High Country Snow Survey

SNOW SURVEY How the experts manage reservoir storage while preventing floods

It's early March, and Phoenix is savoring one of those spring days that Cleveland would kill for: 75° or so, and not a cloud between here and California. On the evening news, the weatherman is predicting that a warm, wet, late winter storm will cross the northern half of Arizona in a couple of days, but only a handful of the metropolitan area's two million people give it a moment's thought.

That handful work either for the U.S. Department of Agriculture's Soil Conservation Service or the Salt River Project, a water and electrical utility; and what they do within the next two days will not only save Phoenix from a devastating flood this week but also help keep the area's lawns, parks, and citrus groves from scorching when June's annual drought arrives.

Exchanging data through computer links and telephone conversations, the specialists sort through the facts. The six reservoirs north and east of Phoenix are holding a combined 1.9 million acre-feet of water, 95 percent of capacity. The snowpack in the mountains beyond the dams is deeper than normal, and it's also unusually dense. When the rain hits it, this pack will melt like ice cream under a warm shower and send more water careening down the Salt and Verde rivers than the reservoirs can hold. When that happens, the water will be released through the dams and down into the normally dry Salt River bed in Phoenix. The best-case scenario: a vast amount of wasted water. The worst: a flood in Phoenix.

But that's if the dams' managers are caught flat-footed. Instead, this happens: several days before the storm arrives, SRP hydrologists forecast the runoff and predict how much water may have to be released from the reservoirs. As the storm develops, workers closely monitor the flow and adjust releases to insure that the smallest quantity of water necessary is released to the river below the dams. Phoenix residents hardly notice. A trickle meanders through town in the riverbed, but it doesn't endanger bridges or threaten to slop over the banks. And the reservoirs will stay close to 100 percent capacity to nurse the city through the summer.

There is a little-known key to making this complicated feat of water management work. It's called snow surveying, and although it uses computers and radio signals designed to carom off meteor trails in the ionosphere, it depends on a bunch of men and women on cross-country skis who, quite simply, love to be out working-and playing-in the snow. It's a still, cool February morning on the Mogollon Rim some 60 miles southeast of Flagstaff. Bruce Gordon, a 39-year-old range conservationist with the Soil Conservation Service, parks the government-issue pickup in a clearing just off State Route 87. The elevation here is 7,300 feet, so there's a good cover of snow. Certainly enough for skiing. We clamp on our skis and glide off to one of the 38 government 'snow courses" sprinkled around the north-central mountains and plateaus of Arizona. It's about half a mile from where we left the truck. When we arrive, surprise! there's the highway again, and the snow course is only a couple hundred feet away from it. We've skied in a semicircle.

"Why didn't we just park there and walk to it?"

"Well, it's a little hard to pull the truck completely off the road here." Gordon looks a trace sheepish. The truth is that he'll seize any excuse to ski to work. So will most of his colleagues. Some of the work of snow surveying,In fact, is automated. In these mountains there are 14 "SNOTEL" sites, which use weight-sensitive snow pillows, depth sensors, and radio transmitters to beam out daily reports. The signals bounce off a layer of ionized particles left by sandgrain-size meteors disintegrating 50 to 75 miles above the earth. The method is said to be more reliable than satellite telemetry, because satellites periodically break down. Meteors will keep falling into the atmosphere forever. But two-legged snow surveying has special advantages. The electronic systems are more than 90 percent accurate, but they don't learn from experience, and they have no intuition or emotion about the outdoors.

"Sometimes we have to use snowmobiles to reach the more remote sites, but frankly, I hate them because of the noise and the fumes," Gordon says. "I've done cross-country skiing for 15 years, and I'll ski to a snow course anytime I can justify it economically. I like the aerobic exercise; I like the crisp, cold air on my face; I like the sound of the skis on the snow.Gordon's boss, District Conservationist Jim Alam, encourages the surveyors to ski On the job; he figures that the more skill they have, the better off they'll be if they're caught in a blizzard.Still on our skis, we unpack the kit of snow surveying tools: a set of calibrated tubes that can be fitted together for deep snow, a scale, a thermometer, a pocket calculator, and a pad of survey sheets. Then we go to work, skiing to the first of seven locations in this course, marked by dots on trees. Gordon twists the tube through the snow to the ground. He removes it and checks the core for specks of soil and dead leaves, indicating it went all the way through the pack. We note the depth of the snow to the nearest halfinch. Then we hang the scale from a handy ski pole (another good reason for skiing in) and weigh the tube with the snow inside. Then we repeat these measurements at six more standard locations in the course so we'll have a reliable average.

Why weigh the snow? Because knowing just the depth of the pack is useless in forecasting runoff. The hydrologists back in sunny Phoenix also need to know its density that is, how much water a given cubic foot of snow will become when it melts. The tube and scale are designed to give us that number with a few pecks at the calculator. The ratio of the core's length to its weight is the key.

As the surveyors explain it, that old rule of thumb we learned in science classthat 10 inches of snow actually contain one inch of water is of no value at all. Freshly fallen snow sometimes measures 10 percent density, but that depends on the storm that produced it. Some storms pack heavy, wet snow; some light, powdery snow-the kind that makes

skiers smile. But a snowpack is a dynamic substance. It's constantly changing as it ages. Even if the temperature remains below freezing, some moisture in the snow crystals will "sublimate," shifting from a solid to a vapor state. Thus the air in the pack becomes more humid. At a certain level of humidity, the moisture will recondense on the crystals, transforming them from lacy, delicate sculptures into bulbous globs. The air space is reduced, and the pack becomes progressively denser.

The decrepit pack we're measuring today proves to be 33 to 39 percent water. At the second Mogollon Rim snow course we check, which lies at an elevation of 7,700 feet, the snow depth averages 30.9 inches. It will melt into 11 inches of water, trickling (or raging, depending on the weather) into the Verde River. It takes about three hours to ski to these two sites and take their measurements. It's noon when we finish, and the temperature has risen to a sun-splashed 28°F. The clean, thin air and the quiet isolation of the forest are wonderfully invigorating. It doesn't even seem cold.

"This is what it's like three-fourths of the time," Gordon says. "But there always comes a moment when you have to pay for it. One day three years ago, we were having problems with the snowmobile, so we skied seven or eight miles to the Inner Basin of the San Francisco Mountains. It was about a 3,000-foot elevation gain, and we were looking forward to a nice, fast ski down. Then a blizzard hit, and we had to hole up in a cabin overnight. By morning there was a full three feet of fresh powder, and the wind was howling like crazy. We finally made it out, but we really had to work very hard at it."

Explains Ron Jones, the SCS water supply specialist who directs the Arizona snow surveys: "We've had crews that were hit pretty hard by storms, and I tell them there's a limit to the risks they should take. We need the information, but it's not worth risking someone's life. We haven't yet had a serious accident in the field. The most hazardous part of the job is still the drive to the site."

Snow surveying is hardly new. It began in 1906 on Mount Rose in Nevada. Dr. James Church, a University of Nevada professor of Romance languages, devised the technique of sticking a pipe into the snow, then melting the sample to find out how much water it contained. It wasn't merely an academic exercise. In a spring following a winter of heavy snowfall, Lake Tahoe's level could rise enough to swamp the cabins around the lake. Church's surveys were used to forecast the rise so that gates could be opened to let water run out of the lake.

The U.S. government started the program under the SCS in 1937, at the peak of the "dust bowl" years. The emphasis wasn't on protecting property, but on conserving water. Throughout the western

states, Jones explains, 70 to 75 percent of the stream and river flow comes from melted snow.

In Arizona the combined watersheds of the Verde and Salt rivers sprawl over 13,000 square miles-an area larger than Belgium. Snowmelt from these mountains and plateaus has nourished agriculture near the rivers for 2,000 years, since the Hohokam and later the Sinagua tribes developed irrigation.

In 1867 pioneer prospector and farmer Jack Swilling organized an irrigation company, dredged what he called an "ancient acequia" (a Hohokam canal) from the Salt River near what is now 40th Street in Phoenix, and began cultivating 100 acres of barley, wheat, and corn. Other farms flourished, and Phoenix soon became the state's agricultural hub.

The Salt River, though, was a capricious provider-often its water arrived either in torrents or trickles. In 1899 a scant half-million acre-feet dribbled into town; in 1900 the flow was 10 times that. For Phoenix to realize its potential, the river had to be controlled. That was achieved in 1911, when the United States Reclamation Service completed Roosevelt Dam, 80 miles east of Phoenix. Six years later, the Reclamation Service transferred operation of the dam to the Salt River Valley Water Users' Association. Soon, valley communi-ties began enjoying the coupled benefits of low-cost hydroelectric power and irrigation.

Today the Association and the Salt River Agricultural Improvement and Power District, operating as the Salt River Project, control 1,276 miles of canals and ditches, six reservoirs that double as recreational lakes, and four hydroelectric generators. The project's constituency has changed. Once its water went almost exclusively to farmers; now about 60 percent serves "urban needs"-households, parks, and industry. The project supplies about half the city of Phoenix and a quarter of the metropolitan area. SRP pumps some groundwater-as much as 40 percent of its total in a dry year-but it's still fair to say that the two small, inconsistent rivers it harnesses are the arteries that have given the nation's tenth largest city its life. SRP also supplies electricity to about half a million metropolitan customers.

By helping SRP regulate the reservoirs more accurately, the snow surveyors are in effect buying time for the sprawling oasis. Without it, Phoenix and its suburbs would have been hit with increasingly frequent water shortages and rationingin between floods, probably. But since the city is expanding and the watershed isn't, thirsty times may be about to arrive anyway.

"The years 1978-88 have been an extraordinarily wet period, and this has lulled people into a false sense of security," says John Keane, SRP's manager of surface water resources. "We expected 1989 to be drier [it was-Ed.], and dry years, like wet years, tend to come in groups. In another year or two, you may see bans on washing of cars, or other new conservation measures.

"If we keep growing, the Valley is going to need either more water importation or more conservation."

Up in frostbite country, the surveyors occasionally ponder the ways Phoenix uses the snowmelt that eventually trickles down to the Valley oasis-the grassy golf fairways, the artificial lakes, the broad lawn-and shake their heads. Most of them have a strong conservationist bent, and they're sure many Phoenicians don't appreciate the value and the increasing scarcity of their most critical resource.

The ironic truth is that Phoenix owes its existence to snow. On a warm, sunny, 75° winter day, few people in the Valley ever think of that.