| As the most populous country and the second largest economic entity in the world,how to sustainably use and manage water resources is a key issue for both China and the world,and is of great significance to global food security,human health and social stability.In order to improve the ecological environment of the Loess Plateau and control soil erosion,the Chinese government has implemented Grain for Green Project since 1999,which is the largest ecological restoration project in the world.The vegetation coverage of the Loess Plateau has doubled by 2013,and the amount of sediment transport in the Yellow River has reduced to the level of the Tang Dynasty farming era.However,the excessive water depletion by reforestation has caused a series of ecological issues including soil desiccation in the water-limited region.Previous studies have investigated the variation of terrestrial water storage and groundwater recharge from the point scale measurements,but few studies have been conducted at regional scales due to the lack of methods and data.To better understand the spatio-temporal changes of water resources at the regional scale of the Loess Plateau.Three studies were performed:?1?the terrestrial water storage products from Texas State University Space Research Center?CSR?,the US Jet Propulsion Laboratory?JPL?,and the Goddard Space Flight Center?GSFC?retrieved with the GRACE satellites based Mascons method?abbreviated to as CSR-M,JPL-M and GSFC-M,respectively?were used to figure out the time series and spatial variation of terrestrial water reserves on the Loess Plateau;?2?data including paired experiments of the forest and grassland,land use change data,normalized difference vegetation index?NDVI?,coal mining data,groundwater level monitoring data as well as the global land data assimilation system?GLDAS?were used to find out the underlying reasons accounting for the terrestrial water storage change at regional scales;?3?a new groundwater storage fluctuation method based on GRACE data was developed to estimate the groundwater recharge?GR?at regional scale.The main results are as follows:?1?The terrestrial water storage of the Loess Plateau calculated by CSR-M,JPL-M and GSFC-M showed a decline trend during the investigated time period with reduced amount of6.1±0.9 mm,6.8±1 mm and 10.7±1 mm,respectively.All the three products can reflect the decline trend of terrestrial water reserves,but their capabilities to represent this trend are gradually weakened from southeast to northwest.A good consistency in regional average and spatial variation was observed between the CSR-M and JPL-M,but the GSFC-M performed quite differently from the other two products.The regional average water storage amplitudes of the three datasets were 13.7±3.6 mm,12.2±3.9 mm,and 10.8±4.2 mm,respectively.While the phases were 11.3±0.5,10.1±0.6,and 10.8±0.7 months,respectively,which showed 12 Months lag compared to precipitation.The regional average amplitude and phase are relatively consistent,but the spatial distribution of these three is quite different.In summary,the GRACE data to calculate regional water storage changes should be applied to large-area,not suitable for application in small areas or a single grid scale.?2?Vegetation has been well restored since the implementation of the GFGP in 1999,but it is interesting to notice that the shallow soil water storage increased by 2.1 mm yr-1?p<0.01?.This is probably attributed to the fact that shallow soils are sensitive to rainfall and there is an increasing rainfall trend during the period investigated.Data from the China National Geological Environment Monitoring Groundwater Yearbook and the Yellow River Water Resources Bulletin showed a long-term trend of groundwater storage change of 0.1 mm yr-1?p<0.01?and-0.1 mm yr-1?p<0.01?,respectively.Compared to the change rate of terrestrial water storage anomalies?TWSA?(-6.1 mm yr-1,p<0.01),the change of groundwater storage less likely contributes to TWSA changes.In addition,the mass change caused by coal mining on the Loess Plateau is-2.7 mm yr-1?p<0.01?,and therefore the contribution of coal mining to the GRACE-derived TWSA cannot be ignored.The depletion of deep soil water storage on the Loess Plateau is the most important factor of TWSA decline on the Loess Plateau with a decrease of-4.7 mm yr-1?p<0.01?accounting for 77%of the decline rate of TWSA.?3?We estimated groundwater recharge?GR?in the Ordos Basin of the Chinese Loess Plateau from 2002 to 2012 with a newly developed method with comparison two other GR methods.The results showed no obvious long-term change trend in GR,but the annual recharge varies significantly from 30.8 to 66.5 mm yr-1,75%of which can be explained by the variability in the annual precipitation.The average GR rate over the 11-year period from GRACE data was48.3 mm yr-1,which is close to the long-term average recharge estimate of 39.9 mm yr-1 from the environmental tracer methods and one-dimensional models.Moreover,the standard deviation of the 11-year average GR is 16.0 mm yr-1,with a coefficient of variation?CV?of33.1%.This indicates that the newly developed GR method is generally comparable to or smaller than estimates from other GR methods.This new method could provide critical information on regional-scale GR estimates for groundwater management and serves as guideline and basis for sustainable use of groundwater resources. |
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