Things You'll Need
Trekking poles (optional)
Some lightweight backpackers eschew tents entirely and use tarps as shelter. In a survival situation, as when a day hike turns into an overnight ordeal, a couple of tarps stowed in the backpack can save your life, or at least make your time in the wild more comfortable. Trees make tarp-shelter setup straightforward, but even away from wooded country, you can assemble a perfectly adequate shelter.
With Trekking Poles
Bring along trekking poles. These modern versions of hiking staffs, so effective in reducing muscle and joint strain, also handily serve as shelter supports.
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Secure the poles vertically in the soil, spaced far enough apart to accommodate the fully extended tarp and your body, with room to spare.
Affix the guyline. Stake each end of the line firmly to the ground, then run the line to the top of the pole, and attach it with an adequate knot, such as a half-hitch, or tight wrap. The line should be fully taut along its length from stake to stake, providing a high-tension spine for the A-frame.
Throw the tarp over the guyline, and secure its corners to the ground. This can be done by tying the corners to stakes or to rocks or logs of adequate heft. As noted by Rick Curtis in "The Backpacker's Field Manual," you can also fold the ends of the tarp under your ground-sheet to create a more snug, weather-resistant nest. In this configuration, the corners can be secured underneath the ground-sheet with rocks placed inside the shelter. In balmier conditions, attaching the tarp corners at 45-degree angles, with a few inches of air exposed between ground-sheet and roof, allows for good ventilation.
Without Trekking Poles
Find a natural dip in the terrain. In many of the semiarid or arid landscapes with natural deficiencies of timber, dry gullies or washes are common topographic features. You can use these for shelter if you're fully versed in flash-flood safety. Sometimes a parched arroyo can swell suddenly with dangerously swift and debris-choked currents, courtesy of rain far upcountry.
Erect your shelter against a large boulder, or several, as an alternative. This might be a glacial erratic stranded by long-retreated ice sheets, or part of the cobble loosed from slopes above by weathering and mass wasting.
Secure the tarp, angled down from the lip of the slope or the crest of the boulder, to the lower ground level. You can do this with a guyline spine to attempt an A-frame shelter, or simply tie down the tent corners for a basic roof.