How the map is made, and what it cannot see
Each nearby CGWB well keeps its own year-by-year record. Its depth is the median of its last three readings, carried forward with its own long-term trend. The old method mixed readings from 2000 with readings from 2024 at the same well. Fixing that is the main gain.
The map is drawn on 100 m cells, but the information in it is set by the wells, which are a median 4.7 km apart. We tested whether terrain (Copernicus 30 m) and long-lived surface water (Landsat 1984–2021) add detail between wells. Across 447 hidden wells they did not, even in hilly country, so the map stays a smooth surface between wells and does not invent field-to-field differences.
CGWB monitoring wells mostly measure the shallow, open aquifer. A borewell in hard rock often draws from deeper fractures that no satellite can see, and where the water table is below 15 m every method here misses by about 8 m. For the exact spot and depth, a resistivity survey and the drilling record of neighbouring borewells are what improve on this.
GRACE and GRACE-FO weigh the water stored under a region about 300 km across. They cannot place water inside a farm, but they show whether the region is gaining or losing water up to a few months ago, after the last well reading. The suggested drilling depth plans for the faster of the two declines.
Hard-rock areas store water in fractures that no satellite can see. In those areas, a local water-divining survey or a resistivity survey before drilling is still worth the cost.
Wells: Central Ground Water Board seasonal water-level reports. Terrain: Copernicus DEM GLO-30. Surface water: EC JRC / Google Global Surface Water. Gravity: NASA JPL GRACE/GRACE-FO mascons. Imagery: Esri, Maxar, Earthstar Geographics.