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 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in situ</i> 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 m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup> ) 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 m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup> or 0.053 m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup> 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 (MD), 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 MD by minimizing representativeness errors.
Highlights
S ATELLITE calibration and validation efforts have been well documented in recent years with the validation of the AMSR-E [1], SMOS [2], and soil moisture active passive (SMAP) missions [3]
Since the NDII and canopy height data for the entire National Ecological Observatory Network (NEON) site covered an area (191 ±73 km2) that was smaller than the SMAP measurement footprint (1089 km2) we used Landsat NDII data as they were available at the SMAP footprint scale
Landsat NDII for 1 and 33 km diameter areas surrounding the NEON sensors were strongly positively correlated [p < 0.001, r2 = 0.89; Fig. 2(c)]. In addition to these correlations, the dominant landcover within the SMAP footprint at each NEON site accounted for a large proportion of the total footprint, which is indicative of a relatively homogeneous area
Summary
S ATELLITE calibration and validation efforts have been well documented in recent years with the validation of the AMSR-E [1], SMOS [2], and SMAP missions [3] This has led to the development of protocols [4] and practices [5] for satellite validation of soil moisture from a variety of sources. The National Ecological Observatory Network (NEON) was recently established by the National Science Foundation to monitor drivers of, and responses to, change in US ecosystems over decadal timescales. It is a distributed network consisting of 81 aquatic (34) and terrestrial (47) sites throughout the US, including Alaska, Hawaii, and Puerto Rico, where co-located measurements are made using standardized protocols and sensors with all data made freely available (see Fig. 1). In addition to providing data, NEON archives approximately one hundred thousand physical samples annually which are available for community use, provides physical infrastructure to support external research projects, and generates outreach material to enhance learning and diversity in ecology and related disciplines
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