Two Hollows Above the Switchback

Field Report: The Switchback Hollows

Above a bend in a mountain trail, ranger Inez studies two small hollows. A hollow is a low place in the ground. Grass Hollow has thick clumps of native mountain grass. Gravel Hollow has mostly bare soil and small stones. The hollows are beside each other, with similar slopes and the same kind of soil. Neither hollow is part of the walking path. Inez wants to compare what happens when rain reaches these two places.

First, Inez marks an equal-sized study area in each hollow. She compares their different plant cover while keeping the study areas, collection equipment, and observation times the same. Matching rain collectors record 8 millimeters during the first rainstorm and 12 millimeters during the second. For each storm, both collectors show the same amount. At the downhill edge of each study area, equipment catches runoff, which is water flowing over the ground. Inez filters soil from the runoff, lets that soil air-dry, and weighs it. The graph shows these measurements.

After each storm, Inez notices narrow grooves in the bare soil at Gravel Hollow. At Grass Hollow, she sees soil grains caught among grass stems. Along one exposed edge, many grass roots form a branching network through the soil. Water runs downhill through both hollows. Small stones split some streams of runoff into different paths. On a later dry, windy afternoon, Inez sees dust lift from a bare patch and travel beyond Gravel Hollow. Grass leaves bend in the same wind, but she sees no dust rise from beneath the thick grass clumps.

A useful model can show more than objects. It can use labeled arrows to show what moves or what changes. In these hollows, rain falls from clouds in the air onto the ground. Roots can take up liquid water from the soil. Water can evaporate from damp ground, and plants can release water vapor from their leaves. Water vapor is water in its gas form. Inez uses land for rocks and soil, water for liquid water and water vapor, air for the surrounding atmosphere, and living things for organisms such as grass.