Secondary information, especially exhaustive secondary information, is widely used in soil carbon mapping. However, the secondary information used in present methods is limited to analytical data and simple topographical indices derived from DEM, which lack the power to account for the immigration and distribution of soil carbon in cultivated fields. Hydrological information is more closely related to the immigration and distribution of soil carbon and so should be considered for soil carbon mapping. An exploration of several indices shows that the wetness index (WI, an index used to quantify the influence of hydrological process based on local topography), which incorporates topographical and hydrological information, using Collocated Cokriging (CC) yields the best result. This trial indicates that CC (with WI-md) not only significantly reduces estimation uncertainties but also reveals more details of the distribution of soil carbon.
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