Monitoring of subsurface runoff using absolute gravimetry in Taiwan

Calibration of subsurface runoff models in a catchment scale requires lots of observation sites/wells due to their scale difference. An observation site could fail to receive the required data due to variations in flow paths in rainfall events.  Therefore, establishing an effective monitoring site for subsurface runoff is a challenging task in hydrology studies. An alternative choice of monitoring equipment for subsurface runoff is using a terrestrial gravimeter. A terrestrial gravimeter has a broader sensible region than a monitoring well or several ERT (Electrical Resistivity Tomography) profiles. In addition, it takes the water(mass) itself as a tracer rather than using biogeochemical proxies and thus the quantity of runoff is estimated through observed gravity changes accordingly. Due to such advantages, in this presentation, we demonstrate the possibility of using gravimetry to monitor subsurface runoff in Taiwan. In one of the study sites at a proximal fan, our studies cover hourly, daily, weekly and monthly time spans, which encounter different intensities of rainfall over 1.5 years. The infiltration coefficient and percolation rate over 30 m length around our study site were determined in a severe rain event. In another study where we placed an absolute gravimeter in land subsidence regions, we estimated water storage changes at different sites after a wet season and rank their capability for being an artificial recharge pond. This presentation demonstrates the possibility of terrain gravimetry used in calibrating subsurface runoff models. We can picture that when quantum gravimeters are well-developed, high-temporal gravity measurements can assist to build a more accurate subsurface runoff model.