The validation of retrieved soil moisture

The validation data are dekad soil moisture collected from 98 agricultural meteorological gauge stations in Eastern China observed from 19811991. The observation has 11 layers in vertical (0-5cm, 5-10cm,10-20cm, 20-30cm, 30-40cm, 40-50cm, 50-60cm, 60-70cm, 70-80cm, 80-90cm, 90-100cm), and is taken 3 times per month (8th, 18th and 28th days of each month).

Comparison of geography distributions between retrieval and observational soil moisture

Retrieved and observed soil moistures at 0-5cm on 8th July 1981 are shown in Fig. 7.17. They are in good agreement with each other, for example the high soil moisture centers in the Northeast and Southwest China, low value center in the Yangtze River-Huai River Valley and to the south of Yangtze River Valley.

"icH mi Trc T7S im observed retrieved

Fig. 7.17 Retrieved and in situ soil moisture at 0-5cm on 8th July 1981

"icH mi Trc T7S im observed retrieved

Fig. 7.17 Retrieved and in situ soil moisture at 0-5cm on 8th July 1981

The regional climate change is not only affected by surface soil moisture anomaly, but that of deeper soil layers, though their effects are at dif-

ferent time scales. It's essential for the retrieving scheme to provide accurate soil moisture at different soil levels.

The spatial correlation between retrieved and observed soil moisture fields at different levels are shown in Fig. 7.18. At upper soil layers (up to 50cm) observation and retrieval results of SRSM are significantly correlated in most area at the significant level of 0.01 (shaded area in Fig. 7.18).

Correlation analysis of 42 stations shows that at upper soil level the retrieved soil moisture agrees better with observation. At deeper levels observed and retrieval soil moistures have similar spatial distribution, though their correlation coefficients are not as high as those at upper levels.

Fig. 7.18 The distributions of correlation coefficients between retrieved and os-erved soil moisture at different layers

Comparison of seasonal variations of observed and retrieval soil moistures

The correlation analysis of retrieved and observed soil moisture at 42 stations also shows that, at first three levels (0-10cm), the correlation coefficients of all 42 stations are significant with significant level of 99%; for 10-20cm soil levels, the percentage of stations that can pass the significant test drops to 98%, at 20-30cm to 83%, at 30-40cm to 62% and 40-50cm to 43%. The results indicate that SRSM is reliable in retrieving soil moisture's seasonal variations above 50cm layer, especially for summertime. For spring and autumn, the temporal variations of observation and retrieval results are different in magnitude. Time series of retrieved and in situ soil moisture at Nanchong are shown by Fig. 7.19.

Above analysis shows that SRSM has ability to reproduce the spatial distribution and temporal variation of soil moisture in Eastern China. It is proved to be reliable and feasible, and can be used to produce the replacement the observation when it is scarce. SRSM can also be used to generate initial soil moisture field for regional climate simulation in Eastern China, and credible data for land surface model validation. The scheme is efficient replacement and supplement for physical models which at present still can not provide objective soil moisture. The analysis also demonstrates that further efforts should be made to improve SRSM's ability, especially the treatment of underground soil water.

1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991

1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991

4035302520151051981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991

1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991

1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991

4035302520151051981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991

40 35 30 25 20 15 10

1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991

Fig. 7.19 Interannual variation of retrieved and observed soil moisture at Nan-chong in the first dekad of July (dotted line: observed; real line: retrieved)

Fig. 7.19 Interannual variation of retrieved and observed soil moisture at Nan-chong in the first dekad of July (dotted line: observed; real line: retrieved)

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