Aquifers are inherently heterogeneous at multiple scales. Limitations of analytical mathematics and our inability to sample aquifers at high density however have dictated adoption of aquifer homogeneity assumption. As scales of our interests become finer and computation and sensor technologies advance, we have developed methods for integrative analysis of multiple pumping tests (i.e., hydraulic tomography or active hydrologic tomography) for characterizing aquifers of tens and hundreds of meters in size at high resolutions. While more field assessments are needed, recent validations of the active hydrologic tomography based on numerical, laboratory and field experiments are promising. They show that not only is the tomography capable of detecting the pattern of hydraulic heterogeneity but also a groundwater flow model with the estimated heterogeneity can accurately predict flows under excitations different from those used in the tomography analysis. As a consequence, model calibration or inverse modeling effort should no longer be viewed as a history or curve matching exercise.

Promising results of active hydrologic tomography encourage the development of basin-scale hydrologic tomography. Basin-scale tomography requires energy sources of great strengths. Spatially and temporally varying natural stimuli are ideal energy sources for this purpose. In our recent study, we explored the possibility of using river-stage variations for basin-scale hydraulic tomographic surveys (i.e., passive hydrologic tomography). Specifically, we use numerical models to simulate groundwater level changes in response to temporal and spatial variations of the river stage in a hypothetical groundwater basin. We then exploit the relation between temporal and spatial variations of well hydrographs and river stage to image subsurface heterogeneity of the basin. Results of the numerical exercises are encouraging. In addition, preliminary analysis of hourly groundwater, river stage, and precipitation data collected by a densely distributed monitoring network in Zhoushuixi alluvium fan (3,000 km square), Taiwan supports the concept of passive hydrologic tomography for characterizing basin-scale aquifers and provides insights to the interaction between streams and groundwater at basin scales.