Groundwater storage

Benefits

• NDVI maybe used as a suitable indicator of groundwater storage conditions in certain areas where the water table is shallow and the vegetation is natural and where in situ groundwater observations are not available.

• This presents a real benefit as NDVI can be measured at relatively high resolution using multiple types of remote sensing observations, while groundwater monitoring via satellite gravimetry (e.g., GRACE) provides only coarse resolution.

Limitations

Correlation between observed groundwater levels and NDVI is not good in agricultural regions, suggesting human influences on these parameters.

Applicability for Northland

Yes, likely applicable.

A method based on Normalized Difference Vegetation Index (NDVI), as an indicator of groundwater storage, could be investigated for natural vegetation covers. Correlations would need to be confirmed in the Northland context and finer spatial resolution imagery (e.g. Sentinel-2) would be more relevant to use. GRACE-derived information would be too coarse for local applications.

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.