Validation of SMAP Soil Moisture at Terrestrial National Ecological
Observatory Network (NEON) Sites Show Potential for Soil Moisture
Retrieval in Forested Areas
Abstract
Soil moisture influences forest health, fire occurrence and extent, and
insect and pathogen impacts, creating a need for regular, globally
extensive soil moisture measurements that can only be achieved by
satellite-based sensors, such as NASA’s Soil Moisture Active Passive
(SMAP). However, SMAP data for forested regions, which account for
~20% of land cover globally, are flagged as unreliable
due to interference from vegetation water content, and forests were
underrepresented in previous validation efforts, preventing an
assessment of measurement accuracy in these biomes. Here we compare over
twelve thousand SMAP soil moisture measurements, representing 88
site-years, to in-situ soil moisture measurements from forty National
Ecological Observatory Network (NEON) sites throughout the US, half of
which are forested. At unforested NEON sites, agreement with SMAP soil
moisture (unbiased RMSD: 0.046 m3
m-3) was similar to previous sparse network
validations (which include inflation of the metric due to spatial
representativeness errors). For the forested sites, SMAP achieved a
reasonable level of accuracy (unbiased RMSD: 0.06
m3 m-3 or 0.053
m3 m-3 after accounting for random
representativeness errors) indicating SMAP is sensitive to changes in
soil moisture in forest ecosystems. Moreover, we identified that both an
index of vegetation water content and canopy height were related to mean
difference, which incorporates measurement bias and representativeness
bias, and suggests a potential approach to improve SMAP algorithm
parameterization for forested regions. In addition, expanding the number
and extent of soil moisture measurements at forested validation sites
would likely further reduce mean difference by minimizing
representativeness errors.