Cheinway Hwang and Min-Fong Peng, EC Kao and Jinyun Guo

1. Lake level change in China from TOPEX/Poseidon altimetry: climate implications Cheinway Hwang and Min-Fong Peng, EC Kao and Jinyun Guo Department of Civil Engineering, National Chiao Tung University, 1001 Ta Hsueh Road Hsinchu 300, Taiwan Jinsheng Ning and Jia Luo School of Geodesy and Geomatics, Wuhan University, 129 Luoyu Road, Wuhan 430079, China Chung-Hsiung Sui Institute of Hydrological Sciences, National Central University, Chungli, Taiwan • 15 Years Progress in Radar Altimetry Symposium • Venice 13-18 March, 2006

3. Background • TOPEX/Poseidon (T/P) and Jason-1 are satellite missions primarily designed to measure sea surface heights. • Archived T/P and real-time Jason-1 altimeter data have also been used to study major lakes of the world • Lake levels of are associated with climate changes and El Niño  Southern Oscillation (ENSO) • T/P over six China inland lakes and to see how the inland lakes respond to climate change, T/P altimeter data are used to compute time series of lake level in selected inland lakes of China

4. The six studied lakes in China

5. TOPEX passes over the six lakes

6. Features of the lakes 1 above the GRS80 ellipsoid with a semi-major axis of 6378136.3 m, derived from T/P altimetry

7. Problems with TOPEX altimeter data over China lakes • The Eurasia continent is the largest landmass in the world, thus orbit maneuvers of T/P are very likely to occur here • Hulun, Bosten, La’nga and Ngangzi Lake are located either at high latitude or at high altitude, thus their surfaces will be covered with ice in winter • The size of the Ngangzi Lake is only 244 km2, here the wet tropospheric delay of range cannot be accurately measured by the TMR • Only one of the lakes, i.e., Hulun Lake, contains TMR water vapor measurements, model values must be used.

8. Altimeter data processingover lakes • Data time span: T/P cycles 10 (December 21, 1992) to 351 (October 20, 2002), spanning about 10 years. Repeat the same ground tracks every 10 days. Lake Hulun is extended to 2005 using Jason-1 • Apply the same geophysical corrections as over the oceans, except ocean tides • Wet tropospheric corrections are not available in the GDRs, NCEP model values are used • Because the lakes under study are relatively small in size compared to large lakes such as the Great Lakes in North America, waveforms are often corrupted, resulting in a large noise in lake levels.

9. Verification of T/P-derived lake level (1): Bosten Lake T/P-derived (dashed) and in situ (solid) lake levels at the Bosten Lake (from Wang et al., 2002).

10. Verification of T/P-derived lake level (3): Hulun Lake

11. Verification of T/P-derived lake level (3): Lake Huron T/P ground tracks over the Huron Lake and comparison of T/P-derived (solid line) and in situ (shaded) lake levels.

12. Lake level change of Hongze and Gaoyu Lake (east China) • Regualted lake levels • Average depth 2 m • Signal is small and noise is large

13. Climate change: difference of mean annual temperature between the period of 1991-1999 and 1951-1980 (Shih et al., 2003)

14. Lake level and climate: Hulun Lake (north China) • Lake level decreased steadily before Dec 1997 with decreasing precipitation. • In the developing stage of the 199-1998 El Nino (From 5/1997 to 11/997), drought occurred in north China; in the decaying stage (from 5/1998 on), floods occurred in north China and the Yangtze River. This description fits the trends of LLA and CPA over the period 5/1997-8/1999. I • In 1997 Hulun lake level dropped continuously until it reached the lowest level in1/1998, which was coincident with the peak of the 1997-1998 El Niño (12/1997). From 5/1998 to 9/1999, the lake level rose steadily. However, from 9/1999 onwards, the lake level again decreased, and this is consistent with the decreasing trend of precipitation (note that the peak of CPA leads that of LLA by 3-4 months). • During the 1996 La Niña, the precipitation dropped abnormally, while the lake level was not affected.

15. Lake level and climate: Bosten Lake (west China) • Lake level of Bosten increased steadily from 1993 to 2001 by more than 2 m. • The correlation coefficient between LLA and CPA at Bosten Lake is 0.78. The rising trend of Bosten lake level is fully consistent with the result reported by Wang et al. (2003). The increase of Bosten lake level was caused by the increased melting snow in the Tianshan Mountain • The driving factor for the increased snow is temperature, as records have shown a steady rise of temperature over this region (Wang et al., 2003).

16. Lake level change: La’nga Lake • Lake level of La’nga dropped by about 3 m over 1993-2002, caused by drought and heavy water consumption • Pattern is different from that of NgangziLake

17. Lake level change: Ngangzi Lake • Lake level of Ngangzi dropped by about 2 m over 1993-1998. • Unlike La’nga’s lake level, Ngangzi lake level reversed its trend in 12/1997 and lake level increased steadily by about 4 m from 1998 to 2001. • The precipitation slope also reversed from negative to positive in 1998

18. Trends and interannual variations of lake level associated with precipitation and ENSO

19. Comparison of La’nga (west Tibet) and Ngangzi Lake (east Tibet) level changes • Signs of slopes of two lake level changes are different after the1997-98 El Nino. • This difference can be explained by the climate pattern in Tibet. The moisture in Tibet largely comes from the Indian Ocean through the river valleys in the southeastern Tibet. The moisture rarely reaches the western Tibet. • The general climate pattern is that the western Tibet is dry while the eastern Tibet is wet. Such a climate pattern is also revealed by satellite data (Ueno, 1998). La’nga Lake is located 550 km west of Ngangzi Lake, so it receives less moisture compared to Ngangzi Lake. • Ngangzi Lake is located in east Tibet, it is more sensitive to climate change in the Indian Ocean than La’nga Lake. This is why the lake level slope of Ngangzi changes rapidly since 1998 while the slope of La’nga remained unchanged. • Thompson et al. (2000), using ice cores retrieved from the southern Tibet, shows that the climate of Tibet is sensitive to the fluctuation of the South Asia monsoon, which is affected by ENSO. • Liu and Chen’s (2000) study indicates that the Tibet Plateau experienced a significant warming in the past decade (1990-2000) and pointed out the Tibet Plateau is one of the most sensitive areas to respond to global climate change.

20. Lake level change and gravity change CPC-derived gravity change, June 2002

21. Gravity change from CPC models and GRACE (Hulun and Bosten Lake) Hulun (north China) Bosten (east China)

22. Gravity changes from CPC models and GRACE (La’nga and Ngangzi Lake ) La’nga (west Tibet) Ngangzi (east Tibet)

23. Conclusions • About 10 years of T/P data were processed to obtain time series of lake levels at six inland lakes over China, which are validated by in situ data. • These lake levels are mostly affected by climate change. • The slope of the Ngangzi lake level changes from negative to positive after the 1997-1998 El Niño; the cause is left to climate modelers. • Grace produces good agreements of gravity change wrt CPC models in Tibet, with a phase shift. • As a future work, corrupted lake heights (e.g.) will be corrected by waveform retracking.