Appraisal of SMAP Operational Soil Moisture Product from a Global Perspective

Abstract

Space-borne soil moisture (SM) satellite products such as those available from Soil Moisture Active Passive (SMAP) offer unique opportunities for global and frequent monitoring of SM and also to understand its spatiotemporal variability. The present study investigates the performance of the SMAP L4 SM product at selected experimental sites across four continents, namely North America, Europe, Asia and Australia. This product provides global scale SM estimates at 9 km × 9 km spatial resolution at daily intervals. For the product evaluation, co-orbital in situ SM measurements were used, acquired at 14 test sites in North America, Europe, and Australia belonging to the International Soil Moisture Network (ISMN) and local networks in India. The satellite SM estimates of up to 0–5 cm soil layer were compared against collocated ground measurements using a series of statistical scores. Overall, the best performance of the SMAP product was found in North America (RMSE = 0.05 m3/m3) followed by Australia (RMSE = 0.08 m3/m3), Asia (RMSE = 0.09 m3/m3) and Europe (RMSE = 0.14 m3/m3). Our findings provide important insights into the spatiotemporal variability of the specific operational SM product in different ecosystems and environments. This study also furnishes an independent verification of this global product, which is of international interest given its suitability for a wide range of practical and research applications.

An Examination of the SMAP Operational Soil Moisture Products Accuracy at the Tibetan Plateau

“…A possible cause for the positive bias reported by this study's results could be uncertainties resulting from ancillary data parameterizations [7,59]. In the SCA-V, brightness temperature (TB) is converted to emissivity by correcting for the effective soil temperature effect, vegetation, and surface roughness.…”

“…A mismatch between the satellite sensor and the measuring depth of the in situ instrument and the discrepancy between the in situ SSM measuring depth and the satellite depth of microwave penetration could explain, in part, the bias displayed in some of the grids. The sensing depth of the L-band is shallow compared with the in situ instruments [7,54]. The ground-based measurements used to validate the products are placed at a depth of approximately 5 cm and usually observe SSM at a depth of 3 to 7 cm.…”

“…The horizontal bar denotes the mean value. Notably, these statistical parameters have been used in similar verification studies implemented in SSM operational products [7,27,48,50].…”

“…It has been recognized as playing a substantial role in seasonal climate and in evaluating impacts of global climate change [4], forecasting extreme events, such as drought, frost, or floods [5]. Furthermore, it has been utilized as a key variable in various models, such as hydrological [6,7], land surface evaporation [8,9], and surface runoff [10].…”

Evaluation of Several Satellite-Based Soil Moisture Products in the Continental US

“…Due to its continuity, SMAP has attracted more attention and is more widely used. Experiments based on several sites have shown that SMAP performed well in North America [14]. SMAP can also be used to compare the performance of different observation networks, but the temporal variation of soil moisture cannot be detected [15].…”