Distribution of organic matter and effect of wetland types and pH on organic matter were studied in sediment of lakeside belt in east Dongting Lake according to taking 56 samples from surface to 20cm and 52 samples from 20cm to 40cm. The results showed that the average content of organic matter was 15.40g/kg from surface to 20cm and 12.02g/kg from 20cm to 40cm in sediment of lakeside belt in east Dongting Lake. Spatial distribution of organic matter could be expressed as middle variation. Wetland types were classed into silt beach, lake marsh beach and reed beach in Dongting Lake region and influenced distribution of organic matter because of difference variance on hydraulic dynamics condition and retarding effect of vegetation on water flow velocity. Order of organic matter content in sediment was the lake marsh beach > reed beach > silt beach in 0-20cm and 20-40cm layers in lakeside belt in east Dongting Lake. Organic matter contents decreased in turn from 0-20cm to 20-40cm in lake marsh and reed beaches and increased in silt beaches. Values of pH in sediment were slightly alkaline in whole studied region. Negative correlation was established between organic matter content and pH value. The results will provide base data and science reference for controlling and recovery of sediment pollution in east Dongting Lake.
With the rapid development of computers and computing techniques, some increasingly complicated computational problems in engineering and science deserve further review and improvement. This work calculated the solution to the problem of Richards partial differential equation with its corresponding initial and boundary condition for one-dimensional unsaturated water movement in soil with changing bulk density by adopting an explicit backward difference method, an implicit forward difference method and an implicit central difference method. Therefore, a temporal-spatial distribution of water content in the process of filtration, evaporation and redistribution of water moisture within the soil column was obtained. Then the computational results were compared and verified, alternatively, by three methods, with a semi-analytical solution. This indicates the methods presented are of high validity, efficiency and accuracy. These methods were used for the first time for the modelling and prediction of the complicated water movement process in soil of changing bulk density. The computations show that the explicit method features a simpler formulation and more capability for complicated modelling, and the authors strongly recommend this method for application in time-related problems. In addition, this work innovatively developed a combination procedure of an implicit difference method and iterative solution method of a large set of linear algebraic equations. It not only avoids solving a large set of linear algebraic equations but also is able to be applied to complicated modelling of soil moisture profiles. Finally, the numerical methods and the technical skills presented in this work can be generalised for two-dimensional or three-dimensional soil moisture movement and more complicated water movement modeling.
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