Forecasting and Managing Groundwater Resources Using InSAR

Groundwater management is highly dependent on the type and quality of field data available describing a given aquifer system. Our increasing reliance on groundwater, especially as traditional surface supplies continue to be overexploited due to rising population and standard of living, requires that we better understand the state of our subsurface supplies and how to best manage them. The dense spatial and temporal variability of subsidence provided by time series InSAR allows us to constrain the extent of an aquifer, its storage coefficient, estimates of hydraulic head, and hydraulic conductivity. We present examples of these parameters associated with groundwater systems in the San Luis Valley, CO, and the Central Valley area of California, as observed by several spaceborne radar systems and validated by comparison with field data.

Groundwater is one component of a water system, which includes surface supplies and all of the various sources and end uses of water in a particular area. Confined aquifers remain the most difficult components of a full water system to characterize and properly manage, as they lie deep underground and are hidden from direct observation. We show that observing subtle deformations of the surface elevations on the order of mm to cm yield important constraints on the underlying aquifer and its hydraulic properties, because variations in the surface height expresses changes in water pressure below.

The fundamental relation between pressure and stress resulting in changes in hydraulic head yields a simple linear relationship between deformation Δb, hydraulic head Δh, and skeletal storage coefficient: S_k = Δb / Δh, so that measuring deformation everywhere above an aquifer over time yields change in head. Using InSAR-observed temporal response of the head (deformation) to changes in forcing by water sources and sinks, and applying the one dimensional diffusion equation resulting from Darcy’s Law and the continuity relation allows us to estimate conductivity in the reservoir. This is important in order the supplement the usually sparse geologic knowledge describing the shape of the various layers in the aquifer system and their compressibility. These tools point toward an approach for forecasting future head levels.