The 1-km illusion in remote sensing for hydrology
Sampling is not actual resolution: the 1 km illusion in satellite hydrology refers to the discrepancy between a dataset's digital sampling grid and its true physical resolution. The "rush" to create high-resolution data has outpaced the ability to validate it on the ground due to missing in situ monitoring networks required to independently validate 1 km algorithms on a global scale. While many modern satellite hydrological products are labeled as "1 km" resolution, this often reflects how the data is stored on a grid rather than what the sensor actually "sees". These products frequently remain "physically blind" to hyper-local anomalies. The fundamental challenge resides in the intrinsic trade-off between spatial resolution and temporal frequency.
With regard to satellite soil moisture products, active radar sensors (e.g. Sentinel-1) provide true 1 km spatial acuity but suffer from temporal gaps, while passive radiometers (e.g. SMAP) offer excellent daily tracking but produce oversampled illusions at the 1 km scale. For practitioners, the selection of a dataset must be dictated by the physical scale of the hydrological event—ranging from farm-scale irrigation to continental-scale drought—rather than the digital label on the file.
In the workshop, a series of real-world stress tests of the "1 km illusion in satellite hydrology" will be presented. These stress tests will demonstrate the hydrological applications of the 1 km illusion, including the mapping of localised summer storms, the estimation of irrigation at the field scale, and the impact of wildfires on water infiltration.
