8. Summary of Good Practice
8.1. ESTIMATION OF 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.
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.
8.2. WHEN TO USE THE EIGENMODEL METHOD
The eigenmodel method performs best for land surface recharge that has a fixed spatial pattern and time-varying magnitude. This property is generally satisfied by natural recharge into an aquifer for which abstraction is a small fraction of the total resource. Under these conditions, the dynamic behaviour of the total groundwater resource can be assessed quite accurately.
As abstraction increases, generally with its own spatial pattern, the eigenmodel becomes less accurate but still provides useful information for management, especially in the forecasting format (Section 7.6). We have some experience of this effect of spatial distortion of recharge in Mid-Canterbury, where the piezometric response to land surface recharge is dominated by soil-water drainage from border-dyke irrigation. In other parts of this same region, pumped irrigation abstraction dominates the piezometric response, but forecasting is still feasible.
The purpose of the eigenmodel method is to provide:
- Whole-aquifer assessment of the useful availability of groundwater;
- Relationships between climate and environmental effects such as low flow in streams;
- Indicative relationships between abstraction and environmental effects;
- Estimates of the piezometric effect of river recharge.
The method is not suitable for assessment of short-range effects such as stream-depletion by groundwater pumping, or upconing of a salt water interface near the coast. However, some salt water intrusion is determined by the rate of natural outflow to the coastal boundary, and the eigenmodel may be useful for estimating the time-variability of this flow.
8.3. THE TRANSITION TO MORE COMPLEX MODELS
The effects of local abstraction stresses on an aquifer are best examined with models that simulate the continuous aquifer space ("continuum" models), such as MODFLOW. These models have the potential to use as much data and geological information as is available, but can require significant modelling resources in terms of expertise and time. Some of the time resource can arise from difficulties in obtaining unambiguous model calibration.
In situations where piezometric data are sparse, the benefits of the continuum model may be achieved only with considerable insight from the modeller. The eigenmodel approach offers the ability to appreciate the "big picture" of the dynamic response of an aquifer to recharge and abstraction. This insight can assist with appropriate reduction of the parameter options in the continuum model.
The small scale of a particular groundwater problem may mean that the areal extent of the continuum model is restricted by computational limits, and does not extend to the natural boundaries of the aquifer. Specification of boundary conditions for these continuum models can be difficult, and sometimes assumptions are made that may undermine the validity of the model. The eigenmodel method can be applied to several observation wells throughout the aquifer to assist in determining the nature of natural aquifer boundaries and how these influence the boundary conditions of the continuum model.
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