4. Limitations of Data and Information
4.1. QUANTIFYING RECHARGE
Aquifers have two sources of recharge: through the land surface, and from rivers.
- Land surface recharge can be estimated from water balance models to an accuracy suitable for most resource management decisions.
- River recharge is very difficult to estimate, and it can be very expensive to conduct measurements of sufficient quality and frequency for useful resource assessment.
4.2. GEOHYDROLOGICAL INFORMATION
Groundwater models usually require information about the nature of aquifer boundaries and the aquifer properties of transmissivity and storativity. The latter are sometimes estimated from pumping tests, but these are subject to a high degree of variability and may not be appropriate for aquifer-scale application. For these reasons, aquifer properties tend to become model parameters for calibration by means of available piezometric data.
Identifying the nature of aquifer boundaries can sometimes be difficult, especially for aquifer-river boundaries in alluvial aquifers. Poor definition of boundaries has a significant effect on the reliability of a mathematical groundwater model.
4.3. PIEZOMETRIC DATA
Measurement of groundwater levels in observation wells provides high quality data about the dynamic behaviour of an aquifer, in terms of its value as a resource. For assessment of the whole-aquifer resource, length of record is generally more important than numbers of wells observed. The principal data limitation in many areas is length of record.
4.4. ABSTRACTION
Most use of groundwater is controlled by the resource consent process. However, these controls are usually specified as maximum rates and volumes. There are few data about actual abstraction from aquifers, but this situation is likely to improve as implementation of water metering is advanced.
4.5. CONCLUSIONS ABOUT GROUNDWATER RESOURCE DATA
The high quality data about an aquifer that are relatively easy to acquire, are estimates of land surface recharge and observations of groundwater level. It would be desirable to use analytical methods that can build on these strengths.
Inadequate data about current abstractions, and the general paucity of observations in many aquifers, suggests that management of these aquifers should proceed in a manner that allows for changes in strategy as more information becomes available.
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