Sidebar by Ed Quillen
San Luis Valley Water – October 1998 – Colorado Central Magazine
A Quick Estimate of a Water Budget for the Closed Basin
The surface of the Closed Basin — 2,940 square miles — gets about 7 inches of precipitation a year. The flanking mountains — Sangres, La Garita Hills, San Juans — are much wetter, getting 15 to 45 inches, mostly as snow.
Now we again indulge in gross oversimplification, and simple arithmetic produces 1,097,000 annual acre-feet of water that lands directly on the Closed Basin.
As for the rest, we’ll start by assuming the Closed Basin is a square, each side about 54 miles long to produce its 2,940 square miles.
On its north, east, and west sides are catchment zones — mountains — whose streams flow into the Closed Basin. We’ll assume each of these three zones is 10 miles wide and 54 miles long, which means 540 square miles in each, or 1,620 square miles for all three, and that’s 1,036,800 acres.
Annual precipitation ranges from 15 to 45 inches on those mountains, so we’ll average that out at 30 inches, or 2½ feet, for 2,592,000 acre-feet a year that might flow into the Closed Basin from the mountains that flank it.
Add that to the 1,097,600 that falls directly on the Closed Basin’s surface each year, and we get 3,689,600 acre-feet of possible Closed Basin recharge water each year.
But that’s just what’s theoretically possible. The forests and meadows in the mountains consume a lot of that water before it even starts to flow down into the Closed Basin, and plenty of it evaporates, too.
Just how much water this takes — the process is called evapotranspiration — depends on wind, temperature, plant species, and similar factors. A reasonable statewide estimate, according to the booklet Colorado Water published by the League of Women Voters, is about 75%.
Apply that to the Closed Basin’s theoretical inflow, and we’re left with 922,400 acre-feet each year that either falls out of the sky directly onto the basin, or flows into the basin from the mountains around it.
Keep in mind that this is just a rough estimate of the “income” side of a “water budget.” Nobody knows for sure, and this is a number that is very much in dispute.
But this amount of annual recharge, approximately 1 million acre feet, is about the same as other estimates that are more informed and scientific.