Soil Moisture Correlation with Small Water Body Extents in Canadian Wetlands: Application to SMAP Soil Moisture Improvement

Wetlands are among the most hydrologically dynamic ecosystems, particularly across northern boreal and subarctic regions of Canada, where seasonal freeze-thaw cycles and precipitation variability drive pronounced fluctuations in surface water and soil moisture. These landscapes play a critical role in carbon storage, ecosystem functioning, and regional hydrology, yet their remoteness severely limits the availability of long-term in situ soil moisture observations. Consequently, satellite-based microwave remote sensing has become an essential tool for monitoring wetland hydrological dynamics at large spatial scales.

NASA’s Soil Moisture Active Passive (SMAP) mission provides global soil moisture estimates at a coarse spatial resolution (~36 km). However, retrieval performance declines significantly in wetland-dominated regions due to the mixed influence of open water, saturated soils, and vegetation within a single footprint. This mixture complicates the interpretation of passive microwave brightness temperatures and increases uncertainties in soil moisture products. Improving SMAP performance in these environments requires a better understanding of how water dynamics influence the satellite signal.

In northern Canadian wetlands, small surface water bodies such as shallow ponds, ephemeral pools, and saturated depressions exhibit substantial seasonal variability, especially during snowmelt and early summer. Although individually below SMAP’s resolution, their aggregated extent may substantially affect observed brightness temperatures and mimic soil moisture variability. This study investigates whether temporal changes in small surface water extent can serve as a proxy for soil moisture variations within SMAP footprints.

We analyzed the relationship between in situ soil moisture, SMAP brightness temperatures, and surface water extent across two wetland regions: the Attawapiskat River (CA-ARB and C-ARF) and the Kinosheo Lakes (CA-KLP). Soil moisture data from eddy covariance flux towers (2017-2021) were used for snow- and ice-free periods (June-October). Small surface water bodies were mapped using Sentinel-1 SAR imagery and the Canadian Digital Elevation Model (CDEM) data through a random forest classification approach, then aggregated to the SMAP footprint scale for analysis.

Results show strong correlations between in situ soil moisture and surface water extent (r > 0.58), as well as between surface water extent and SMAP brightness temperatures (r > 0.77). These findings indicate that surface water dynamics capture spatially representative hydrological variability within SMAP pixels and help address scale-mismatch issues between point in situ measurements and coarse satellite products. The study demonstrates the potential of surface water extent as a proxy variable to support calibration, validation, and future improvement of SMAP soil moisture retrievals in wetland-dominated regions.