Today one of the community garden people wanted to water the spinach bed because it was wilted. She assumed that the wilting meant that the ground was dry. Fair enough assumption, but a wrong one in this case. The soil under the mulch around the spinach was quite moist. What the gardener was seeing was a wilting effect due to transpiration on a hot, sunny day.
Transpiration is the process where plants move moisture from the ground, up through the plant, then out the leaves. Most of the time the process balances out quite well and everything looks just fine. But on a hot sunny day, sometimes the plant loses moisture out the leaves faster than it can replace it from the soil, even if the soil is wet. Plants with big leaves seem to have the most problem with wilting due to too rapid lose of moisture.
Grabbing a hose and watering the plants doesn't make a difference. They will still be wilted until the sun declines.