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Archive January 2012

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?

Edmond Halley, an underappreciated hydrologist Waiology Jan 09

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By Daniel Collins

I remember in 1986 going to the Beverly-Begg Observatory, in Dunedin, to see Halley’s Comet. At the time, I was a young kid fascinated with astronomy. I had discovered a book on the topic the previous year while on holiday in Central Otago, and soon joined an astronomy club. Through the club, I built a basic telescope (with a lot of help) and dreamed of becoming an astronaut. But this dream was short-lived and that career never eventuated. Instead, I became a hydrologist.

HalleyEdmond Halley (1656-1742), after whom the comet is named, is best known for his role in astronomy. But of his 107 or so published papers, only 36% were on the topic. 34% of them were on geophysics, a few of which covered the water cycle and in turn helped usher in a fundamentally new approach to hydrology.

It’s likely that Halley became interested in the water cycle after reading a book by France’s Edme Mariotte, published in 1686. Mariotte argued, with the support of experimental measurements, that rainfall is sufficient to supply the flow of water in rivers. (You think this is obvious? It wasn’t at the time.) But Halley then wondered: Is evaporation sufficient to supply rainfall?

After a basic lab experiment and a back-of-the-envelope calculation, Halley believed it was indeed sufficient. And in doing so he completed the water cycle: rainfall feeds rivers; rivers flow into the oceans; oceans evaporate into water vapour; and water vapour condenses into rainfall.

The fundamentally new approach to hydrology, then, was the quantitative measurement of the water cycle. Halley shares the honour of ushering in this revolution with Mariotte and a second Frenchman, Pierre Perrault. It arguably remains the most important element of hydrology to this day.

Halley’s sortie into hydrology didn’t last long, though. He published only four papers on the topic, the most important in 1687 and 1691, and never put his other interests on hold, hence his fame in astronomy. Yet among historians of hydrology, Halley’s contributions on the water cycle are very much appreciated.

References:

Dooge J.C.I. 1974. The development of hydrological concepts in Britain and Ireland between 1674 and 1874. Hydrol. Sci. Bull., 19(3): 279-302.

Malin S.R.C. 1993. Edmund Halley — geophysicist. Q. J. R. astr. Soc., 34: 151-155.

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