Characteristics And Impacting Factors Of The Spatial-temporal Variations Of The Shallow Groundwater Depth In The North China Plain

Groundwater is an important component of water resources.At present,the North China Plain is an important industrial and agricultural economic region in China,and with rapid urbanisation,it has become one of the regions with the greatest pressure on water resources use in China.The semi-humid climate has resulted in a lack of surface and underground water resources in the North China Plain,and long-term groundwater exploitation has led to a series of problems such as falling water levels,declining water quality,salinisation and surface subsidence.The long-term exploitation of groundwater has brought about a series of problems such as falling water levels,declining water quality,salinisation and surface subsidence.Groundwater problems in northern China have become a hot issue in China,and the investigation of the spatial and temporal evolution of groundwater depth(or groundwater level)and its relationship with the influencing factors can provide theoretical support for the rational exploitation of groundwater resources.This study analyses the spatial and temporal variation characteristics of groundwater depth in the North China Plain based on the groundwater depth observation data from 2005-2016,and investigates the response relationship between groundwater depth and factors such as temperature,precipitation and human activities.The following main conclusions were obtained:(1)The average groundwater depth in the North China Plain from 2005 to 2016 showed an overall trend of increasing year by year,with the maximum annual average groundwater depth occurring in 2015,and the groundwater depth exhibiting seasonal characteristics of low in winter and high in summer,with relative stability in winter and maximum monthly averages all occurring in June-July.Spatially,the groundwater depth in the pre-mountain plain along the eastern foot of the Taihang Mountains is greater than that in the central region of the North China Plain.(2)The empirical orthogonal function decomposition method was used to decompose the groundwater storage changes in the North China Plain into several main modes,with the first two modes explaining 86% of the overall changes.Among them,the explanation rate of the 1st mode is about 71%,reflecting the trend of decreasing water level in the pre-mountain plain region east of Taihang Mountains year by year,while the water level in Beijing,central Lu and southeastern Henan regions shows an increasing trend or less change.(3)By analysing the variation pattern of land water storage(GTWS)in the GRACE satellite inversions of the North China Plain,it is found that the land water storage also shows a fluctuating decline,with a gradual decrease of 0.9 cm/a overall.In addition,the GTWS correlates well with the depth to groundwater table(-0.64 and-0.84 on the annual and monthly scales,respectively),indicating that the GRACE satellite data can better reflect the changes in groundwater table.(4)Analysis of the effects of precipitation and average air temperature on groundwater depth in typical regions of the North China Plain revealed that the lags also varied between well sites in different regions,for example,groundwater levels located in the Dabie Mountain region responded with some lag to both precipitation and average air temperature one to two months earlier.Temperature and precipitation had large interannual variability over the period 2005-2016,but did not reflect a significant trend,suggesting that the main cause of the continued decrease in groundwater volume over this period may have been primarily human activity.