From AMS Glossary
Small drainage areas (up to about 25 km2) in which periodic measurements are conducted on a long-term basis.
Observations are made on both geomorphological and hydrological characteristics, including channel changes, valley-floor features, hillslopes, reservoirs, precipitation, runoff, and vegetation. The purpose is to document changes in the landscape and its hydrology over time, especially over a period of decades, and to make the data available to present and future generations of scientists. A formal Vigil Network was established on an international basis in the 1960s by U.S. Geological Survey scientists L. B. Leopold, W. W. Emmett, and R. F. Hadley.