Water goes 'missing' with snow loss

Mt Shasta, California, in December 2006. Snow-fed water resources are critically important to the western USA Natural '"water towers" - places that hold and release water for society

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A new study finds that if temperatures rise and more precipitation falls as rain rather than snow, it will reduce the total amount of water in rivers.

It is a surprising observation. One might expect the timing of water flow to change but not the overall volume.

But this is precisely what scientists discovered when they examined the histories of 420 catchment basins in the US spanning the period 1948-2001.

The researchers report their work in the journal Nature Climate Change.

The study compares places of similar climate and precipitation, but with differing fractions of that precipitation falling as snow and as rain.

And it seems there is a significant difference in total streamflow if the fraction shifts from snow to rain.

"At first we thought there could be all kinds of explanations for this, so we better dig a bit deeper and make sure that this difference between places wasn't being caused by something else," explained Dr Ross Woods from Bristol University, UK.

"We also checked out what happened at the same place between different years, and we found that the years with more snow also produced more streamflow than the years with less snow," he told BBC News.

Frozen ground

The team is now working through a number of possible explanations.

One idea is that the ground is also changing in response to temperature.

Colder conditions will lead to frozen ground, meaning when the snows do melt the water can run direct into streams and rivers.

In contrast, with warmer conditions, more water can be held in the ground for longer, to be later evaporated or transpired by plants. Less of the overall volume of water would then make it to streams and rivers.

"This issue is important because quite a lot of the snow at the moment is in the places we call the 'water towers' - the places that provide water to the great bulk of society," said Dr Woods.

"If the amount of water in the river on average goes down then those people find themselves under more pressure. Irrigation systems or hydropower systems or municipal water supply systems - the total amount of water available to those systems would be less.

"Those communities or systems would need then to adapt to that - to find a way to get by with less water, or else find alternative water sources."

Of course, under a generally warming climate, the overall patterns of precipitation are likely to change as well, with some locations receiving perhaps significantly more or less precipitation, irrespective of the fraction that is falling as snow or as rain.

The effect observed in this study may not therefore have too much relevance in some locations if their total water balance is rising. But in other, currently snowy places where overall precipitation is declining as well, the effect could compound problems.

The team hopes in future work to tease out the mechanisms involved.

Lead author Wouter Berghuijs said: "Hydrologists can take a large number of measurements at an individual site where they can determine for example what part of the flow in the river is due to a recent precipitation event and what part is due to ground water, by taking isotopic measurements of the stream.

"So, by going into great detail at individual places, it would be possible to understand the small-scale processes that underlie the behaviour that we observe," the researcher from Delft University of Technology, The Netherlands, told BBC News.

The study was also presented at the recent European Geosciences Union General Assembly in Vienna, Austria.

Headwaters of Tsitika River watershed on Northern Vancouver Island, British Columbia, December 2005. Temperature increases will affect the hydrological system in particular in rain/snow transition zones. It is possible that ground conditions are playing an important role

Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos

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