Abstract. Spatiotemporally continuous soil moisture (SM) data are
increasingly in demand for ecological and hydrological research. Satellite
remote sensing has potential for mapping SM, but the continuity of
satellite-derived SM is hampered by data gaps resulting from inadequate satellite coverage, snow cover, frozen soil, radio-frequency interference, and so on. Therefore, we propose a new gap-filling approach to reconstruct
daily SM time series using the European Space Agency Climate Change Initiative (ESA CCI). The developed approach integrates satellite observations,
model-driven knowledge, and a machine learning algorithm that leverages both
spatial and temporal domains. Taking SM in China as an example, the
reconstructed SM showed high accuracy when validated against multiple sets
of in situ measurements, with a root mean square error (RMSE) and a mean absolute error (MAE) of 0.09–0.14 and
0.07–0.13 cm3 cm−3,
respectively. Further evaluation with a 10-fold cross-validation revealed median values of the coefficient of determination (R2), RMSE, and MAE
of 0.56, 0.025, and 0.019 cm3 cm−3, respectively.
The reconstructive performance was noticeably reduced both when excluding
one explanatory variable and keeping the other variables unchanged and when removing the spatiotemporal domain strategy or the residual calibration
procedure. In comparison with gap-filled SM data based on a
satellite-derived diurnal temperature range (DTR), the gap-filled SM data
from bias-corrected model-derived DTRs exhibited relatively lower accuracy
but higher spatial coverage. Application of our gap-filling approach to
long-term SM datasets (2005–2015) produced a promising result (R2=0.72). A more accurate trend was achieved relative to that of the original
CCI SM when assessed with in situ measurements (i.e., 0.49 versus 0.28,
respectively, in terms of R2). Our findings indicate the feasibility of
integrating satellite observations, model-driven knowledge, and
spatiotemporal machine learning to fill gaps in short- and long-term SM time
series, thereby providing a potential avenue for applications to similar
studies.