Study on the Correlation Between GNSS Vertical Time Series and the Space-Time Distribution of Groundwater in California

摘要

The agriculture in California mainly depends on irrigation, but it is one of the regions that suffer serious groundwater loss in the world. The authors obtain the four-year data of 48 survey stations uniformly distributed by PBO network in California and the vertical time series of each station based on GAM1T/GLOBK software. Based on the time series, the contour map of surface elevation variation in California from 2014 to 2017 can be obtained, and it is found that there is an obvious subsidence funnel in southern California. In order to analyze the cause of subsidence funnel, the authors introduce the burial depth data, which shows that the groundwater depth in this area decreases 25.65 cm annually in the past four years. The groundwater loss caused by agricultural irrigation is the main reason for the formation of settlement funnel in this area, while rainfall can effectively alleviate the exploitation of groundwater for agriculture. Through analyzing the TRMM rainfall data of this area, the authors find that the cumulative rainfall in northern California from 2014 to 2017 is significantly higher than that in the south. The maximum reaches about 6000 mm, but the minimum rainfall occurs in the south, less than 1000 mm. According to the GPS vertical time series, TRMM rainfall data and groundwater depth analysis, there is a high correlation between the P566 survey station with severe surface subsidence and the groundwater depth data of nearby wells. In addition, the rainfall data shows that the rainfall in southern California is significantly lower than that in the north. The experimental results indicate that the time series of the surface elevation change acquired by the GPS can effectively reflect the groundwater timing change; the ground surface settlement monitored by the GPS is highly correlated with the measured groundwater depth data, which proves that the surface settlement of the southern California is mainly caused by the exploitation of the underground water; the loss of groundwater in the southern region can not be effectively alleviated due to inadequate rainfall, which further demonstrates that the surface settlement in California is mainly caused by the excessive exploitation of groundwater.