GEOFF BENNETT: The demand for water from the Colorado River is of paramount importance out West and the focus of some big battles.

It's been especially critical for farming and agriculture.

And in California's Imperial Valley, there are growing questions over the use of that resource and whether bigger changes are needed.

Our science correspondent, Miles O'Brien, has this report.

MILES O'BRIEN: All right, well, let's try one.

It was a cool, clear morning in the citrus grove when Gina Dockstader sliced open a Minneola orange for me, flooding my taste buds.

Oh, that's delicious.

And blowing my mind.

Oh, my gosh.

It's almost magical when you consider where we are, the middle of a blazing hot low desert, where Southern California meets Northern Mexico, a place that gets unlimited sunshine, but only two to three inches of rain each year.

Here we are standing in this lush orchard in the middle of the desert.

GINA DOCKSTADER, Chair, Imperial Irrigation District Board of Directors: It's amazing.

MILES O'BRIEN: Welcome to the Imperial Valley, a half-million acres of highly productive, irrigated farmland, America's winter salad bowl.

If the Imperial Valley didn't exist, what we would see in produce sections in the winter would be entirely different, wouldn't it?

GINA DOCKSTADER: Entirely different, with salads, with the lettuce, with broccoli.

I mean, we just wouldn't have the vegetables, we wouldn't have the fruit that we have now.

MILES O'BRIEN: About two-thirds of the nation's winter vegetables are grown right here.

And it would not happen without water from the Colorado River.

J.B. HAMBY, Chair, Colorado River Board of California: This is Imperial Dam.

We're straddling the Colorado River right now.

MILES O'BRIEN: J.B. Hamby is California's Colorado River commissioner.

J.B. HAMBY: This is one of the final stops on the Colorado River and one of the lowest stretches here in the desert, originating all the way up in the Rocky Mountains as snowfall makes its way here to this liquid gold, which is water that supports our farms and communities all throughout this region.

MILES O'BRIEN: Hamby is also a director of the Imperial Irrigation District, a powerful and controversial water agency that controls the single largest entitlement to Colorado River water, about three million acre feet -- that's about a trillion gallons -- a year.

It's 70 percent of California's share.

J.B. HAMBY: The doctrine of prior appropriation, first in time, first in right, putting water to use first, secures your ability to use it.

MILES O'BRIEN: It all began in the early 1900s, when private developers dug the first canals to divert Colorado River water into this desert valley.

Despite the arid climate, the land is astonishingly fertile.

Thousands of years ago, the river flooded through here, leaving behind rich alluvial soil, more than 150 feet deep in some places.

Much of the valley lies below sea level, so no pumps are needed.

Gravity does the work.

When a farmer requests irrigation, a zanjero, or ditch rider, simply opens a gate to let the water flow in.

And so how long will this gate be open today?

JON SHIELDS, Imperial Valley Farmer: Well, this gate, it'll probably run until morning to irrigate that whole field.

MILES O'BRIEN: Farmer Jon Shields showed me how it works.

Smaller canals fill up, more gates are opened, and the field is flooded.

JON SHIELDS: You can see that's the water's running out nice and even out there.

MILES O'BRIEN: It's simple and cheap, but with so much loss to evaporation, hardly efficient.

And the cost of water here provides little incentive to conserve.

It's only $20 an acre foot, six-one-thousandths of a cent per gallon.

For all practical purposes, it's free.

But this free resource is becoming more precious.

Since 2000, persistent record-setting drought, fueled by climate change, has cut the Colorado's flow by 20 percent.

The result?

A growing fight over water and many eyes fixed on the Imperial Valley and its outsized share of the pie.

TINA SHIELDS, Imperial Irrigation District: With that water right come certain obligations and lots of targets on your back, because you are the solution to everybody else's problem.

MILES O'BRIEN: Tina Shields, Jon's wife, is a water department manager in the district.

TINA SHIELDS: Our goal really is to keep our community whole, keep food and production to feed the nation.

But we know that we have obligations and stewardship roles as the largest irrigation district to keep California on solid ground from a water supply perspective.

MILES O'BRIEN: So the district has struck a series of deals with Southern California's municipal water suppliers.

The cities pay hundreds of dollars per acre foot for water.

The revenue helps fund local infrastructure upgrades and advance conservation practices, everything from laser-leveling fields to drip irrigation and micro-sprinkler systems, like the ones Gina Dockstader now uses.

The perception in cities sometimes is that farmers have senior rights and don't want to share.

Is that accurate?

GINA DOCKSTADER: No, I don't think that's accurate at all.

There might be a little resentment when you're going to go put in another golf course using that water.

MILES O'BRIEN: The resentment runs both ways when the talk turns to alfalfa.

The Imperial Valley may be known for growing winter produce, but its dominant crop is alfalfa, spreading across about 120,000 acres.

Grown mainly to feed livestock, the perennial requires a lot more water relative to its value.

AMANDA STARBUCK, Research Director, Food and Water Watch: What we need are, first of all, moratoriums on the expansion of alfalfa farms.

MILES O'BRIEN: Amanda Starbuck is the research director for the environmental advocacy group Food and Water Watch.

AMANDA STARBUCK: And we need resources to help farmers shift to farming systems that are more in line with the climate reality.

MILES O'BRIEN: As global demand for beef and dairy rises, increasing amounts of alfalfa are shipped overseas.

In the past decade, California exported between 20 and 40 percent of its alfalfa hay production, primarily to Asia and the Middle East, some of it grown right here in the Imperial Valley.

George Frisvold is an economics professor at the University of Arizona.

GEORGE FRISVOLD, University of Arizona: That's been a source of controversy because people are looking at, well, water is being used.

It's exported in body in the crops that we're exporting.

MILES O'BRIEN: There are ways to grow alfalfa using a lot less water.

In the summer, when temperatures often reach 120 degrees, alfalfa grows more slowly and requires twice as much water.

ROBERT GLENNON, Professor Emeritus, University of Arizona Law School: We have to stop growing alfalfa during the summer.

MILES O'BRIEN: Robert Glennon is a professor emeritus at the University of Arizona Law School.

ROBERT GLENNON: If you turn off the irrigation systems in June, you can turn them back on in August or September and the alfalfa plants have simply gone dormant.

They haven't died.

MILES O'BRIEN: The Imperial Irrigation District is trying to implement that idea.

Jon Shields volunteered for a deficit irrigation program that paid him to stop watering some of his alfalfa fields in the summer.

But his yield losses were higher than he anticipated.

JON SHIELDS: The fields just didn't come back as good as I wanted them.

Every single one of them, I have had to come in and replant.

It's not like the old days where we could just water at will.

MILES O'BRIEN: Jon Shields is a fourth-generation farmer.

Like so many here, his roots are as deep as the alluvial soil.

But without the Colorado River, the fertile ground is only desert.

So farmers here are scrambling for ways to save their liquid gold and their way of life.

For the "PBS News Hour," I'm Miles O'Brien in the Imperial Valley.