Joint Satellite Altimetry And Satellite Gravity Data To Monitor Water Level Changes In The Caspian Sea

Water is an indispensable resource for the survival and development of all things,and the lake water cycle is an important part of the global water cycle.With the gradual exposure of global water scarcity,research on the balance of the water cycle and how to efficiently monitor changes in water resources has become increasingly important.Traditional hydrological station monitoring methods have shortcomings such as limited measurement stations,uneven distribution of stations,and insufficient data acquisition.They are increasingly unable to meet the real-time,all-weather,high-precision,high-resolution,and sustainable water level monitoring requirements of present society.The emergence and development of Satellite altimetry technology and GRACE(Gravity Recovery and Climate Experiment)satellites provide two new global water resource change monitoring methods that can meet the current water resource monitoring needs.The Caspian Sea is the world's largest inland water body,and it also has the characteristics of oceans and lakes.It contains rich mineral resources,which is of great significance for creating a safe,reliable and diversified new energy channel in China.Whether it is now or in the future,The Caspian Sea area occupies an important position in China's foreign strategy,and monitoring the changes in the level of the Caspian Sea has important practical significance.The Caspian Sea area is covered by multiple altimetry satellites,and the basin area is much larger than the spatial resolution of GRACE satellites(?300km).The mass migration dynamics can be detected by GRACE satellites,and it is feasible to monitor the sea level changes in the combined satellite altimetry and satellite gravity data.The main work of this paper is as follows:Using the Jason-1/2/3 Sensor Geophysical Data Record(SGDR)from the three altimetry satellites from 2002 to 2020 to reconstruct the Caspian Sea average sea surface height model and the long-term water level change time series.Before waveform retracking,Empirical Mode Decomposition(EMD),Singular Spectrum Analysis(SSA)and EMD+SSA combined denoising methods are used to remove noise in the height measurement waveform data.In this paper,the combined denoising method is used to denoise the height measurement waveform for the first time.The results show that the denoising effect of the combined method is better than that of EMD and SSA alone.After the waveform denoising processing,combined with the 50%threshold waveform retracking algorithm to correct the Caspian sea level,the research results show that the combined denoising method combined with the 50% threshold retracking algorithm solves the Caspian sea level change trend that is closest to that of Hydroweb,respectively.-7.33±0.32cm/yr,-7.40±0.34cm/yr,which shows that the Caspian sea level change sequence and the average sea level model calculated by the combined denoising method are better than other methods.Use GRACE/GRACE-FO RL06 2002?2020 data and based on the constrained forward modeling method(Forward Modeling,FM)to invert the Caspian Sea land water reserve changes.The detected land water reserves of the Caspian Sea showed an upward trend from 2002 to 2006,and the rate of change was 9.77±1.31cm/yr.From 2006 to 2020,there is a downward trend,and the rate of change is-8.98±0.34cm/yr.The overall trend is down,and the rate of change is-7.65±0.38cm/yr.Based on the CSR MASCON data,the Caspian Sea boundary was expanded by 200 km and combined with the NOAH hydrological model data to correct the leakage error.The calculated land water reserves of the Caspian Sea showed an upward trend from 2002 to2006,a downward trend from 2006 to 2020,and an overall downward trend,with a rate of change of-8.02±0.41 cm/yr.Based on the Argo data product data,the Caspian sea level change is estimated.The time series fitting results show that the thermosteric height changes shows an upward trend,with a rising rate of +0.45mm/yr.Comparing GRACE and satellite altimetry results,it is shown that the total sea level change calculated by "CSR MASCON(200km)+temperature and salinity" is the closest to Hydroweb,and the correlation coefficient is as high as0.9854.The contribution of evaporation,precipitation,and runoff to the Caspian sea level is analyzed in detail.The contribution rate is drawn: evaporation is greater than runoff and precipitation.The evaporation contribution is-77.41cm/yr,the simulated runoff contribution is+43.66cm/yr,and the precipitation contribution is +27.82cm/yr.The overall Caspian Sea shows a downward trend of-5.93cm/yr,which is equivalent to the downward trend of-7.33±0.32cm/yr and-7.40±0.34cm/yr of the total sea level calculated by Hydroweb and EMD+SSA+50%threshold.Analyzing separately from 2002 to 2006,the contribution of evaporation to the Caspian Sea is about-75.92 cm,the contribution of runoff to the Caspian Sea level is about+49.31 cm,and the contribution of precipitation to the Caspian Sea level is about +29.32 cm.The estimated Caspian sea level change is about +2.71cm/yr,the Caspian sea level showed an increasing trend from 2002 to 2006,which was consistent with the change trend of land water reserves detected by GRACE.The main reason for the increase was the increase in the runoff of the Caspian Sea.Looking at the long-term time series of the Caspian Sea from 2002 to 2020,among the three factors,evaporation accounted for the main part of the changes in the Caspian sea level.Excessive evaporation of the Caspian Sea(outflow is greater than inflow)is the main reason for the decline in the Caspian sea level in recent years.