5. Monitoring Methods
5.1. GROUNDWATER LEVELS - THE KEY INDICATOR
Groundwater level, as monitored at observation wells, is the most important indicator of the state of the resource in terms of availability for use and the likely effects on the environment. Environmental effects, such as the low-flow regime of streams and salt water intrusion, are determined primarily by piezometric levels in the connecting aquifers.
The dynamic variability of groundwater levels at observation wells located throughout an aquifer has some components common to all the wells. This means that even one well, suitably located, can provide a significant amount of information about the overall state of the resource. The best locations are furthest from outflow boundaries to surface waters, because the amplitude of level variation is greatest in relation to measurement noise and other influences. It is also desirable to select sites that are less likely to be affected significantly by local abstraction, or local recharge from surface irrigation and flood events in streams perched above the aquifer.
The value of groundwater level observations increases more with length of record than with number of observation sites, because of the common dynamic components.
5.2. LAND SURFACE RECHARGE
The defining characteristic of an aquifer as a groundwater resource is its dynamic behaviour as a leaky storage for natural recharge. This behaviour can be determined from the observed response of groundwater levels to land surface recharge, because the influence of river recharge is usually attenuated to a steady piezometric effect within a few kilometres of the recharge zone.
Land surface recharge is estimated as the soil-water drainage component of soil-plant-atmosphere processes at the land surface, in response to climate. There are a number of water balance models available, but these are not critiqued in the present report. The emphasis here is on necessary and sufficient aspects of these models:
- Water balance must be calculated on a daily basis, and then totalled to the selected time interval such as a month;
- Particular crop-soil combinations, can be expressed as a water-holding capacity;
- Only significant areas (as a percentage of total) of a particular soil-crop combination need be considered.
The strongest recharge signal for analysis of an aquifer comes from winter recharge when abstraction (for irrigation) is least. Therefore, estimation of land surface recharge need not take abstraction into account, for initial assessment, even if the aquifer is not in a "virgin" state.
5.3. ABSTRACTION
Abstractions are limited to maximum rates and volumes by means of the resource consent process, but this information is not directly useful for management. There is an increasing trend to requiring significant water users to meter their abstractions. For the purpose of managing groundwater for sustainable use, abstraction data recorded as monthly volumes would usually be adequate.
Contact for Enquiries
MAF Information Services
Pastoral House
25 The Terrace
PO Box 2526
Wellington, NEW ZEALAND
Fax: +64 4 894 0721
Contact this person
