Soil indicators of hillslope hydrology in the Bedford and Weatherley catchments

Abstract

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.

Afrikaans: Daar is ‘n interaktiewe verhouding tussen grond en hidrologie. Water speel ‘n primêre rol in die genese van meeste grondeienskappe, terwyl dié eienskappe hidrologiese prosesse beïnvloed en beheer. Hierdie prosesse word in hidrologiese modelle geïnkorporeer om sodoende waterbronne effektief te bestuur. Hidrologiese prosesse is egter dinamies van natuur en varieer oor klein afstande; dit maak metings van die prosesse onakkuraat, tydrowend en duur. Dit is dus nodig om die prosesse te probeer voorspel. Hierdie voorspellings is veral noodsaaklik in opvanggebiede wat nie oor hidrologiese data beskik nie. Siende dat grondeienskappe beide die oorsaak en gevolg van die interaktiewe verhouding is, kan die identifikasie en interpretasie van die eienskappe waardevolle informasie oor die prosesse beskikbaar maak. Die hipotese was dus dat grondeienskappe as ‘n kenteken van die hidrologiese prossese kan dien. Indentifisering en interpretering van die eienskappe en hulle relatiewe verspreiding in ‘n heuwelhang, kan bydrae tot ‘n beter begrip van heuwelhang reaksie t.o.v. hidrologie. Dit kan ook help om konseptuele hidrologiese heuwelhang modelle te ontwerp. Hierdie modelle kan baie waardevol wees ten opsigte van die voorspelling van hidrologiese prosesse in opvanggebiede sonder hidrologiese metings. Daar word onderskei tussen drie tipes gronde gebasseer op hulle hidrologiese gedrag, naamlik: aanvullings-, deur-vloei- en respons gronde. Data van vorige studies is gebruik om die verskille tussen die onderskeie gronde te beklemtoon. Kriteria om tussen twee verskillende stoormeganismes (seisoenale en tydelike grondwater) in Suid-Afrikaanse gronde te onderskei, word ook voorgestel. Twee opvanggebiede in die Oos-Kaap provinsie in Suid-Afrika was geselekteer vir hierdie studie: ‘n Heuwelhang in die boonste opvanggebied van die Weatherley opvanggebied is geselekteer om die impak van grondtipes op die hidrologie te ondersoek. ‘n Konseptuele model wat gebasseer is op morfologiese grondeienskappe en hul relatiewe verspreiding is ontwerp. Die eienskappe wat gebruik is sluit in diepte, vlekke, en klei-inhoud. Hierdie model toon dat daar ‘n aanvullings-, deurvloei- en respons area in die heuwelhang teenwoordig is. Die konseptuele model is vervolgens getoets met die hulp van klimaat, tensiometer, neutron water meter, stroomvloei sowel as evapotranspirasie (ET) data. Die konseptuele model en ander grond inligting is ook gebruik om die bydrae van verskillende stroomvloei ontwikkelings meganismes te bepaal. In die groter Bedford opvanggebied is drie sub-opvanggebiede (B3, B4 en B5) opgemeet vir hidrologiese doeleindes. Al die grondeienskappe wat verband hou met hidrologiese prosesse is geïdentifiseer en gekoppel aan die hidrologiese gedrag wat hulle kan veroorsaak in die heuwelhang. Die grondeienskappe sluit o.a grond tipe, diepte, verwering van onderliggende materiaal en teenwoordigheid van CaCO3 in. Konseptuele modelle van verteenwoordigende heuwelhange in die geselekteerde opvanggebiede is ontwerp gebasseer op die geïnterpreteerde grond inligting. Die dominante faktore wat stroomvloei in opvangebiede B4&5 beheer is vlak gronde met relatief ondeurlaatbare moedermateriaal, wat oorland vloei bevoordeel. Daarteenoor bevoordeel die dieper gronde in B3 infiltrasie, deur-vloei in die grond asook aanvulling van water tafels (regionaal en seisoenaal) Twee vlakke van grond inligting (Land Tipe data: vlak 1 en geobserveerde data: vlak 2) is gebruik om die impak van grond inligting op hidrologiese modellering te toets. Die resultate is gebruik om die bydrae van grond inligting en die effektiwiteit van die konseptuele model te takseer. Die bydrae van sommige stroomvloei ontwikkelings meganismes is ook gedoen. ‘n Metode om gronde te klassifiseer op grond van hulle hidrologiese gedrag word voorgestel. Toekomstige navorsing moet fokus op verskeie aspekte (grond water inhoud, ET, dreineer kurwes, hidroliese geleiding, vloeipaaie en stoor meganismes) wat die hidrologie van Suid Afrikaanse gronde beskryf. So ‘n klassifikasie sisteem kan ‘n groot bydrae lewer tot hidrologiese modelering, veral in opvanggebiede wat nie oor hidrologiese data beskik nie.