Vertical hydraulic gradient (VHG) provides detailed information on
3D groundwater flows in alluvial fans, but its regional mapping is complicated
by a lack of piezometer nests and uncertainty in conventional well data.
Especially, determining representative depth of well screen in each well is
problematic. Here, a VHG map of the Toyohira River alluvial fan, Sapporo,
Japan, is constructed based on groundwater table elevation (GTE), using
available well-data of various screen lengths and depths. The water-level data
after 1988, when subway constructions are mostly completed in the city, are divided
into those of shallow wells (≤20 m deep), and those of deep wells (>20 m
deep). First, the GTE map is generated by kriging interpolation of shallow well
data with topographic drift. Next, the individual VHG value of each deep well
is calculated using its top, middle, and bottom elevations of the screen
depths, respectively. The VHG maps of three cases are then obtained using
neighborhood kriging. The VHG map of the bottom screen depths has proven most
valid by cross-validation. The VHG map better visualizes that downward flows of
groundwater are predominant over the fan. Positive area of VHG is mostly
vanished around the fan-toe, indicating urbanization effect such as artificial
withdrawals. A negative peak of VHG corresponds to
recharge area, and is seen along the distinct losing section in the river. The
negative peak also expands upstream to the fan-apex where a basement is
Cite this paper
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