The overall size and timing of monsoon floods in Bangladesh are
challenging to measure. The inundated area is extensive in lowlying
Bangladesh, and observations of water storage are key to
understanding floods. Laser-ranging instruments on Gravity
Recovery and Climate Experiment (GRACE) Follow-On spacecraft
detected the peak water storage anomaly of 75 gigatons across
Bangladesh in late July 2020. This is in addition to, and three times
larger than, the maximum storage anomaly in soil layers during
the same period. A flood propagation model suggested that the
water mass, as shown in satellite observations, is largely influenced
by slow floodplain and groundwater flow processes. Independent
global positioning system measurements confirmed the
timing and total volume of the flood water estimates. According
to land surface models, the soils were saturated a month earlier
than the timing of the peak floodplain storage observed by GRACE
Follow-On. The cyclone Amphan replenished soils with rainfall just
before the monsoon rains started, and consequently, excessive runoff
was produced and led to the early onset of the 2020 flooding.
This study demonstrated how antecedent soil moisture conditions
can influence the magnitude and duration of flooding. Continuous
monitoring of storage change fromGRACE Follow-On gravity measurements
provides important information complementary to river
gauges and well levels for enhancing hydrologic flood forecasting
models and assisting surface water management.