Abstract
Surface-water divides can be delineated by analyzing digital elevation models. They might, however, significantly differ from groundwater divides because the groundwater surface does not necessarily follow the surface topography. Thus, in order to delineate a groundwater divide, hydraulic-head measurements are needed. Because installing piezometers is cost- and labor-intensive, it is vital to optimize their placement. In this work, we introduce an optimal design analysis that can identify the best spatial configuration of piezometers. The method is based on formal minimization of the expected posterior uncertainty in localizing the groundwater divide. It is based on the preposterior data impact assessor, a Bayesian framework that uses a random sample of models (here: steady-state groundwater flow models) in a fully non-linear analysis. For each realization, we compute virtual hydraulic-head measurements at all potential well installation points and delineate the groundwater divide by particle tracking. Then, for each set of virtual measurements and their possible measurement values, we assess the uncertainty of the groundwater-divide location after Bayesian updating, and finally marginalize over all possible measurement values. We test the method mimicking an aquifer in South-West Germany. Previous works in this aquifer indicated a groundwater divide that substantially differs from the surface-water divide. Our analysis shows that the uncertainty in the localization of the groundwater divide can be reduced with each additional monitoring well. In our case study, the optimal configuration of three monitoring points involves the first well being close to the topographic surface water divide, the second one on the hillslope toward the valley, and the third one in between.
Highlights
Groundwater divides are curves separating different subsurface catchments
To find the optimal placement of piezometers in order to delineate a groundwater divide, we apply the optimal experimental design method PreDIA, which we briefly review in the given context
In this work we have presented a framework to identify the best piezometer configuration from a set of possible layouts to delineate local groundwater divides
Summary
Groundwater divides are curves separating different subsurface catchments. Water entering the subsurface on one side of the groundwater divide ends up in a different receptor than water infiltrating on the other side of the divide. Delineating groundwater divides is important for the analysis of aquifer water budgets, for investigating contaminant fate, and other applications of groundwater management. Groundwater divides represent attractive geometries for setting second-type boundaries of hydrogeological models, since the water flux across the divide is zero A natural stream network contains many nested surface water and groundwater divides of different order (i.e., a catchment can be subdivided into sub-catchments). That is why for the mentioned research areas, it is always important to define the scale of investigation to identify which groundwater divides are relevant and which sub-catchments can be attributed to a higher-order catchment
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