Existing literature suggests that Darcy's law is not valid in layers of different degrees of decomposed peat soil. The present study attempts to validate the applicability of Darcy's law by comparing the velocity predicted by it and the velocity obtained through experiment for a peat soil column and also to assess the changes of hydraulic conductivity, k, with the depth of peat soil layers. The suitability of Izbash's law to predict the flow through peat soil column of different degrees of decomposition was tested by determining the n value (Izbash's parameter). Izbash's law (n = kiⁿ) is preferred because it is in continuity with Darcy's law. Soil column studies were set up by applying different values in hydraulic gradient in order to obtain discharge velocity, n , of the sample. From the experimental data, the suitable Izbash's parameter, n, for each depth of peat soil profile was determined. The result shows that Darcy's law is only appropriate for the upper layer and as the layers become deeper, deviation from Darcy's law becomes larger. Izbash's law provides a much better approximation of water flow through much deeper peat layers.

The type of peat, depending on the degree of decomposition and bulk density, could provide a good basis for the assessment of its hydraulic conductivity properties (Boelter, 1974). In terms of the relationship of hydraulic conductivity to the Von Post humification scale, there is a decrease in conductivity with increasing humification/undecomposition (Rycroft et al., 1975). A study on core samples of peat across Peat Bay, South Carolina, had found that, in general, the highest hydraulic conductivities tended to be found where the peat layers were higher in fiber and lighter in color. On the other hand, the lowest hydraulic conductivities were found where the peat layers were more oxidized/humified and darker in color. It was also found that in all the core samples from Peat Bay site, the hydraulic conductivities tended to increase with depth for the first 25 cm and then decrease with depth for the rest of the core interval, perhaps as a result of either autocompaction of the peat or changes in original environments of deposition (climate, hydrology, etc.), which affected the depositional processes swamp-wide and consequently the physical composition of the organic sediments (Rizzuti et al., 2004). The conductivities of the different peat types converge as they reach a high degree of humification.

Conventionally, the empirical relation, known as Darcy's law, is used to model the relationship between the specific discharge of water, and the hydraulic conductivity and gradient in peat (Basak, 1977). However, existing literature suggests that such flow violates Darcy's law in humified peat and casts a doubt on the applicability of existing models for flow through porous media when applied to peat lands (Hemond and Goldman, 1985). A number of studies suggest that Darcy's law is only applicable to the upper layer of peat (Rycroft et al., 1975; and Hemond and Goldman, 1985). The cause of departure from Darcian behavior is associated with changes in pore geometry of peat soil due to the different degrees of decomposition.

Soil And Water Sciences Journal, Darcy's law, Degree of Decomposition, Izbash's Law, Peat Soil Column, Water Flow, Organic Sediments, Hydraulic Conductivity, American Society for Testing and Materials, ASTM, Linear Regression Analysis, Hydraulic Gradient.