4.2 Uncertainty
If one salient property of information and knowledge on large aquifer systems has to be mentioned and highlighted here, then it is uncertainty. Anyone who has been professionally involved in detailed field exploration and assessment of a groundwater system knows that:
- field observations meant to determine parameters and variables tend in practice to be limited in number (due to financial and other constraints);
- sampling locations and depths (e.g., for exploratory boreholes) are often not optimal;
- commonly used methods of observation tend to produce outcomes of only limited accuracy;
- significant methodological, observational, processing and interpretation errors are not uncommon; and,
- the conversion of point observations to spatially continuous maps of aquifer properties and conceptual models requires assumptions based on sound professional expertise, thus is inherently subjective.
Uncertainty may be reduced to some extent (but not eliminated) by additional data acquisition programs, but especially for large and deep aquifer systems these involve major efforts and major financial investment. Observation by remote sensing techniques provides useful complementary information but is similarly subject to methodological, observational, processing and interpretation errors. The GRACE examples presented in Section 3.3 illustrate this.
In summary, uncertainty is part and parcel of information and knowledge on aquifer systems. It is inherent to virtually all components, ranging from assumptions on aquifer delineation and hydraulic continuity of the rock masses within the defined system boundaries to numerical values obtained by using advanced and sophisticated observational techniques. Therefore, critical thinking is needed regarding all information presented in the literature on aquifer systems, and the presence of significant margins of error should always be taken into account.