapouri hydro scheme). But seeing as Kiwis seem to know less about our aquifers than our rivers, I’d like to turn now to the issue of where we can take our water from — rivers, lakes, aquifers or reservoirs (or storage lakes).

SWGWAllocation

About two thirds of the water NZers are allowed to take is from surface water — rivers and lakes. This is for all non-hydro schemes and other non-consumptive uses; the data come from a 2010 report from Aqualinc Research for MfE. About a third comes from groundwater. Five percent comes from reservoirs, which are largely fed by rivers.

Looking across New Zealand, we see that the relative importance of surface water, groundwater and reservoirs differs among regions. In Otago, surface waters are relied upon for 84% of the allocated water supplies; in Auckland, it’s 4%. In Hawke’s Bay, groundwater accounts for 74%; in Otago and Taranaki it’s 7%. And in Auckland, reservoirs account for 74%; in Waikato and Manawatu-Wanganui it’s zero.

So why do these proportions vary so much?

There are three key factors behind this: whether there’s enough water in the rivers; whether there are accessible and productive aquifers; and how important reliability of supply is to the water user.

Otago, for example, only has a smattering of significant aquifers, so they get most of their water from rivers. Hawke’s Bay, Southland and Tasman each have productive aquifer systems available. In the Auckland region, where over a third of NZ’s population needs to be provided with a reliable supply of drinking water, they turn to reservoirs and more recently to a reliable supply from the Waikato River via a tunnel. On the Canterbury plains, water from rivers and aquifers are both readily accessible and both highly used, though it is typically easier and cheaper to take water from rivers, even if the reliability of supply isn’t as high. You can see maps of NZ’s key aquifer systems here.

So we can see, yet again, that the physical environment has a huge effect on the availability and reliability of water supply in New Zealand. In posts to come, we’ll see what exactly this water is used for.

Recent NZ research from climate change to tussock | Journal of Hydrology (NZ): 50(2) Waiology Nov 24

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By Daniel Collins

For those of you who don’t receive New Zealand’s own hydrology journal, or for those who want to save some time, here’s an overview of this month’s edition — the journal’s 100th.

1. Barry Fahey (Landcare Research) et al. use their water balance model, WATYIELD, to assess whether tussock in Otago’s uplands can really intercept appreciable amounts of fog, turning it into runoff. Their conclusion: no. This is actually one in a long line of studies that have considered the same question, a question that has turned out to be a veritable controversy, with different papers firmly coming down on opposing sides.

2. Suzanne Poyck (formerly NIWA) et al. use their catchment hydrology model, TopNet, to forecast the impacts of climate change on the Clutha River basin, with a particular focus on changes to snow. Annual precipitation is forecast to increase, as is streamflow (mainly in winter and spring), while the role of snow diminishes.

3. Michael Stewart (Aquifer Dynamics and GNS Science) et al. use isotopic analysis to identify the sources and ages of nitrate in the Waimea Plains, near Nelson. Two kinds of contamination were identified: diffuse contamination from inorganic fertilizers and manure, and point source contamination from a large piggery (now closed). This has been a problem because Ministry of Health guidelines for drinking water have been exceeded for some years. And while input of nitrogen has been decreasing, best practices and nutrient budgeting are still encouraged.

4. Luke Sutherland-Stacey (University of Auckland) et al. test a mobile rain radar device in Tokoroa, central North Island, during 2008 and 2009. Their X-band radar system was able to make observations with high spatial (~100 m) and temporal (~15 s) resolutions, which helps resolve rainfall patterns during convective weather systems at least compared with existing rainfall monitoring systems. But as accuracy declined with distance, particularly over 15 km, the device is best suited for small study areas.

5. Tim Kerr (NIWA) et al. develop a new map of mean annual precipitation for the Lake Pukaki catchment, which includes Aoraki/Mt Cook, using data from 1971-2000. The catchment average is 3.4 m/yr, with over 15 m/yr falling in the north west of the catchment.

6. Clare Sims (BECA) et al. study the dynamics of snowmelt in the Pisa Range, Central Otago. Their focus was how the meteorological conditions that develop over fault-block mountain ranges in the region affect snowmelt. They showed that net radiation (basically sunlight) was slightly more important than sensible heat flux (basically wind), resulting in a sustained pulse of meltwater. They went on to suggest that changes in winter snow could have a significant effect on summer river flows.

The importance of groundwater Waiology Nov 14

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By Tiejun Wang

Groundwater is one of the most important natural resources in New Zealand, which by one estimate accounts for about 80% of the water present at any one time across and beneath the country. According to a 2010 review of water allocation [2], the number of groundwater consents accounts for 68% of all national consents. In terms of the allocated volume for consumptive use, groundwater allocation is about 12% (3.3×109 m3/year; equivalent to 6% of the water in Lake Taupo) of the total annual allocation (26.9×109 m3/year). Over the last decade, the demands for groundwater have increased dramatically in New Zealand, which is manifested in the volume of groundwater allocated and the percentage of groundwater allocation in the total allocation [2]. Most of the groundwater resources have been allocated to irrigation purposes in NZ. For example, in the Canterbury region alone, 55% of the agricultural lands are irrigated by groundwater.

tsingtao_logoCompared to surface water, groundwater possesses some unique features. For example, groundwater is typically a more reliable resource, particularly in dry regions, and sometime has higher quality (e.g., temperature and minerals). My hometown (Qing Dao) is famous among other things for its beer (Tsingtao beer). The beer is made of groundwater that is brought up to the surface by springs that are rich in minerals. In fact, it’s the number one branded consumer product exported from China, which you can occasionally find here in NZ. However, the growing demand for groundwater resources in China has raised concerns about the sustainability of both agriculture and the economy. The overdraft of groundwater not only has economic consequences, but also may pose serious threats to the environment.

To start off, dropping groundwater tables, for example due to pumping, can significantly increase the cost of irrigation because the energy required to bring groundwater up to the surface increases with the depth to the groundwater. Secondly, there are numerous cases regarding the adverse impacts of overdraft of groundwater on the environment, which we should learn from. To name one example, the excessive pumping of groundwater, particularly in arid and semi-arid regions, can produce massive depression cones (e.g., in the North China Plain, more than 20 depression cones stretch over 50,000 km2), which can cause the Earth’s surface to subside just like earthquakes (most of the time it is an irreversible process!!), and intensify groundwater contamination.

The water problems that New Zealand is facing right now require us as hydrologists to take immediate actions on how to sustainably manage our water resources. As Ross mentioned earlier, groundwater and surface water are tightly coupled. Therefore, in order to more effectively manage water resources, hydrologists from different sub-disciplines (e.g., surface water hydrology, groundwater hydrology, and ecohydrology) should work together to tackle the problems. This is one of the major goals for the Waterscape project.

The low down on groundwater Waiology Nov 10

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By Daniel Collins

Several posts back I reported on a public perceptions survey that showed that New Zealanders seem to know less about groundwater and wetlands than about rivers and lakes. (Hydrogeologist Michael Campana, OSU, lamented a similar bias towards surface waters among US and international water resource management circles.) I expect this is because rivers and lakes are much more in the public eye — more noticeable when driving around the landscape or on a map, or easier to cover on prime time news. So I’d like to start shedding some light on a part of the water cycle that is basically always in the dark. This should put readers in good stead when reading about groundwater on Waiology or seeing it in the news.

So, groundwater — what is it? The name is almost a dead giveaway really. It’s water contained in the gaps in rock, soil or some other geological layer. But there’s a catch: to count as groundwater and not as soil moisture, all the gaps must be filled with water — that is, the geological material must be saturated.

And where does this water come from? Groundwater is just another part of the water cycle, recharged by water percolating from the soil, rock or rivers above. Over time, groundwater can return to the surface when the water table reaches the ground surface, when it discharges into a surface water body, or when tapped by plant roots or wells.

If a geological layer is particularly good at containing and conveying groundwater, it’s called an ‘aquifer’, from the Latin ‘aqui-‘ (water) and ‘ferre’ (to bear or convey). Its first known occurrence in the English language was in a 1901 article in the journal Science (PDF):

‘The artesian system shows four or five aquifers, or water-bearing strata, more or less completely separated from one another.’

In French, ‘aquifere’ seems to have been used nearly 100 years earlier by the biologist Lamarck when distinguishing between air-conducting passages and water-conducting passages in the body. It attained its hydrogeological meaning by 1835, when Arago described the physics of natural springs.

For a geological layer to be capable of bearing an appreciable amount of water, it must have enough gaps or fractures in it to allow water to flow. If there isn’t much space for water, or if it can only flow very slowly, the layer is called an aquiclude.

How conductive the layer is depends on how it was formed, geologically, and with what material. Gravel deposited by ancient rivers or glaciers tends to form good aquifers; fine sediments deposited beneath lakes or estuaries tend to become aquicludes. In some regions, like the coastal parts of the Canterbury Plains, you can actually find alternating layers of geological material: aquifer, aquiclude, aquifer, aquiclude, etc. In Canterbury’s case, this came about by sequential glacial advances and sea level rises over geological time.
Aquifer2
The upper-most aquifer, when not disconnected from the soil by an aquiclude, is termed an ‘unconfined aquifer’. Aquifers that lie beneath aquicludes are ‘confined’. In some instances, the water in confined aquifers can be under such pressure that they give rise to artesian springs or wells (named after the region in France, Artois) — where water flows unaided above the top of the aquifer or even the ground surface. And as for the water table, this is the depth where the ground switches from being partly saturated to fully saturated, and only applies to unconfined aquifers.

In aquifers, groundwater flows from gap to gap, or from fracture to fracture. While water on the Earth’s surface flows downhill, water in aquifers flows down the pressure gradient which could be upwards. Compared with rivers, however, groundwater flow is much, much slower. It might take decades or centuries for water to cover the distance in an aquifer that river water might cover in a day.

So there you have it – the lowdown on groundwater. It’s an important resource, so we had better understand how it works.

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