Snowpack, Streamflow, and the Siskin Basin Forecast

Snow collects in the Siskin Basin through the winter, and the town of Starnes drinks that same water the following summer. Connecting the two is the work of the Siskin Valley Water Authority, a fictional utility whose forecasting job is typical of mountain systems.

The basin is a bowl of high country about forty miles above town. Rain falls there in autumn, but most of the year's water arrives as snow. The snow sits through the cold months and melts in the spring. Meltwater soaks into the ground, feeds small creeks, and gathers in the Siskin River, which fills the authority's reservoir. Very little of the water the town uses between June and September falls during those months, because most of it arrived as snow much earlier in the year.

Because of that delay, the authority can know a great deal about its summer supply while there is still ice on the road. What it needs is a good measurement of the snow. Depth alone will not do, since fresh powder and old settled snow can stand at the same height and hold very different amounts of water. So surveyors measure snow water equivalent instead. A technician drives a hollow tube down through the snowpack to the ground, weighs the core, and converts the weight to the depth of water the snow would make if it melted where it stands. A reading of eighteen inches of snow water equivalent means that if the snow at that spot melted in place, it would leave eighteen inches of water.

Crews take those readings at fixed points called snow courses, at the same spots in the same order every month. Automated stations report daily between visits. Neither method covers the whole basin, which is why the authority treats its readings as samples of a much larger surface rather than as a census of the snow.

The first day of April matters more than any other date. By then the basin has usually collected most of the snow it is going to get, and serious melting has not yet begun. The April measurement is therefore the closest thing to a full accounting of the year's deposit. The authority compares that April total against records from earlier years, finds the years with similar readings, and looks at how much water actually ran down the river in each of those years between April and July. That comparison produces the forecast.

The forecast is published as a range. A basin holding a certain amount of snow water might yield thirty thousand acre-feet of runoff in one year and forty thousand in another, because the snow is only half of the equation. What happens to the snow on its way down matters just as much.

Three conditions do most of the damage to a promising snowpack. The first is dry ground. If the previous autumn was dry, the soil under the snow enters spring thirsty, and it will absorb a large share of the meltwater before any of it reaches a creek. The second is a warm, windy spring. Some of the snowpack then turns straight from ice to vapor without ever becoming liquid, a process called sublimation, and that water leaves the basin as humidity rather than as streamflow. The third is dust. Wind carries fine soil from the dry country to the west and lays a gray film across the white surface. Dark snow absorbs more sunlight than clean snow, so a dusty year melts early and fast, which sends water down the river in a rush during weeks when the reservoir may already be full and spilling.

The authority uses the forecast to make three decisions. It sets how much water to hold in the reservoir through the spring rather than release downstream. It picks a start date for deliveries to the irrigation districts below town. And it decides whether to ask households for voluntary conservation, which it prefers to announce early, before lawns are planted.

One recent year showed how the process handles a surprise. The April reading came in slightly above the long-term average, and the authority published a comfortable forecast. May was hot and clear. The snow left the basin about three weeks early, the river peaked at 310 cubic feet per second and then fell off sharply, and the June measurement found bare rock where snow usually lingers. The authority moved its conservation request forward by a month and delayed the irrigation start by two weeks. Both decisions drew complaints when they were announced, though by August the authority's reading of the season had proved correct.

Forecasters describe their product as a statement of odds rather than a prediction. They can measure the snow that has already fallen with reasonable confidence, while the share of it that reaches the river depends on weather that has not happened yet.

1. What does snow water equivalent measure, and why is it more useful to the authority than snow depth?
2. According to the passage, why does the authority treat the first day of April as the most important measurement date?
3. The passage names three conditions that can make a good snowpack produce less streamflow than expected. Choose two, and explain how each one reduces the water that reaches the river.
4. Explain why the authority publishes a range instead of a single number.
5. In the year described near the end of the passage, what two decisions did the authority change after the May weather, and what evidence prompted the changes?
6. Write a paragraph of six to eight sentences explaining to a Starnes resident how the authority turns a winter snow measurement into a summer water plan. Use at least three details from the passage.