Global daily 9 km remotely sensed soil moisture (2015–2025) with microwave radiative transfer-guided learning

Feng, S., Li, A., Zhou, R., Butterbach-Bahl K., Guan, K., Jin, Z., Looms., M.C., Wang, S., Igel, C., Olesen, J.E., Wang, S. Improved global SMAP daily 9 km surface soil moisture dataset with microwave radiative transfer process-guided machine learning (2015 - 2025). Scientific data (under review).

Abstract

Accurate estimation of surface soil moisture (SM) in terrestrial ecosystems is essential for understanding hydroclimate dynamics. The L-band Soil Moisture Active Passive (SMAP) mission has provided 9-km global daily surface SM since April 2015, using a microwave radiative transfer model (RTM)-based algorithm. However, the performance of this algorithm is often limited, especially in regions lacking dense in-situ SM measurements.  Based on the microwave RTM, a process-guided machine learning (PGML) was developed to estimate global surface SM at SMAP descending overpass time (6 AM, local time). The validation results show that PGML SM has strong agreement with global in-situ SM measurements, with R=0.884 and unbiased RMSE=0.052 m3/m3.

This dataset contains 11 years (April 2015 - April 2025) global spatio-temporal consistent surface SM with 9-km spatial resolution at a daily scale. The projection of the PGML_SM is EASE-Grid2.0, and the SM unit is m3 / m3.

Data Format and Usage Note

This dataset is stored in Network Common Data Form (NetCDF) format with one file per day, defined by two dimensions (latitude and longitude) and a variable surface SM.
The file name is designed as “PGML_SM_yyyymmdd_Vx.nc”, where “yyyy” stands for year, “mm” stands for month, “dd” stands for day, and ‘Vx’ stands for version ID. For example, “PGML_SM_20170101_V1.nc” contains the global surface SM distribution on the first day of January 2017 at version 1.

Naming convention: PGML_SM_yyyymmdd_Vx.nc
Projection: 9-km ESAE-Grid 2.0 (EPSG: 6933)
Variable: 
lat_center: latitude
lon_center: longitude
pgml_sm: surface soil moisture, valid range is from 0 to 1 m3 / m3
pgml_sm_std: standard deviation of surface soil moisture
qc: quality control flag (8-bit flag)
    00000001: Polluted by water, urban or ice/snow.
    00000010: Frozen surface (surface temperature<0℃)
    00000100: Dense vegetation (vegetation water content>5kg/m2)
    00001000: soil moisture exceeds the soil water capacity