Soil indicators of hillslope hydrology in the Bedford and Weatherley catchments

English: There is an interactive relationship between soil and hydrology. Water plays a primary role in the genesis of most soil properties and soil properties influences and governs hydrological processes. Incorporation of these processes into hydrological models is essential for water resource management. Hydrological processes are dynamic in nature with strong temporal variation, making measurements expensive, inaccurate and time consuming. Predictions of these processes, especially predictions in ungauged basins (PUB) are therefore essential. Since soil properties are both a cause and result of this interactive relationship, identifying and interpreting relevant soil properties, can reveal information on key hydrological processes. The hypothesis is then that soil properties can serve as signatures of hydrological characteristics. Identifying these and interpreting them and their relative distribution at hillslope scale can lead to better understanding of hillslope hydrological response and facilitate the formulation of conceptual hillslope hydrological models. These models can aid in the prediction of 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 transient groundwater) 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 impact of soil types on hydrological response. A conceptual model was developed based on soil morphological properties and their relative distribution. These morphological properties included soil depths, mottling, and clay contents. These properties indicate that there are definite recharge, interflow and responsive areas in this hillslope. The conceptual model was then evaluated with the use of climate, tensiometer, neutron water meter, hydrograph and evapotranspiration (ET) data. The conceptual model and soil information were utilised to calculate the relative contribution of streamflow generation mechanisms. Base and 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 for hydropedological purposes. All the soil properties which might influence or be influenced by the hydrology were identified and related to hydrological hillslope response. These properties include: soil type, soil depth, weathering of underlying material, and presence of CaCO3. Conceptual models of representative hillslopes in the selected catchments were developed based on the interpreted soil information. The dominant factors governing the streamflow in catchment B4&5 was shallow soils on bedrock with restricted permeability, which facilitated overland flow. In B3 the deeper soils and permeable bedrock facilitated infiltration, interflow as well as recharge of water tables (regional and perennial). Two levels of detail of soil information namely; Land Type data: level 1 and Observed data: level 2, were used to test the impact of soil information on hydrological modelling. The results were assessed 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. Future research should focus on several aspects (soil water regime, ET, drainage curves, hydraulic conductivity, flowpaths and storage mechanisms) which describe the hydrology of soil of South Africa. Such a system can benefit hydrological modelling, especially in PUB’s.