Groundwater is an essential source of water supply in many countries around the world, especially for irrigation but also for industrial and domestic use. Monitoring of groundwater from space is challenging. The use of Earth Observation in support for Groundwater mapping and exploitation therefore most often relies on proxy indicators on groundwater resource potential and vulnerability.
Yet, one satellite mission provides invaluable measurements of groundwater, although on relatively coarse spatial scale: NASA’s GRACE mission. It provides measurements of the changes in the Earth’s gravitational field at basin scale which can often be attributed to changes in groundwater resources. In most cases, however, the usage Earth Observation in support for Groundwater mapping and exploitation uses proxy indicators on groundwater resource potential and vulnerability. The depletion of groundwater resources when leading to ground subsidence can be measured by processing synthetic aperture radar satellites data using the InSAR (Interferometry of Synthetic Aperture Radar) techniques.
Ground water availability maps can provide a starting point for discussions about (a) ground water development potential; (b) broad areas of vulnerability In terms of the likely availability or non-availability of ground water during drought; and (c) the fit between water security-insecurity and food security-insecurity zones. GRACE can measure seasonal changes in groundwater storage, albeit at regional levels only, while SAR based interferometry techniques can be used to measure ground subsidence as consequence of receding ground water levels, and with possible impacts in terms of damages or even collapses of buildings.
As already indicated this field is mainly hampered by the inability to make direct observation of the groundwater storage. Although, the GRACE mission does provide estimates of total water storage changes (including groundwater) it is only within large footprints of 300-kilometer, and hence only for regional applications. Similar, and although InSAR can potentially derive very accurate terrain deformation maps (mm scale) then it will not work in heavily vegetated areas and very steep terrain.
For the immediate future EO for groundwater mapping and exploitation will continue to rely mainly on proxy indicators, but there are a few notable future enhancements which could bring the field forward. GRACE-II will extend and improve on the first GRACE mission, by providing observations in an increased resolution of around 100 kilometers, and with the potential to support finer scale applications. Further, the denser time series of Sentinel-1A and B provide compatible free of charge C-band acquisitions every 6 days that foster operational use of InSAR (and SAR) in ways not possible with previous missions such as Envisat and ERS-2 (cf. 35-day repeat-cycle).