Measurement of Fluxes Over Land: Capabilities, Origins, and Remaining Challenges

In the first half of the twentieth century the measurement of rates of exchange of momentum, heat, and especially water between the terrestrial surface and the atmosphere was an elusive challenge. A wide variety of methods evolved, ranging from reliance on drag plates, evaporation pans, and weighed lysimeters to a variety of methods based on measurement of vertical gradients in the air. None of these provided great confidence in the results. The promise of direct measurement by eddy covariance was held at bay by the lack of suitable fast-response instrumentation and the analytical requirement to average the products of pairs of these fast-response signals. Developments starting in the 1950s have now largely solved the problems that previously limited eddy covariance. Here, the history of flux measurement is summarized, and the path to now-standard capabilities is explored. The currently active major international programs making use of the modern instrumentation are addressed (with the focus on CO2 and CH4, as befitting global concerns regarding climate change), with particular attention to the emerging need to revisit the fetch and footprint constraints that confronted early workers. Finally, some areas of continuing uncertainty are mentioned, with an implicit request for increased experimental attention.