An investigation into the influence of soil pattern on preferential flow and groundwater recharge in fractured bedrock and cover sand aquifers

Stander, McLachlan Du Toit (2011-12)

Thesis (MScAgric)--Stellenbosch University, 2011.

Thesis

ENGLISH ABSTRACT: Increased pressure on groundwater sources due to increased population size and threats of climate change is driving research to better understand the process of aquifer recharge. Soil pattern is of interest as it serves to partition rainwater into different flowpaths destined for surface runoff, evapotranspiration and deep percolation. The challenges inherent to studying these flowpaths are almost universal as uncertainties concerning spatial and temporal heterogeneity in catchments make the upscaling of models complex. This research addresses these challenges as it aims to improve the catchment scale hydrological models of two aquifer systems: One a fractured bedrock system at the Kogelberg Nature Reserve, Kleinmond, and the other a cover sand system in Riverlands Nature Reserve, Malmesbury. This study focussed on strengthening the link between what is known about a given soil form and the hydrological assumptions that can be drawn from that classification, and formulating the results so that they may ultimately be used to calibrate the recharge prediction models for the respective catchments. The research was done in two parts: The first phase was to conduct soil surveys in both reserves during which soils were classified according to South African Soil Classification. Samples were collected at representative observation points which provided textural data for use in pedotransfer functions (PTFs). These PTFs were used to estimate plant available water (PAW) and hydraulic conductivity (K) for the observed profiles. Infiltration experiments were subsequently done to investigate the infiltration patterns of distinctly different soil forms at two sites from each reserve. The experiments included double ring and mini disc infiltration, volumetric water content determination and flow path visualisation using a staining dye. A statistical comparison between the hydrological properties (K and PAW) of the different soil forms suggest that hydraulic properties differed between the deep sandy soil forms (Fernwood, Pinegrove and Witfontein in Kogelberg and Witfontein, Concordia and Lamotte in Riverlands) and the shallow rocky soil forms (Cartref and Glenrosa in Kogelberg). Thus grouping of hydrological similar units (HSUs) could be done on the basis of the soil forms present within the given catchments. The infiltration study showed that shallow, rocky soils that grade into bedrock would have infiltration rates far greater than those estimated using PTFs in Kogelberg. This is due to the prevalence of continuous preferential flow (PF) of water between coarse fragments in these profiles. Recharge estimates would thus be inaccurate in such soils and calibration using locally derived data is recommended. On the contrary, PTFs produced accurate infiltration estimates relative to measured infiltration rates in deep sandy soils in Kogelberg and Riverlands. The Lamotte soil form is an example of such a soil form. It should however be noted that an increase in PF in these soils had subsequently higher K values than estimated, thus illustrating the link between PF and accelerated infiltration rates. These results confirm that using soil survey information, in the form of a soil map, and calibrated hydrological properties, one can delineate HSUs that encompass a large degree of heterogeneity in a given catchment.

AFRIKAANSE OPSOMMING: Verhoogde druk op grondwaterhulpbronne weens die groeiende bevolking en klimaatsverandering dryf tans navorsing om akwifeer hervulling beter te verstaan. Die grondlaag is van belang sienend dat dit reënwater verdeel in oppervlak afloop, evapotranspirasie en diep dreinering. Die uitdagings in hidrologiese navorsing is universeel as gevolg van onsekerhede oor ruimtelike en tydelike variasie wat lei tot komplekse grondwatermodelle. Diè navorsing mik om die tekortkominge in akwifeer hervulling aan te vul deur groundwatermodelle van twee akwifeersisteme te verbeter: Die een is 'n gebroke rots sisteem in die Kogelberg Natuur Reservaat, Kleinmond, en die ander is 'n sand-bedekde sisteem in Riverlands Natuur Reservaat, Malmesbury. Die navorsing streef om die verhouding tussen 'n spesifieke grondvorm en sy hidroliese vloeipaaie te bestudeer en om die gevolgtrekkings so te formuleer dat dit kan gebruik word om die onderskeie grondwatermodelle te kalibreer. Die eerste fase van die navorsing was om 'n grondopname van die onderskeie reservate te doen waartydens die gronde geklassifiseer was volgens die Suid Afrikaanse Grondklassifikasie Sisteem. Grondmonsters is by verteenwoordigende observasiepunte geneem en geanaliseer om tekstuurdata vir pedo-oordraagbare-funksies (PTFs) te kry. Die PTFs was gebruik om plant beskikbare water (PBW) en hidrouliese geleiding (K) te voorspel vir die verskeie observasiepunte. Infiltrasie eksperimente was daarna gedoen om die infiltrasie patroon van twee verskillende grondvorms van elke reservaat te bestudeer. Die eksperimente sluit dubbel- en minidisk-infiltrasie, volumetriese waterinhoud bepaling en vloeipad visualisering met die gebruik van 'n kleurstof in. Die statistiese vergelyking van die hidrouliese eienskappe (K en PBW) en grondvorm dui aan dat die hidrouliese eienskappe verskil tussen die diep, grondvorms met 'n oorwegende sand tekstuur (Fernwood, Pinegrove en Witfontein in Kogelberg en Witfontein, Concordia en Lamotte in Riverlands) en die vlakker, klipperige grondvorms (Cartref en Glenrosa in Kogelberg). Groepering van hidrologies soortgelyke eenhede (HSE's) kan dus op die basis van die teenwoordige grondvorms in 'n opvangsgebied gedoen word. Die infiltrasie studie het bewys dat vlak, klipperige gronde wat tot die rotsbodem gradueer 'n baie hoër infiltratsie tempo sal hê as die PTF voorspelde waardes. Dit is as gevolg van die voorkoms van aaneenlopende voorkeurvloei (VV) van water tussen die growwe materiaal in die profiele, veral die gebroke rots ondergorond. Voorspellings van akwifeer hervulling sal dus onakkuraat wees en kalibrasie met plaaslike data word dus aanbeveel. In teendeel met die begenoemde, het die PTFs akkurate voorspellings gemaak relatief tot die gemete infiltrasie tempo's in die diep sanderige grondvorms in Kogelberg en Riverlands. Dit was duidelik met metings dat 'n toename in aaneenlopende VV hoër gemete K waardes getoon as die voorspelde waardes. Die verband tussen VV en verhoogde infiltrasie tempo word dus hiermee geillustreer. Die resultate bevestig dus dat grondopname data, in die vorm van 'n grondkaart en gekalibreerde hidrouliese eienskappe gebruik kan word om hidrologies soortgelyke eenhede uiteen te sit wat die meerderheid van die variasie in 'n gegewe opvangsgebied insluit. Die HSE's kan gebruik word om grondwatermodelle meer akkuraat te laat funksioneer en dus beter voorspellings te genereer.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/18089
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