A Few Thoughts on Water

Years ago, I frequently commuted between Los Angeles and Seattle by air.  The contrast between the two cities was always a bit jarring, particularly in July and August -- high summer on the west coast of North America -- when the lawns in Seattle are brown while all the residential yards in Los Angeles are a beautiful emerald green.  Summer rainfall in Seattle is usually about 1.8 inches spread over those two months, while Los Angeles is essentially dry.

A couple of weeks ago I flew into LAX from the east coast and got another perspective on water use there.  My first glimpse of the basin was the smog lapping up against the rim of the San Gabriel Mountains. I managed to snap a quick photo after we had flown over the ridge (the smog is on the lower left, though the contrast was more impressive when we were looking from the east side).

IMG_0477.JPGEven in May it looks a little dry 'round those parts.

A few minutes later, I noticed large green patches covering the sides (usually the west side) of hills.  This continued all the way to downtown LA, and we were high enough for most of that time that I couldn't figure out why the locals were spending so much of their precious water keeping the sunset sides of hills green.  Then, finally, we passed over one low enough that the purpose jumped out at me.


Even in death, Los Angelinos maintain their homage to William Mulholland by keeping him eternally damp.  And in death, Los Angelinos continue to contribute to the smog shown above -- the grass covering the land of the dead is trimmed quite short.  Many, many square miles of it.  A cushy life, have those dead people.  And to be fair to Los Angeles (which, admittedly, is hard for me), Seattle, too, uses a great deal of water and hydrocarbons to keep our decaying ancestors covered with a trim layer of green.  It happens everywhere here.  Welcome to America.

Even the way the US irrigates land to feed the living represents a profligate use of water.  According to the USDA, 80% of the water consumed in this country goes to agriculture. (Note that "use" and "consumption" are often confused.  Agriculture and thermoelectric power generation both "use" about 40% of the nation's freshwater, but while almost 100% of the water used for power generation is returned to where it was taken from -- albeit somewhat warmer than when it was taken -- much of the of water put on crops is does not reach the roots or is evaporated and lost to the atmosphere.)  Notice that I did not use the word "waste", because some of the leakage winds up back in groundwater, or otherwise finds its way into the environment in a way that might be classified as "beneficial".

And pondering water use here in the US, and the impact on our economy, my thoughts turn to water use in Asia.  Much ado was made in the last couple of years about the IPCC report of anomalous melting of Asian glaciers, followed by the discovery that there was no actual data behind the assertion.

A recent paper in Science adds some much needed analysis to the story.  Walter Immerzeel and colleagues set out to understand the relative importance of meltwater and rainwater to river flows in Asia.  It is interesting to me that this sort of analysis wasn't done before now: "Earlier studies have addressed the importance of glacial and snow melt and the potential effects of climate change on downstream hydrology, but these are mostly qualitative or local in nature."

For five large river basins the authors used a combination of precipitation data, snow melt models, and evaporation rates, to calculate the Normalized Melt Index (NMI).  The NMI is the ratio of snow and glacier discharge to downstream discharge.  If all the water in a river downstream is from melting, then this ratio is obviously one; if the ratio is less than one, rainfall contributes more than meltwater, and if it larger than one, more water is lost through evaporation or other processes (like agriculture) and meltwater is more important for total flow.

Here are the results.  For each of the rivers, the authors calculated the percentage of the total discharge generated by snow and glacial melt:











In other words, water supplies in the Indus river valley are largely dependent on meltwater, whereas the large river systems in China appear to be less dependent on meltwater.  That is a very interesting result, because the story told by lots of people (including myself) about the future of water in China is that they are in big trouble due to glacial melting in the Himalayas.  Assuming this result holds up, China may be better off in a warmer world that I had anticipated.

The authors also used various projections of snow and rainfall to estimate what water supplies would look like in these rivers in 2050.  As you might expect, a warmer world leads to less snowfall, more melting, and lower river flows.  But as the warmer world brings increased rainfall, the impact is smaller than has been widely assumed.  I am not going to bother putting any of the numbers in here, because, as the authors note, "Results should be treated with caution, because most climate models have difficulty simulating mean monsoon and the interannual precipitation variation, despite recent progress in improving the resolution of anticipated spatial and temporal changes in precipitation."

But they went one step further and tried to estimate the effects of potential decreased water supply on local food supplies.  Couched in terms of crop yields, etc., Immerzeel et al estimate that the Brahmaputra will support about 35 million fewer people, the Indus will support about 26 million fewer people -- that's food for 60 million fewer people in India and Pakistan, if you are counting -- and the Yellow about 3 million more people.  Finishing up, they write:

We conclude that Asia's water towers are threatened by climate change, but that the effects of climate change on water availability and food security in Asia differ substantially among basins and cannot be generalized. The effects in the Indus and Brahmaputra basins are likely to be severe owing to the large population and the high dependence on irrigated agriculture and meltwater. In the Yellow River, climate change may even yield a positive effect as the dependence on meltwater is low and a projected increased upstream precipitation, when retained in reservoirs, would enhance water availability for irrigated agriculture and food security.

I am perplexed by the take on these results over at Nature News by Richard Lovett.  His piece carries the title, "Global warming's impact on Asia's rivers overblown".  I'll give Lovett the out that he may not have written the actual headline (Editors!), but nonetheless he sets up the Immerzeel paper as a big blow to some unnamed group of doomsayers.  Perhaps he imagines that Immerzeel completely undermines the IPCC report?  This is hardly the case.  As I wrote last January, sorting out the mistake over Himalayan melting rates is an example of science working through a blunder.  Instead overturning some sort of vague conspiracy, as best I can tell Immerzeel is simply the first real effort to make quantitative assessments of something to which much more attention should have been paid, much earlier than it was.

And even Lovett appears to acknowledge that reducing the human carrying capacity of the Brahmaputra and Indus river valleys by 60 million people is something to be concerned about.  From Lovett: 

The findings are important for policy-makers, says Jeffrey Kargel, a glaciologist at the University of Arizona in Tucson. "This paper adds to mounting evidence that the Indus Basin [between India and Pakistan] is particularly vulnerable to climate change," says Kargel. "This is a matter that obviously concerns India and Pakistan very much."

Indeed.  As they should concern us all.