Responding to the need of more knowledge and technology for sustaining an optimal global environment, a new interdisciplinary subject Hydropedology is in its initial stage of development. Its utility is to link the disciplines of pedology including soil macro- and micro-morphology, vadose zone hydrology and other disciplines dealing with interfaces between land, air and water. Examples of the application of this emerging interdisciplinary subject in which the pedotaxon and soil forming processes as defined phenomenologically are presented. If a pedotaxon characterized by Π* is represented by a set of soil properties {Πi}, a change in its characteristics is the consequence of a change of all properties Πi. And recognizing that each property Πi depends upon the actions of soil-forming factors Fj, the change of all of the i-th properties is dP* = ΣiΣj (dΠi/dFj) Fn≠j where index Fn≠j denotes that all other factors except Fj are kept constant. Hence, the pedotaxon is characterized by soil forming processes and these processes are usually linked with their rates differing by orders of magnitude. Moreover, as a consequence of considering soil polygenesis, the equilibrium concept between the properties of soil taxons and soil forming factors is not applicable. Indeed, soil taxon features are characterized by processes including those on a broader scale of the history of landscape evolution. Consequently, the coupling of soil processes exists across various tensorial orders. For flow J, straight and coupled coefficient L and conjugate force X, we have Js = LssXs + LsvXv where index s pertains to scalar and v to vector. Similar equations describe vector flow Jv or tensor flow Jt. Because such a general coupling can exist only in an anisotropic system, it must be concluded that anisotropy is a general characteristic of all soils. The links between pedology and hydropedology are demonstrated by the necessity of combining knowledge of soil micromorphology with that of the physical formulation of transport parameters. This aim is attainable when the pore size distribution is reflected by soil hydraulic functions, i.e. by the soil water retention equation and the unsaturated hydraulic conductivity function only being accurate when the bi-modality or n-modality of soil porous systems is accounted for and respected. As a first attempt, relevant equations are presented to evaluate both functions for soils with the distinct bi-modality. We anticipate that future research shall relate derived transport parameters to quantified soil micromorphic characteristics. A close cooperation of hydropedology with soil chemistry and microbiology will produce an insight into the role of organic substances upon the change of soil hydraulic parameters. These substances appear either as the consequence of soil pollution, or due to soil organic matter transformation. An example is presented on the change of saturated hydraulic conductivity due to the adsorption of organic cations. Similar results are expected from the cooperation of hydropedology with plant physiology when the role of plant exudates upon the change of soil hydraulic functions is studied.

Keywords: pedology; soil genesis; soil morphology; soil hydrology; soil hydraulic functions.