Grouping genetic soil horizons in functional hydrological units, serving hydrological modelling

Hydrological modelling is hampered by a lack of reliably structured conceptual hydrological models indicating flowpaths, flow directions and flow rates. The majority of South African soils are matured and the horizonisation advanced, leaving a variety of signatures of pathway characteristics. The horizons have typical soil physical properties controlling the horizon's hydrology. Specific combinations of these horizons control pedon hydrology. They are also indicators of the control of pedon hydrology and together with topography, soilscape hydrology. This interactive was investigated. Long-term measurements of soil water contents has a good relationship with diagnostic horizons and raised soil water contents expressed as the duration of the degree of saturation exceeding 0.7 of porosity. This norm serves as an indicator of significant amounts of drainable water moving at significant rates. Pathway indicators have a limited number of characteristics which can be defined for horizons. Horizons can be defined as hydropedologically functional units. These units include horizons with: fast vertical downward flowpaths typical of red and yellow-brown apedal subsoils and the overlying topsoils; slow vertical flow typical of argilic horisons with strong angular blocky structure; fast lateral flow (LF) typical of albic horizons; slow vertical flow typical of aquic conditions; etc. As soils are not distributed randomly, neither are horizons distributed randomly, hydrological functional units linked to soil polipedons and soilscapes lends itself to the development of conceptual hydrological models indicating flowpaths in hillslopes and the distribution of the models in a catchment depending the distribution of typical catenas. The catenal relationship lends itself to predictive distribution of hydrological soilscapes in catchments. This should be very useful for developing hydrological models in ungauged basins. These results make the transfer of pedological data, available in normal soil survey reports, appropriate for hydrological modelling. It can increase the application of soil data and improve the efficiency of hydrological modelling.