The Global Soil Moisture Data Bank - Benchmark Soil Moisture Observations

Soil moisture is a crucial component of the global hydrological cycle. Summer desiccation is a potential threat from some climate model simulations of global warming, but actual in situ soil moisture observations are needed to examine long term soil moisture changes as well as to develop accurate land surface models. Here we present a set of benchmark in situ soil moisture observations, from long-term observing programs in the United States, Europe, and Asia. The longest time series started in 1958, the same year as the Keeling Mauna Loa CO2 record, and is based on gravimetric observations from 141 stations at agricultural fields with winter and spring cereal crops in the Ukraine with a temporal resolution of 10 days (3 measurements per month) during the growing period, from April 8 to October 28. We discuss methods of observation, of quality control, and of achieving homogeneity. Shorter records from other locations will also be presented. These benchmark soil moisture observations are being used by the international research community to study climate change, as ground truth for remote sensing, to develop and evaluate land surface models, and to evaluate general circulation model and reanalysis calculations of soil moisture variations. While land data assimilation of remotely-sensed forcing and soil moisture, utilizing a validated land surface model, will be necessary to produce a global soil moisture data set, this goal cannot be achieved without these actual in situ observations.