When it comes to commodities, hard assets like gold and oil tend to be on investors’ hit lists. One of the things that’s furthest from their minds is water. That’s a shame, though, as “blue gold” and water stocks could be equally compelling long-term investments.

With the world’s population continuing to surge upwards, water demand has followed suit in a big way. While water scarcity studies are (ahem) scarce, according to a 2009 study from the United Nations’ Food and Agriculture Organization, withdrawals from freshwater stores have tripled in the past 50 years.

Current demand for fresh water is increasing by around 64 billion cubic meters every year. That means by the time the clock rolls over to 2030, global water demand will already be about 40% higher than accessible & reliable supplies. Considering that there’s plenty of folks already living in drought conditions and lacking clean water, this going to be a huge deal.

Which is why investors may want to consider adding water stocks to their portfolios.

There’s a whole ecosystem of water stocks and firms that provide filtration, desalinization, pumping and other needed equipment to help stem the water crisis. Over the longer term, these companies should continue to see their stars — and earnings — rise as the world is required to get serious about its water issues.

Here are four water stocks to make a big splash in your portfolio.

Water Stocks to Buy #1 – Watts Water Technologies (WTS)

For investors looking for a “jack of all trades” with regards to owning water stocks, then Watts Water Technologies (WTS) IS a great starting point. The firm makes a whole host of products for the water industry. This includes everything from industrial pumps to rainwater collection devices.

That broad spectrum of water products is a good business to be in. WTS’ long-term estimated EPS growth rate is nearly 15%.

Huge demand for its water products in China has been driving that double-digit earnings growth rate. WTS estimates that it will realize a 15% to 20% increase in sales in its Asia-Pacific as it expands into China. The U.N. estimates that 80% of China’s lakes and rivers are too toxic for fish and wildlife — let alone human consumption. So there’s plenty of work for water stocks like WTS to do in the nation. Additionally, rising construction activity in the U.S. has management at WTS giddy.

Not that investors in WTS really need to worry about earnings growth. Watts recently reported second-quarter earnings increased by 21% year-over-year to 69 cents per share.

At a forward P/E of 20, WTS represents one of the cheapest broad water stocks to own.

Water Stocks to Buy #2 – American Water Works Company (AWK)

Turning waste water into usable drinking water isn’t a very sexy or exciting business. But it is a pretty stable one. And very few in the water utility sector do it better than American Water Works Company (AWK). AWK is the largest water utility in the U.S. — operating in 40 states and parts of Canada.

And yet, AWK continues to grow.

The water stock has been a serial acquirer of smaller municipal water systems. The problem is that budgets at many small towns and municipalities have been stretched to the limit. In order combat this, many have been forced to sell off their waste-water treatment operations. AWK can buy these already operational assets, “tuck them in” to their various business segments, trim the fat and make a tidy profit. Those profits have grown steadily over the last few years — even during the recent recession. AWK’s latest earnings increased 7% from the same period last year.

And those profits have been flowing back to investors in the way of big dividends. American Water recently upped its dividend by 11% back in April to 31 cents per share. That increase has the water utility yielding 2.5% — beating the S&P 500’s dividend yield.

Water Stocks to Buy #3 – Flowserve (FLS)

As far as water stocks go, one of the biggest issues is moving the stuff around. Flowserve (FLS) is there to provide all the necessary pumping, values and other equipment. And as with other water stocks — like WTS and AWK — that boring business means some pretty big dollars for FLS.

Aside from water treatment pumps, FLS is a huge player in the pumping equipment needed for desalination plants. Its various products have become the standard when trying to move massive quantities of water long distances. And with desalination becoming the go-to for some nations to stem their water crunches, FLS has continued to see its earnings and revenues rise.

For the latest quarter, FLS saw a 7.1% jump in earnings per share.

However, unlike most water stocks FLS has an ace in the hole. It’s also a huge player in the oil & gas industry. Flowserve makes a variety of pump that cross all the sectors of the industry. Whether it’s oil extraction or moving production through pipelines, FLS makes some sort of pump doing the work. Revenues and profits from this division continue to grow faster than its water operations. Ultimately, that will provide some extra oomph for investors looking at the boring water stocks.

FLS stock yields 0.9% and trades at forward P/E of 17.

Water Stocks to Buy #4 — Guggenheim S&P Global Water ETF (CGW)

With so many good water stocks on the market, it can be hard to decide the best way to play the sector. To that end, Guggenheim S&P Global Water ETF (CGW) maybe the best choice.

The $367 million exchange-traded fund (ETF) tracks 52 of the biggest water stocks from around the world, tracking the S&P Global Water NR Index. Its holdings include both WTS and AWK as well as other leaders like Veolia Environnement S.A. (VE). As expected, the vast bulk of its holdings are industrials and utilities.

CGW is truly a global fund — only around 38% of its holdings are domiciled in the U.S. The remainder come from a variety of nations, with France, the U.K., Switzerland and Hong Kong rounding out the top 3. That global focus has also helped it beat out other ETFs that track water stocks by a wide margin. CGW is up nearly 3.3% this year.

Over the longer term, CGW has managed to produce 12% annual returns over the last five years. While that has slightly underperformed the S&P 500, investing in water stocks can be seen as one of the ultimate long-term plays. Investors shouldn’t be swayed by the relatively short-term performance picture.

Expense for CGW run 0.70% — or $70 per $10,000 invested.

As of this writing, Aaron Levitt did not hold a position in any of the aforementioned securities.