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Figure 1 L o c a t i o n o f R e s e a r c hcatchments
1 WATER QUALITY RESPONSE TO LAND USE FOR SHORT DURATION, HIGH INTENSITY STORMS
BRUCE KELBE and NINA-MARIE SNYMANDepartment of Hydrology,
University of Zululand,Private Bag X1001,
Kwa Dlangezwa, 3886.
ABSTRACT
An integrated approach has been adopted to determine the water quality response from different landuses in a small rural, urban fringe catchment following the runoff from short duration, high intensitystorms. A Geographic Information System (GIS) and Digital Elevation Model (DEM) have beenused to delineate areas of different land use (potential pollutant sources) in the catchment in orderto estimate the time and stage within the discharge hydrograph where the anticipated water qualityresponse would be observed. These are compared to measured responses from the catchment.
INTRODUCTION
One of the major problems facing South Africa is environmental degradation and its influence ondeteriorating water quality (Stocking, 1984). This degradation and deterioration is increasing inresponse to expanding population pressures on rural areas and changing catchment managementpractices. Rates of soil erosion are exceeding the rate of soil formation in many areas (Stocking,1984) and consequently there is an urgent need for improved understanding of conservation practiceswhich are effective and also acceptable to the catchment inhabitants. Conservation practices in turnrequire an improved understanding of the hydrological response to the catchment degradation andthe development of effective prediction techniquesfor evaluation and implementation of possibleremedial and mitigating methods.
The hydrological response from a disturbedcatchment in the urban fringe area near Empangeni- Richards Bay - Eshikawini (Figure 1) has beenidentified with changing land use (Kelbe et al,1991). However, it is not clear which aspects ofthe land use (roads, cultivation, housing, livestock,etc) are the main contributors to the indicatedhydrological response. This catchment providesthe opportunity to investigate the hydrologicalprocesses which are producing the recognizedhydrological response. Identification of theprocesses affected by degradation and denudationwould assist with research on remedial ormitigation (conservation) methods which couldinfluence the sustainable nature of water resources
Figure 2 Three stage model hydrographs and composite storm hydrograph derived from fivehistorical storm events.
in Natal.
The tributary of the Ntuze river flows through several gauging stations which include W1H017nested within W1H016 (3.2 km2). The gauging stations have been instrumented to provide highresolution temporal data of the water quantity and quality emanating from these catchments. Theprinciple objective of this paper is to relate several aspects of the discharge characteristics under shortduration, high intensity storms to hydrological processes affecting water quality from a rural, urban-fringe catchment along the Natal coastal belt with the ultimate aim of modelling the effects of landuse on water quality.
HYDROGRAPH CHARACTERISTICS FOR SHORT DURATION HIGH INTENSITYSTORMS AT W1H016
The hydrographs at W1H016, from short duration, high intensity storms show several distinct peaks(Kelbe et al, 1991) which have been attributed to the geomorphic features of the catchment byMulder and Kelbe (1992). These storms generally produce triple peak hydrographs (Figure 2) forspatially uniform rainfall. These hydrographs can be considered as the convolution of three separateunit hydrographs emanating from three specific areas of the catchment. The convoluted hydrograph,after including attenuation and lag, is also shown in Figure 2.
The estimated time to each peak from the start of the storm hydrograph and the corresponding meanhydraulic path-length of the central point in the sub-catchments (see subsequent sections) providedan indication of the characteristic velocity of runoff from each area within the catchment. This wasestimated to be less than 0.09 m/s (range 0.07 to 0.10 m/s). This is not a typical channel or streamvelocity but is more representative of overland flow rates.
Figure 3 Storm discharge (#) and conductivity(x) for DOY 317.
Figure 4 Storm discharge (#), turbidity (o)and suspended solids (x) for thestorm on DOY 317.
The discharge hydrograph following a shortduration (< 1 hour) and relatively highintensity (>30 mm/h) storm on November12, 1992, is presented in Figure 3. Since thecontinuous monitoring systems were installed(mid 1992) at the outlet weirs (W1H016 andW1H017), this is the only storm to haveoccurred which produced sufficient runoff fordetailed water quality analysis. There wereseveral other longer duration storms duringthis severe drought period but they were notsuitable for analysis.
WATER QUALITY CONDITIONS FORSHORT DURATION HIGH INTENSITYSTORM ON DOY 317 AT W1H016
Continuous water quality measurements ofconductivity, turbidity, pH and temperaturewere supplemented by flow related samplesof nitrates and phosphates for the triplepeaked storm presented in Figure 3. Thefirst peak corresponds to an immediate dropin conductivity (Figure 3), and a strong peakin turbidity and suspended solids (Figure 4).All these changes were more pronouncedthan any changes during subsequent peakflows. This immediate response could be dueto the stream channel flow or it could bederived from the surface runoff emanatingfrom the sub-catchment close to the weir.Since the estimated flow velocities are low(0.09 m/s) it is assumed that the initial fastresponse must emanate from the catchment inthe immediate vicinity of the weir. Also, theinitial stream (base) flow should be similar topre-storm conditions.
The continuous measurements also show animmediate drop in pH and a rapid rise innitrate concentration. These changes occurin the period associated with the first peakand could be linked to local sources. Inparticular the nitrate rise could be due to theclose proximity of commercial sugar canefields with normal fertilizer applications (seelater section).
The turbidity and discharge volume both reach peak values within the same minute (Figure 4). Theturbidity then decreases rapidly to pre-storm conditions while showing two minor intermittent peakswhich correspond closely to the discharge peaks. The third peak in turbidity was measured at approx50 mV which is very close to the peak turbidities in the upper sub-catchments at weir W1H017. Thisagrees with the above concept of three contributing hydrographs and suggests that the third peak isthe routed hydrograph from W1H017. The nitrate concentrations on the other hand continue to riseand peak at approximately the same time that the surface runoff ceases and the discharge becomespredominantly return or through flow.
Changes in conductivity and pH during the hydrograph also show differences between the threecontributing areas. The conductivity reaches a minimum just after the peak discharge (Figure 3)and then starts to rise with several possible discontinuities associated with the other peak discharges.The conductivity ceases to change just after the third discharge peak when the surface runoff hasstopped and flow becomes predominantly return or through flow. The pH shows a similar declinefrom 7.0 to nearly 5.0 at peak flow. It then starts to slowly recover to 6.0 after about 25 hours.
In order to describe all these difference in the water quality response from the three sub-catchmentsthe land use, soils and topographical features have been examined through spatial analysis using GISand DEM.
PHYSICAL FEATURES OF THE NTUZE CATCHMENT
The topographical features were captured and transformed into maps of slope and aspect which wereused to derive hydraulic flow paths from each area (10x10 m2 pixel) to the catchment outlet (weir).These were then used to determine the hydraulic distance of each dominate land use feature withineach sub-catchment in order to estimate where that feature would be expected to exhibit the strongestresponse to changes in water quality within the discharge hydrograph.
The physical features of the research catchment have been derived from various maps and analyzedusing either IDRISI, a Geographical Information System (GIS) or LAMONT, a Digital ElevationModel (DEM). Both systems have their unique features but they also had several commoncomponents which were compared and the best results used in the analysis.
The topographical image of the research catchment was captured through IDRISI by digitizing the10 m contours from a 1:10 000 topographical map of the Ntuze. To reduce the numerical problemassociated with crest divides in areas with no slope, several intermediate (5 m) contours fromorthophotos were also digitized for problematic areas of the catchment boundaries. The vector imageof the topographical contours was used to create a DEM in IDRISI for comparison with the DEMcreated in LAMONT. Both were compared to catchment boundaries derived from visual
Figure 5 The research catchment topography and stream networkFigure 6 Rasterized images of land use in W1H016 from 1991 aerial-photos.
observations. The topographical features and stream network are shown in Figure 5.
PATH LENGTH AND TRAVEL TIMES
The hydraulic pathway that a molecule of water will travel from its point of entry into the catchment(rainfall) to the outlet weir was derived using the numerical techniques (GIS and DEM). Thesemethods used the aspect of each pixel to define the direction of surface flow from one pixel to thenext. The hydraulic pathway was then used to estimate theassociated travel time from each pixel to the catchment outletby applying Manning's Equation for surface runoff from shortduration, high intensity storms. R is the mean hydraulicdepth of surface runoff, S is the slope of each surface element(pixel) and n is the Manning coefficient. The Manning coefficient depends on surface roughness andis a function of land use. The n values chosen for each land use were extracted from publishedliterature. The mean hydraulic depth was assumed to be constant and was estimated from averagecatchment conditions to be 0.4 mm. Under these conditions the mean transit time across each pixel(10 m) is approximately 2 to 3 minutes at a velocity of 0.07 m/s on a representative slope of 15%(Mulder and Kelbe, 1992). The application of this model requires the specification of land use foreach pixel in order to define the Manning coefficient.
LAND USE
The land use in this research catchment comprises indigenous and exotic forests, commercial sugarcane production, subsistence agriculture and livestock breeding by inhabitants who live in smallcommunal settlements which comprise several mud and thatched huts which are generally constructedin a circular arrangement containing the animal stockade in the centre. Nearly all the inhabitants
collect water from springs or boreholes and have created a network of irregular foot paths betweenthe settlements and water sources (Kelbe et al ,1991). There is only one road through the catchmentwhich follows the catchment divide in many areas.
Figure 7 Rasterized image of isochrones from weir W1H016.
Figure 8 Distribution of all isochrones and those from sugar cane fields in W1H016.
The present land use was captured (Figure 6) from aerial photographs taken in August 1991. Theseimages were combined with the hydraulic distance to create the rasterized images of the transit timesacross each pixel . These transit times were then integrated along each hydraulic pathway to givean image of the travel time from specific land use areas to the outlet weir (W1H016). The spatialarrangement of travel times (isochrones) along the hydraulic pathways to the outlet of bothcatchments is shown in Figure 7. These rasterized images were used to derive the frequencydistribution of hydraulic travel times (isochrones) within each catchment (Figure 8). The frequencydistribution of isochrones shows three distinct peak which are separated by ranges of isochronevalues with very low frequency. These zones of low frequency indicate a restricted flow regimeseparating three separate sub-catchments along the course of the stream network. Consequently, theyare expected to correspond to the model of three convoluted hydrographs shown in Figure 2 for shortduration, high intensity storms when surface runoff occurs.
If there is a distinct difference in land use within each sub-area, then the hydrological response ofthat area would be expected to occur within the appropriate portion of the discharge hydrographassociated with that sub-area. The distribution of sugar cane production is predominantly in the firstarea close to the weir (Figure 8). Consequently, any response to sugar cane production on waterquality should be detectable in the first half of the discharge hydrograph. For example, harvestingor cultivation of the cane fields during periods of short duration and high intensity storms would beexpected to produce increased erosion and subsequent sediment transport at the beginning of thesubsequent hydrographs. This was evident in the only storm analyzed. However, fertilization of canemay produce excessive nutrient leaching from within the soil which would be expected to appear inthe return or through flow component of the discharge hydrograph. The rapid response in nitrateconcentration indicates a surface source in this area.
CONCLUSIONS
Continuous measurements of water quality parameters have provided a means of evaluating thetemporal variations of runoff from disturbed catchments during specific stages of the dischargehydrograph. The nature of the catchment and discharge hydrograph have enabled specific periodsof runoff to be linked to specific areas of the catchment. Spatial analysis (GIS and DEM) haveprovided a means for identifying specific land uses within sub-catchments to be linked to dischargecharacteristics which may be used to identify the effects of land use on water quality emanating fromrural catchments. An association between land use and hydrological response was based on onestorm. Further storms are required for confirmation and further investigation of the processesinvolved. However, the techniques presented may be a suitable method for setting up a program formonitoring non-point source pollution in small catchments.
This approach indicates that, if a particular pollutant is observed with any one of these peaks, thenthat pollutant should be linked to the hydrological processes operating within that particular area ofthe catchment if it is assumed that there is no lag effect which could delay its travel time along thepath-way. Assuming that the distributions of path-lengths and isochrones represents the dischargehydrograph for short duration, high intensity storms then it follows that any constituent carried bythe discharge should also appear at the weir outlet at the same instant. Consequently, if the runoffis polluted to a significant extent along a particular pathway, then that pollutant should be detectableat the weir at the same point in time (stage of the hydrograph) as the discharge volume reaches thefull extent of its path-length. For example, if a conservative nutrient is dissolved at a particular fieldin the catchment which has a specific path-length to the weir, then that nutrient should be observed
as an increase in that nutrient at the same time that the discharge reaches the outlet. This approachwould then provide a method of predicting where a water quality response should be detectable andassist with sampling programmes for non-point source pollution experiments.
ACKNOWLEDGEMENT
This paper represents part of the research currently funded by the Water Research Commission. Wewould like to express our gratitude to the WRC for their support and to our colleagues in theDepartment of Hydrology for their valuable assistance, particularly Mr Anton Verwey.
REFERENCES
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