09/29/2016

Soil Moisture Data: When Is There Enough?

Scientists examine long-term measurements of soil moisture, including data from two ARM sites, to determine the observational record length needed for robust statistics.

The Science

Soil moisture modifies energy and moisture fluxes into the boundary layer, thereby influencing near-surface air temperature, humidity, and boundary layer instability, and, in some cases, determining if, where, or when precipitation occurs. Understanding land-atmosphere interactions driven by soil moisture anomalies is crucial for subseasonal-to-seasonal climate prediction as well as forecasting of extreme climatic events. A recent study looked into how long of a soil moisture record is needed for robust statistics.

The Impact

Existing soil moisture datasets do not have consistent record lengths; therefore, the ability to use these databases for large-scale model validation or investigation of land-atmosphere interaction processes across a range of land types is contingent on properly standardizing soil moisture observations from a variety of in situ sources. This study uses data from 15 long-term measurement sites, including two sites operated by the Department of Energy’s Atmospheric Radiation Measurement (ARM) Climate Research Facility, to determine what observational record length is sufficient to produce a stable soil moisture distribution. The authors find that between 3 to15 years of data are required to produce stable distributions, with the majority of stations requiring only 3 to 6 years of data. However, more years of data are required to obtain stable estimates of the distribution extremes (5th and 95th percentiles). These results have important implications for the design of soil moisture observational networks and model evaluation studies.

Summary

The ability to use in situ soil moisture for large-scale soil moisture monitoring, model and satellite validation, and climate investigations is contingent on properly standardizing soil moisture observations. Percentiles are a useful method for homogenizing in situ soil moisture. However, few stations have been continuously monitoring in situ soil moisture for 20 years or longer. Therefore, one challenge in evaluating soil moisture is determining whether the period of record is sufficient to produce a stable distribution from which to generate percentiles. In this study, daily in situ soil moisture observations, measured at three separate depths in the soil column at 15 stations in the United States and Canada, are used to determine the record length that is necessary to generate a stable soil moisture distribution. The Anderson-Darling test is implemented, both with and without a Bonferroni adjustment, to quantify the necessary record length. The team evaluates how the necessary record length varies by location, measurement depth, and month. They find that between 3 and 15 years of data are required to produce stable distributions, with the majority of stations requiring only 3 to 6 years of data. Not surprisingly, more years of data are required to obtain stable estimates of the 5th and 95th percentiles than the first, second, and third quartiles of the soil moisture distribution. Similarly, the required number of years increased with depth, with more years necessary for observations taken between 50 and 60 cm than those taken between 20 and 30 cm and 5 and 10 cm depths. Overall, the results suggest that 6 years of continuous, daily in situ soil moisture data are sufficient in most conditions to create stable percentiles. These results may not apply to locations with climatic or edaphic conditions that differ from those used in this study.

Principal Investigator(s)

Trent Ford
Southern Illinois University
twford@siu.edu

Funding

This work used data from the Oklahoma Mesonet network, which is jointly operated by Oklahoma State University and University of Oklahoma. Additional data was provided by the U.S. Department of Energy’s Atmospheric Radiation Measurement (ARM) Climate Research Facility (Pawhuska and Lamont, Oklahoma sites). Finally, this work used soil moisture data acquired by the FLUXNET community and, in particular, by Fluxnet-Canada (supported by the Canadian Foundation for Atmospheric Sciences, Natural Sciences and Engineering Council, BIOCAP, Environment Canada, and Natural Resources Canada).

References

Ford, T. W., Q. Wang, and S. M. Quiring. 2016. “The Observation Record Length Necessary to Generate Robust Soil Moisture Percentiles,” Journal of Applied Meteorology and Climatology 55(10), 2131-49. DOI: 10.1175/JAMC-D-16-0143.1.