Abstract
Balancing the demand of groundwater resources and the mitigation of land subsidence is particularly important, yet challenging, in populated alluvial fan areas. In this study, we combine multiple monitoring data derived from Multi-Temporal InSAR (MTI), GNSS (Global Navigation Satellite System), precise leveling, groundwater level, and compaction monitoring wells, in order to analyze the relationship between surface displacement and groundwater level change within the alluvial fan of the Choshui River in Taiwan. Our combined time-series analyses suggest, in a yearly time scale, that groundwater level increases with the vertical surface displacement when the effect of pore water pressure dominates. Conversely, this relationship is negative when the effect of water-mass loading predominates over pore water pressure. However, the correlation between the vertical surface displacement and the groundwater level change is consistently positive over the time scale of two decades. It is interpreted that the alluvial fan sequence in the subsurface is not fully elastic, and compaction is greater than rebound in this process. These findings were not well reported and discussed by previous studies because of insufficient monitoring data and analyses. Understanding the combined effect of groundwater level change and vertical surface displacement is very helpful for management of land subsidence and usage of groundwater resources. The spatial and temporal integration of multi-sensors can be applied to overcome the limitations associated with the single technique and provides further insights into land surface changes, particularly in highly populated alluvial fan areas.
Highlights
Alluvial fans are major reservoirs of groundwater in piedmont areas around the world
The land subsidence rates in the coastal zone derived from the ERS-1/2 images were underestimated in comparison with the data derived from the precise leveling, which were interpolated by the inverse distance weighting (IDW) method
The average subsidence rate derived from ALOS-based Persistent Scatterer Interferometry (PSInSAR) results in the main subsidence area in Tuku of the Yunlin County is greater than 63 ± 4.1 mm/year
Summary
Alluvial fans are major reservoirs of groundwater in piedmont areas around the world. The withdrawal of groundwater from fan aquifers usually causes compaction of unconsolidated deposits and land subsidence in the time scale of decades due to the decrease of pore-water pressure (e.g., [1,2,3,4,5]). The combined effect of pore-water pressure and hydrological mass loading on the vertical surface displacement in an alluvial fan has not been fully explored due to the lack of spatially and temporally well-documented monitoring data. The study area is the alluvial fan of the Choshui River in western Taiwan, which has suffered serious land subsidence over the past 30 years. The alluvial fan of the Choshui River is located in the Changhua and Yunlin Counties, western central Taiwan. Aeslaabrguenpdoarnttiognrooufnthdewparteercirpeistoautirocnesoicncuthrse fdrorymseMasaoynt.o September and provides abundant groundwater resources in the dry season
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