Saturated / unsaturated zones
Satellite(s)MODIS, SRTM, SAR, GRACE. | Monitoring elementLand reflectance, land gravity. |
|---|---|
Description techniqueWesterhoff et al (2018) derived a nationwide water table (NWT) using multiple New Zealand wide and satellite-derived datasets by applying the Equilibrium Water Table (EWT) model, described in Fan et al (2013). Outputs consist of water table depth (m below ground level) and elevation (m asl) at a resolution of 200m. Hydraulic head has been estimated via a D-InSAR technique, with the difference in measured surface elevation between successive observations was calibrated to measure water head at across a number of wells, resulting in a model that can predict hydraulic head (Reeves et al, 2014). Researchers have applied the Gravity Recovery and Climate Experiment (GRACE), instrument to detect changes in the water table depth (Swenson et al, 2006). GRACE was deployed to monitor variations in the Earth's gravitational field, and can resolve monthly changes in terrestrial water storage. | Accuracy / ResolutionVariable spatial and temporal resolution according to sensors. |
Case studyThe nationwide water table model developed by Westerhoff et al (2018) might be used as a first approximation of the water table in areas where this is unknown. Also, components of the water table can be used as starting points / initial estimates for more local groundwater or hydrogeological models. Comparison with two regions where ground water level is available shows very good correlation. Reeves et al (2014) utilized D-InSAR processing of ERS-1 and ERS-2 data to predict hydraulic head in Colorado. 59% of the predicted values matched the measured values within the uncertainty range of 1cm. Swenson et al (2006) compared ground station measurements of soil moisture anomaly and water table depth to data derived from GRACE across a 280,000km2 area in Illinois. The results suggested good correlation once the GRACE data had been properly filtered. | |
Also fits domainFreshwater. | |
Benefits
| Limitations
|
Applicability for NorthlandWesterhoff et al. (2018) provide an estimate of the water table depth for the Northland region. Local calibration, including the recently captured regional LiDAR as input elevation data, should improve the accuracy of this model. This model has also been used as input by Rissmann et al. (2019) in combination with radiometric data to perform wetland delineation. Techniques applying optical data will be limited in coverage and temporal granularity by the persistent cloud cover in the region, particularly during the winter months. Mature cloud-masking techniques are directly available for open access multispectral data (e.g. Landsat and Sentinel-2). When using commercial data, care must be taken to ensure that there is sufficiently cloud free imagery available, as cloud masking is not as mature and ordering a large volume of imagery to ensure complete cloud free coverage between multiple observations can become cost prohibitive. | |
Publication references Westerhoff, R., White, P. and Miguez-Macho, G., 2018. Application of an improved global-scale groundwater model for water table estimation across New Zealand. Hydrology and Earth System Sciences, 22(12), pp.6449-6472. Fan, Y., Li, H. and Miguez-Macho, G., 2013. Global patterns of groundwater table depth: Supplementary material. Science, 339(6122), pp.940-943. Reeves, J.A., Knight, R., Zebker, H.A., Kitanidis, P.K. and Schreüder, W.A., 2014. Estimating temporal changes in hydraulic head using InSAR data in the San Luis Valley, Colorado. Water Resources Research, 50(5), pp.4459-4473. Swenson, S., Yeh, P.J.F., Wahr, J. and Famiglietti, J., 2006. A comparison of terrestrial water storage variations from GRACE with in situ measurements from Illinois. Geophysical Research Letters, 33(16). https://escholarship.org/content/qt4dh9t2vs/qt4dh9t2vs_noSplash_fdfcd98e98fd8b75a3fbaa92e2725d87.pdf
| |