|dc.description.abstract||Groundwater, the water stored in the subsurface, plays an essential role in water provision for domestic, industrial and agricultural use. Groundwater is also vital for ecology and environment, since it provides base flow to many streams, rivers and wetlands. Groundwater demand in New Zealand has grown strongly in recent years and assessment of groundwater volumes, fluxes and flows requires more advanced methods. Exploration of groundwater, the least-known water resource, often requires the combination of multiple research disciplines, such as well-drilling, geophysical techniques and groundwater models. These combined products often are uncertain in areas where ground-observed data are sparse. This uncertainty limits our future scientifc capability to better assess our complex groundwater resources.
Satellite geophysical data can help tackle the problem of data sparsity in groundwater research because they commonly cover several regions or a whole nation in one image. On a global scale, in recent years, use of satellite-derived data in hydrogeology has gained considerable attention. Although data access is relatively easy and data are mostly free, similar in-depth hydrogeological studies have not been performed for New Zealand. This is mainly because satellite data can contain much noise, as the signal has to travel far before it reaches the Earth, and because data are often too coarse for the New Zealand aquifer scale. Therefore, satellite data for hydrogeological research in New Zealand are not proven technology and they are generally not used by New Zealand scientists, consultancies or water managers. A break-through is needed.
This thesis researches the added value of satellite data to better estimate groundwater volumes and flows, including their uncertainties, in New Zealand. The thesis uses the concept of `efficient combination', where satellite data are combined with existing information from ground observations or models, so that they can fill in gaps and trends in data-sparse areas. The satellite data used are those on evapotranspiration, vegetation properties, and soil moisture (the amount of water stored in our top soils). These water budget components are important input for the estimation of rainfall recharge to groundwater: the amount of rainfall that percolates through the soil to eventually replenish the groundwater. Rainfall recharge to groundwater is an important component for groundwater flow models. This thesis shows that satellite data can be used as a tool with other data (e.g., with ground observations and models) to better spatially estimate evapotranspiration, soil moisture, rainfall recharge and groundwater table, including their uncertainty estimates. Satellite data thus help to better estimate groundwater volumes, fluxes, and flows, at both the regional and national scale. This thesis has resulted in the first New Zealand-wide gridded estimates of rainfall recharge and groundwater table.
With my thesis, I hope to have paved the way for further use of satellite-derived data sources in groundwater research. I am confident that the results and recommendations of this thesis will lead to a further increase of New Zealand research and applications of satellite data for groundwater and freshwater studies.||