Soil moisture varies in space and in time. However, a large number of studies have observed a time invariance of the spatial pattern of these variables under various soil and climate conditions. This phenomenon was termed as time stability or temporal stability which is the time-invariant association between spatial location and the classical statistical parametric values of a given soil property. Hu and his coauthors also confirmed the existence of time stability of soil moisture at the watershed scale on the Loess Plateau in China (Hu et al., 2009). One of the most important applications of the time stability concept has been the identification of time-stable locations in order to obtain a mean soil water content for an area of interest.

Traditionally, the identification of time stable sites is based on the value of standard deviation of relative difference (SDRD) over time. However, two problems exist: firstly, the value of SDRD may be affected to some extent by the magnitude of the mean relative difference; and secondly, the value of SDRD cannot be directly related to the estimation error of the mean value. Therefore, the identification of time stable sites based on SDRD may not be the best choice from the view point of an accurate mean estimation or prediction. With the purpose of estimating or predicting the mean soil water content or soil water storage more accurately, Dr. HU Wei, Institute of Geographic Sciences and Natural Resources Research (IGSNRR) and his coauthors developed a new criterion - the mean absolute bias error(MABE) to identify time stable location (Hu et al., 2010a). Soil water storage of 0-1 m, 1-2 m, 2-3 m, and 3-4 m taken over four years on a hillslope were used to evaluate the performance of the new criterion. The results showed that MABE was more suitable for the identification of time stable sites for mean soil water storage estimation or prediction. Since the absolute prediction error (APE) of drier sites is more sensitive to the changes of relative difference in terms of mean soil water storage prediction, the sites of wet sectors should be preferable for mean soil water storage prediction for the same changes in relative difference.

Most efforts on time stability analysis of soil water content have focused the surface soil layer and very few reports refer to the whole soil profile. The spatial pattern of soil water content has been well recognized, to date, however, no report referred to the geostatistical analysis of the spatial pattern of temporal stability indicators. In the study of Hu et al (2010b), time stability of soil water content was analyzed with two indices (SDRD and MABE) using neutron probe soil water content (0 to 0.8 m), evaluated at 20 dates during a year in the Loess Plateau of China, in a 20 ha watershed. Specific concerns were (a) the relationship of temporal stability with soil depth, (b) the effects of soil texture and land use on temporal stability, and (c) the spatial pattern of the temporal stability. Results showed that temporal stability of soil water content at 0.2 m was significantly weaker than those at the soil depths of 0.6 and 0.8 m. Soil texture can significantly (P<0.05) affect the stability of soil water content except for the existence of an insignificant difference between sandy loam and silt loam textures, while temporal stability of areas covered by bunge needlegrass land was not significantly different from those covered by korshinsk peashrub. Geostatistical analysis showed that the temporal stability was spatially variable in an organized way as inferred by the degree of spatial dependence index.

Reference:

1. Hu, W., Shao,M.A., Reichardt, K. Using a new criterion to identify sites for mean soil water storage evaluation. Soil Sci. Soc.Am.J., 2010a, 74:762-773.

2. Hu,W., Shao, M.A., Han, F.P., Reichardt, K., Tan J. Watershed scale temporal stability of soil water content. Geoderma, 2010b, 158:181-198.

3. Hu, W., Shao,M.A., Wang, Q.J., Reichardt, K. Time stability of soil water storage measured by neutron probe and the effects of calibration procedures in a small watershed. Catena, 2009, 79:72-82.