| Climate change and human activities have profoundly changed the global land water storage model and seriously threatened the availability of social water resources.It is of great significance to explore the change characteristics and internal evolution mechanism of land water storage,not only to understand the water cycle in the basin but also to understand the sustainable development of human beings.Taking the Yangtze River Basin as an example,this paper evaluated the performance of GRACE products from different institutions and generated fusion products using GRACE gravity satellites,Water GAP Global hydrological model(WGHM),surface hydrological and meteorological observations,global reanalysis data and remote sensing products.Based on this,the temporal and spatial variation of land water storage in the basin during the historical period(2002-2021)was analyzed,and the spatial and temporal variation of land water storage in the basin was decomposed into climate and non-climate driven parts for attribution analysis,and the future change of land water storage under various climate conditions was predicted.Finally,an application example of land water storage change was demonstrated.It aims to promote the understanding of the water cycle in the watershed and provide reference for the future operational use of GRACE.The main work of the paper is as follows:(1)The performance of multiple GRACE products was evaluated against the WGHM hydrological model and in-situ measurements of terrestrial water storage changes(ΔS).The relative uncertainty was assessed based on the generalized three cornered hat method and subsequently the GRACE data were fused based on their uncertainty.The overall variations of GRACE products from different institutions are highly consistent.By comparing with the WGHM results,the GRACE products from CSR and GSFC may have better accuracy than the data from JPL institutions,while the comparison results between different GRACE products and in-situ observations reveal similar performance.The relative uncertainties of GRACE products from different institutions are 5(CSR),7.2(JPL)and 3.9(GSFC)mm/m,respectively.The NSE between the fused GRACE products and WGHM data reached 0.71,which was between the GRACE products from three institutions(CSR/JPL/GSFC=0.66/0.61/0.69),and the NSE between the GRACE products and the observation-based results reached 0.31,which was higher than that of the individual products.It shows that the fusion process can reduce the uncertainty of estimation of land water storage to a certain extent.(2)Based on the fused GRACE products,two sets of global GRACE reconstruction datasets(GRACE＿REC＿Li and GRACE＿REC＿Mo)were compared and optimistically selected to fill in the missing GRACE data,analyze the spatio-temporal variation characteristics of terrestrial water storage in the basin,and explore the relationship between them and extreme hydrological events.By comparing the GRACE＿REC＿Mo dataset with GRACE observations,the prediction performance of GRACE＿REC＿MO dataset is better than that of GRACE＿REC＿Li product.The GRACE data based on GRACE＿REC＿Mo showed that the monitored land water stocks increased widely,mainly in the central and eastern parts of the basin,but decreased slightly in the western and northern parts of the basin.At the basin scale,the overall change trend of land water storage was 4.1 mm/a(p<0.05).The annual distribution of land water storage in the basin was mainly controlled by precipitation,which gradually increased from February to July and then steadily decreased until the lowest value in January.Based on two extreme hydrological events(drought in 2006 and flood in 2020),a good response relationship between land water storage and precipitation was also found.(3)The terrestrial water storage anomaly in the Yangtze River Basin was decomposed into climate-deriven(CTWSA)and non-climate-driven(NCTWSA)using two statistical models and a hydrological model,and the impacts of different climate and non-climate factors on terrestrial water storage components were analyzed from the perspective of longterm trends.Two types of method expressed the consistent spatial pattern of CTWSA,with found significant(p < 0.05)increasing trends in the downstream river basin ranging between5 to 10 mm/a,and the southern and southeast parts also has significant increasing trends.In addition,the statistical results also found slight upward trend in the river source area,while the slope is less than 5 mm/a,However,the trend revealed by WGHM did not pass the significance test at 0.05 level.A downward trend was also found in the southwest and northeast regions of the basin,but the statistical method had a faster rate of decline than the hydrological model method,even less than-10 mm/a in some areas.For the whole basin,CTWSA increased at rates of 1.4 mm/a(statistical model)and 1.3 mm/a(hydrological model,p<0.05).Precipitation well explained the variation trend of CTWSA at the spatial and temporal scales,and the variation trend of precipitation in the basin reached 13.8 mm/a(p<0.05).Both methods revealed an increase in the middle and downstream regions and a decrease in the upstream regions,with a trend of 2.7 mm/a(statistical model)and 0.6 mm/A(hydrological model),respectively.The rise of local land water storage in the middle and lower reaches of the basin corresponds to the impoundount process of many large and medium-sized reservoirs.The rising trend of human water use was found in the middle and upper reaches of the basin and the northern part of the basin,which may reveal the decline of some local terrestrial water storage.(4)Combined with the WGHM hydrological model and the global climate model,the variation of land water storage under various future climate scenarios(RCP2.6,RCP6.0 and RCP8.5)was discussed and the TWS-DSI index was constructed to evaluate the dryness/wetness conditions of the basin.The ensemble mean value of WGHM results driven by different GCM performed better than the single model when compared with GRACE.Terrestrial water storage has a large inter-annual difference between GCMs,but the change of terrestrial water storage under different scenarios does not show obvious differences.The seasonal pattern is relatively uniform,showing an obvious single-peak pattern.At the end of this century(2080-2099),terrestrial water storage in dry season under RCP6.0 and RCP8.5 scenarios will decrease relative to the base period(2003-2022),while it will increase in the wet season,but it is the opposite under RCP2.6 scenario.In the middle of this century(2040-2059),land water storage in the southwest and the lower reaches of the basin increased under the RCP2.6 scenario,while slightly decreased in the upper and middle reaches of the basin.In the lower reaches of the basin under the RCP8.5 scenario,terrestrial water storage also decreased,although there were some discrepancies reported by the RCP6.0 scenario.The increase of terrestrial water storage in the northern part of the basin is in contrast to the loss in the upstream and downstream,with the average change of terrestrial water storage in the whole basin reaching 1.1(RCP2.6),-4(RCP6.0)and-6.4(RCP8.5)mm,respectively.At the end of the century,land water storage decreased in more regions,except in the RCP8.5 scenario,where terrestrial water storage increased in the upper and middle reaches of the basin.At the basin scale,the shift values were-1.8(RCP2.6),-5.6(RCP6.0)and-3.8(RCP8.5)mm,respectively.The spatial pattern revealed by TWSDSI is highly consistent with the change of terrestrial water storage,and the depletion in the upper Yangtze River basin is further emphasized,especially under the RCP6.0 and RCP8.5 discharge scenarios.(5)The Yangtze River Basin is selected as an example to demonstrate the application case of terrestrial water storage in the water balance studies,including the use of GRACE data to estimate the actual evaporation,estimate and analyze the variations of groundwater storage anomaly,and estimate basin flood potential.The results were compared with the insitu measured data,official water resources bulletins,and different auxiliary products.The actual evaporation derived from GRACE tended to be significantly overestimated in the wet season,while underestimated in the dry season.The variation amplitude of "human-induced evaporation" derived from GRACE was significantly higher than that of WGHM,but the obvious seasonal characteristics could still be found,which showed an obvious upward tendency from 2002 to 2016.The correlation coefficient between groundwater storage and WGHM data obtained based on GRACE reached 0.7(p<0.05),and the correlation coefficient between groundwater supply volume of the basin was-0.55(p<0.05),indicating that human exploitation of groundwater plays an important role in regulating groundwater storage dynamics.It shows a significant(p<0.05)rising trend in the source region of the Yangtze River and the middle part of the basin from 2003 to 2016,with the rate generally between 6-12 mm/a,while the decline of groundwater reserves in the southwest and northeast of the basin is likely caused by the increasing water demand of human beings.The trend of groundwater storage anomaly is 2.6 mm/a for the basin.The correlation coefficient between the flood potential index(FPI)calculated based on GRACE and the measured runoff reached 0.9(p<0.05),and it had a good response process to many actual reported flood events. |
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