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Posts Tagged Etymology

What makes wetlands wet lands? Waiology Feb 04

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By Daniel Collins

The simple answer is, of course, water. But that says little about the natural history of wetlands, or what physical conditions are necessary to maintain, restore or even engineer them. For that, we need to take a closer look at wetland hydrology.

Wetlands are tracts of land that are water-logged at least seasonally. They may be spongy bogs, mountain tarns, verdant swamps, or many other types. They remain wet because the inputs of water from rain, rivers or groundwater compensate any losses.

The various types can be distinguished based on their hydrology. In their book on wetland restoration, Bev Clarkson and Monica Peters (2010) quantify this continuum with the “gumboot test”. Short “red bands” are usually okay for keeping you dry in bogs, taller gumboots are needed for fens, thigh waders for swamps, and waist waders for marshes.

The continuum of wetland types in New Zealand, after Clarkson and Peters (2004).

A kettle hole at O Tu Wharekai/Ashburton Lakes. Kettle holes, formed by glacial deposits, are fed by rainfall and groundwater and can fluctuate from wet to dry depending on groundwater levels. (Photo: M. Beech, DOC)

Controlling this water balance are climate, geomorphology and even the vegetation itself. Wetlands typically form in gently sloping or topographically convergent portions of a landscape, where surface and ground waters meet. The vegetation plays several roles here, including the build-up of peat, changing evaporation and water flows, and by controlling erosion and hence the shape of the local landscape to some degree. Kettle holes, such as those in the Ashburton Lakes, are an example of the climate and glacial geomorphology controlling the hydrology, which in turn controls the ecology.

Each plant species is adapted to a particular range of wetting and drying. Too dry for too long, and terrestrial plants can invade. This is particularly important to bear in mind when conserving, restoring or engineering wetlands. It’s not enough to simply add water – the hydrological regime must match the desired ecosystem’s needs.

Some of the hydrological effects of wetlands are in essence also ecosystem services – benefits conferred to society by the wetlands. Reducing flooding and augmenting low flows are two services often cited, though they are not true for all wetlands (Bullock and Acreman, 2004). Science is actually a little in the dark as to which biophysical features of wetlands confer or degrade the various hydrological services.

And as we consider the hydrological origin of wetlands and differences between wetland types, it is also interesting to consider the etymological origin of wetland words. The word “swamp”, for example, can be traced back to the Old Norse word for “sponge”, sharing a common ancestry with “sump”. “Marsh”, “morass” and possibly “moor” have origins in the Proto-Germanic word for “sea”, and are in turn related to the words “marine” and “maritime”. “Mire” comes to us from the Proto-Indoeuropean (PIE) word for “damp”, and shares this root with “moss” and “must” (as in “musty”). “Bog” came to us via Gaelic, with a meaning of “soft” or “moist”, and earlier still from the PIE word for “bend” (as does “bow”). And lastly “fen”, which remains truest to its roots, goes back to the PIE word for “swamp”.

Ordering these words on a scale from less to more wet, in terms of their etymological roots, we get: bog – fen/swamp – mire – marsh. As a testament to biophysical basis of words, this aligns nicely with the order of wetland types illustrated above.

Much, however, remains to be learned about the hydrological origins, needs and impacts of wetlands. Continued research in this front will assist in conservation and restoration, the use of ecosystem services, and in the broader understanding of the water cycle – all very useful as NZ seeks to balance resource use with environmental protection.

References:

Bullock A. and Acreman M. (2003). The role of wetlands in the hydrological cycle. Hydrol. Earth Syst. Sci., 7, 358-389, doi:10.5194/hess-7-358-2003.

Clarkson B. and Peters M. (2010). Wetland types. In B. Clarkson and M. Peters, eds., Wetland restoration: A handbook for New Zealand freshwater systems. Manaaki Whenua Press, Lincoln. Pp. 273.


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

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.

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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