Building a Desert Observatory: Where Are the Water, Gravel and Heat?

A Club Night at Coyote Flat

1On clear nights, Lena Marchetti's astronomy club drives out to Coyote Flat, a desert basin far from city lights and about 300 km from the ocean. A university plans to build a new observatory there. On one club night, engineer Tobias Quarshie explained what the builders need from the ground: fresh water for the staff, coarse gravel for the concrete pier that holds the telescope steady, and a way to heat the buildings on cold desert nights.

2Coyote Flat lies between two mountain ranges. The mountains get about 60 cm of rain and snow a year, but the basin floor gets only about 15 cm. Streams rush out of the mountain canyons. Where they reach flat ground, they slow down and drop their load in wide, fan-shaped piles. The biggest stones drop first, near the mountains. Sand travels farther, and the finest silt and clay are carried all the way to the low center of the basin. There, water collects after storms in a shallow lake that dries up each summer and leaves a white crust that tastes salty.

3The builders drilled four test wells in a line from the eastern mountains toward the dry lake (Table 1). They measured how fast water flowed into each well and how much salt was dissolved in it. Water with more than 500 mg of salt per liter tastes salty and is not used for drinking. The observatory needs at least 100 L of fresh water per minute.

4Along the foot of the eastern range runs a large fault, a long break in Earth's crust where huge blocks of rock have slipped past each other. Warm springs bubble up at several places along it. The builders drilled three test holes, each 100 m deep, and measured the temperature at the bottom (Table 2). Mr. Quarshie explained that rock gets hotter with depth because Earth's interior is hot, and that groundwater can sink deep, warm up and then rise quickly through broken rock.