Soil indicators of hillslope hydrology in the Bedford and Weatherley catchments

[摘要] English: There is an interactive relationship between soil and hydrology. Water plays a primary role in thegenesis of most soil properties and soil properties influences and governs hydrologicalprocesses. Incorporation of these processes into hydrological models is essential for waterresource management. Hydrological processes are dynamic in nature with strong temporalvariation, making measurements expensive, inaccurate and time consuming. Predictions ofthese processes, especially predictions in ungauged basins (PUB) are therefore essential. Sincesoil properties are both a cause and result of this interactive relationship, identifying andinterpreting relevant soil properties, can reveal information on key hydrological processes.The hypothesis is then that soil properties can serve as signatures of hydrologicalcharacteristics. Identifying these and interpreting them and their relative distribution at hillslopescale can lead to better understanding of hillslope hydrological response and facilitate theformulation of conceptual hillslope hydrological models. These models can aid in the predictionof hydrological processes in ungauged basins (PUB).Hydrologically there are three main soil types namely recharge, interflow and responsive soils.Data from previous studies were utilized to accentuate the differences between these soil types.A criterion for distinguishing between two storage mechanisms (perennial and transientgroundwater) in the soils of South Africa is also proposed.Two catchments in the Eastern Cape of South Africa were selected for this study:A hillslope in the upper catchment (Uc) of the Weatherley was selected to determine the impactof soil types on hydrological response. A conceptual model was developed based on soilmorphological properties and their relative distribution. These morphological properties includedsoil depths, mottling, and clay contents. These properties indicate that there are definiterecharge, interflow and responsive areas in this hillslope.The conceptual model was then evaluated with the use of climate, tensiometer, neutron watermeter, hydrograph and evapotranspiration (ET) data. The conceptual model and soil information were utilised to calculate the relative contribution of streamflow generation mechanisms. Baseand peakflow calculations gave a very good estimation of the actual streamflow.In the greater Bedford catchment, three sub-catchments (B3, B4 and B5) were surveyed forhydropedological purposes. All the soil properties which might influence or be influenced by thehydrology were identified and related to hydrological hillslope response. These propertiesinclude: soil type, soil depth, weathering of underlying material, and presence of CaCO3.Conceptual models of representative hillslopes in the selected catchments were developedbased on the interpreted soil information. The dominant factors governing the streamflow incatchment B4&5 was shallow soils on bedrock with restricted permeability, which facilitatedoverland flow. In B3 the deeper soils and permeable bedrock facilitated infiltration, interflow aswell as recharge of water tables (regional and perennial).Two levels of detail of soil information namely; Land Type data: level 1 and Observed data: level2, were used to test the impact of soil information on hydrological modelling. The results wereassessed to evaluate the contribution of soil data and the effectiveness of the conceptual model.The contribution of some streamflow generation mechanisms was also calculated.A method for classifying soils based to their hydrological behaviour was proposed. Futureresearch should focus on several aspects (soil water regime, ET, drainage curves, hydraulicconductivity, flowpaths and storage mechanisms) which describe the hydrology of soil of SouthAfrica. Such a system can benefit hydrological modelling, especially in PUB's.

[发布机构] University of the Free State