Temporal and spatial scales of observed soil moisture variations in the extratropics

Scales of soil moisture variations are important for understanding patterns of climate change, for developing and evaluating land surface models, for designing surface soil moisture observation networks, and for determining the appropriate resolution for satellite-based remote sensing instruments for soil moisture. Here we take advantage of a new archive of in situ soil moisture observations from Illinois and Iowa in the United States, and from Russia, Mongolia, and China, to evaluate the observed temporal and spatial scales of soil moisture variations. We separate the variance into two components, the very small scale, determined by soils, topography, vegetation, and root structure, and the large scale forced by the atmosphere. This larger scale, determined by precipitation and evaporation patterns, is of interest for global climate modeling. We characterize the small scale as white noise for our analysis, keeping in mind that it is an important component of soil moisture variations for other problems. We find that the atmospheric spatial scale for all regions is about 500 km. The atmospheric temporal scale is about 2 months for the top l-m soil layer. The temporal scale for the top 10-cm layer is slightly less than 2 months. The white noise component of the variance for temporal variations ranges from 50% for the top 10 cm to 20-40% for the top 1 m. For spatial variations the white noise component is the same for all depths but varies with region from 30% for Illinois to around 70% for Mongolia. Nevertheless, the red noise (atmospheric component) can be seen in all regions. These results are for Northern Hemisphere midlatitudes and would not necessarily apply to other latitudes. The results are based on observations taken from grassland or agricultural areas, and may not be similar to those of areas with other vegetation types. In China, a region with substantial latitudinal variation, the temporal scale for the top 1 m varies from 1 month in the south to 2.5 months in the north, demonstrating the control of potential evaporation on the temporal scales. Seasonal analysis of the scales of soil moisture for Illinois shows that during the winter the temporal scales are long, though the spatial scales are short. We suggest that these variations are both attributable to the seasonal cycle of potential evaporation.