Land Use Change Impacts On The Amount And Quality Of Recharge Water In The Loess Tablelands Of China

Land use change?LUC?has dramatically altered land cover at the loess plateau of China?CLP?,impacting local water resources such as soil moisture and groundwater recharge.The former is important for sustaining primary productivity,the latter for downstream water supply sustainability.While research on LUC and its effects on soil moisture are plenty,we know very little about the relationship between LUC,groundwater recharge,and water quality.Bridging this knowledge gap is critical for making holistic predictions about LUC effects on water resources at the CLP.Here we investigated the relationship between LUC,groundwater recharge,and water quality at three sites?Luochuan,Changwu,and Baicao?with different land cover at CLP.Using chloride as a tracer of water movement and NO3-N as a proxy for water quality in soil water and groundwater,our main results are as follows:1.Land use change affects soil moisture content and distribution.Using multivariate analysis,we found that soil moisture content was strongly associated with the combined effects of climate,soil properties,and vegetation.Soil moisture distribution was significantly correlated with rainfall in all three sites.Between sites,soil moisture depletion at Baicao was more severe than at the relatively wet Changwu and Luochuan sites.While soil moisture depletion was evident in all three sites,punctuated by dry horizons at varying depths between sites,the dry horizon at Baicao was thicker than at the other two sites.Differences in soil properties also played a role in that soil moisture content was higher in areas with high clay and silt content.Notwithstanding,soil moisture content and distribution were most strongly associated with differences in vegetation.Soil moisture in deeper layers decreased between 27and 47%after converting farmlands into apple orchards at both Luochuan and Changwu sites.We also found a positively strong relationship between soil moisture depletion and age of apple orchards.2.Land use change affects infiltration rate and groundwater recharge.Converting grasslands or farmlands into apple orchards at Luochuan and Changwu resulted in significant decreases in groundwater recharge.A strongly negative correlation between recharge amount and age of apple orchards was also found.Receiving the least amount of precipitation,Baicao was found to show strong and continuous soil moisture depletion,effectively disconnecting soil moisture from groundwater reservoirs to an extent that was unambiguous when compared with Luochuan and Changwu.Modeling groundwater recharge via piston flow,we found that the time it takes for precipitation to reach the groundwater reservoir varied considerably:167-834,106-568,and 298-3345 years for Luochuan,Changwu and Baicao sites,respectively.We therefore conclude that the influence of LUC on soil moisture,resulting from land conversions in recent decades,has yet to significantly affect groundwater resources at these sites.Notwithstanding,we caution that if this trend continues then the sustainability of groundwater resources may ultimately be affected.3.Different land use types lead to different characteristics of soil nitrate accumulation and leaching.Using NO₃-N as proxy for groundwater quality,we found that groundwater NO₃-N content in apple orchards was greater than that in grasslands or farmlands at Luochuan and Changwu.There are obvious NO₃-N accumulation peaks.The width and depth of peaks have a increasing trend with the orchards age increase.The four sampling sites on Baicao tableland,except for the farmland,have low NO₃-N concentration,and there is no obvious NO₃-N accumulation peak.Land use change also leads to the differences in the speed and quantity of NO₃^-entering into the groundwater.Total input amount of NO₃^-content under grasslands or young orchards on Luochuan tableland are higher than that under old orchards.The amount of NO₃^-input in farmlands are slightly higher than 20-year-old apple orchards on Changwu tableland.While there is little difference in the total amount of NO₃^-input under different land use types on Baicao tableland.Assessed the effect of the recharge rate and solute concentration on groundwater quality comprehensively,groundwater recharge is found to be the primary factor that affect groundwater quality in arid and semi-arid areas...