CORRELATION BETWEEN RAINFALL, STREAMFLOW AND GROUNDWATER LEVELS FOR THE ZACHARIASHOEK SUB- CATCHMENT NEAR WEMMERSHOEK, WESTERN CAPE

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1 CORRELATION BETWEEN RAINFALL, STREAMFLOW AND GROUNDWATER LEVELS FOR THE ZACHARIASHOEK SUB- CATCHMENT NEAR WEMMERSHOEK, WESTERN CAPE Nicolette Vermaak 1, Henry de Haast 2 & Gideon Steyl 3 1 Department of Water Affairs, Geohydrology Section, Bellville Regional Office Bellville, Western Cape, South Africa; VermaakN2@dwa.gov.za 2 Department of Water Affairs, Geohydrology Section, Bellville Regional Office Bellville, Western Cape, South Africa; DeHaastH@dwa.gov.za 3 Golder Associates (Australia) Milton, Queensland, Australia; GSteyl@.com.au Abstract Zachariashoek catchment was one of the study areas looking into the hydrological characteristics of winter rainfall catchments in the Western Cape. Nearly thirty years of historical data are available for the Zachariashoek area. This data include rainfall, gauge plate readings for the weirs, and waterlevels for the boreholes in the area. Numerous articles and reports had been written about the research that had been done in the area, concentrating mostly on the effects of fire on streamflow and vegetation. This article will look at patterns that can be observed from the data record and correlate the different data sets for the Zachariashoek sub-catchment. It will use the data from the two weirs, three rain gauges and at least three of the boreholes that was drilled in this sub-catchment. The information gained from this comparison can then be use to evaluate possible future hydrological patterns and the interaction between the various components of the hydrological system. 1. INTRODUCTION Various catchments studies had been done around the world, with the Jonkershoek studies having the claim to be the first one in the Western Cape (Wicht, 1967; Blight, et al, 25). The Zachariashoek studies were done together with the Jonkershoek study and the study at Jakkalsrivier (van der Zel, 1974). This article will look at the data collected for the Zachariashoek research since the inception of the study in 1964 till it was terminated in 1991 due to a lack of funding. We will look at the correlation between the rainfall as measures at three stations, groundwater levels from four boreholes and streamflow in the form of gauge plate readings at two weirs. We also tried to find hydrological patterns and the various components of the hydrological system. Study Area The Zachariashoek research catchment was divided into three sub-catchments. These sub-catchments were part of the La Motte State Forest. It is situated between the Klein Drakenstein and the Wemmershoek mountains in the Western Cape (van der Zel, 1974; Lindley, et al., 1988). The sub-

2 catchments were Zachariashoek, Bakkerskloof, and Kasteelkloof. Zachariashoek and Kasteelkloof were further subdivided into upper and lower catchments with two weirs each. The Kasteelkloof catchments border on the North East on the catchment of the Wemmershoek Dam, one of the main sources of water for the City of Cape Town (van der Zel, 1974). This article will only look at the data collected for the Zachariashoek sub-catchment, with only the most complete data sets used for the analysis (See Figure 1). Figure 1: Satellite photograph showing the Zachariashoek sub-catchment. Note that the rain gauges are designates RG1, RG8 and RG2 in this figure (Google Earth, 212). 2. METHODLOGY Data was collected for the Zachariashoek experimental setup from June 1964 till July 1991, a period of 27 years. The rainfall data was plotted against the waterlevels in the boreholes and the gauge plate readings of the weirs respectively, to compare the correlation between rainfall, groundwater levels and stream flow. Further analysis was done with the data set. The analysis of the Zachariashoek data only begins in April 1969 as the rain gauges were only installed at that time and to avoid big data gaps. Boreholes HB676/9 and HB676/14 are also excluded from the analysis, as only very short data records are available for these boreholes. The two boreholes next to the weirs, HB676/1 and HB676/2, had to be excluded too. These boreholes were equipped with recorders and very infrequent control readings were taken. The cumulative deviation from was calculated for each variable. The cumulative deviation from for the rain gauges was plotted against the same for the four remaining boreholes and the two weirs. This was done to be able to compare patterns and behaviour in the precipitation, groundwater levels and streamflow. Yesertener (28) found that downward trending cumulative deviation from curves for rainfall represented dry periods or periods of below average rainfall, while upward trending curves represented periods of higher rainfall or wetter periods. 3. RESULTS

3 Figure 2 shows the rainfall from the three rain gauges plotted against the waterlevels in the boreholes in the Zachariashoek sub-catchment. The waterlevels are given as meters below collar. The rainfall is plotted on the primary axis while the waterlevels are plotted on the secondary axis. Two of the boreholes (HB676/9 and HB676/14) are not included in this graph, as their waterlevels are in a different range and the data record very short. A data gap stretching from 2 November 1967 to 24 April 1969 can clearly be seen. The boreholes (HB676/1 and HB676/2) next to the weirs that was equipped with recorders are the only ones for which some data exist for this period. The seemingly cyclical trend where waterlevels in some boreholes decline for a number of years, making a quick recovery followed by another period of declining waterlevels, was the subject of a previous article (Vermaak et al, 211), and will not be discussed in this paper Waterlevel [mbc] HB676/1 HB676/2 HB676/1 Figure 2: Graph showing the correlation between the waterlevels in the boreholes and the rainfall. Figure 3 shows the rainfall on the primary axis plotted against the gauge plate readings for the two weirs on the secondary axis. Notice that the same data gap exists for the weirs as for the boreholes. The rain gauges were only installed in Gauge plate reading [m] Figure 3: Graph showing the correlation between rainfall and gauge plate readings on the weirs.

4 Figure 4 shows the cumulative deviation from for the two rain gauges closest to borehole HB676/1 compared to the cumulative deviation from for the waterlevels in the borehole. The turning point at the end of the dry period for HB676/1 was in July 1974 while the turning point for was two months earlier in May The wet period ends in September 1977 while the cumulative deviation from for the borehole only reaches its highest turning point in December 1979, more than two years later Waterlevel [mbc] HB676/1 HB676/1 HB676/1 Figure 4: Graph showing the rainfall at and compared with the waterlevels in HB676/1. The cumulative deviation from is given for all three variables. Figure 5 shows the cumulative deviation from for the rain gauges above plotted with the cumulative deviation from for the waterlevels in the borehole. There is a longer data record for this borehole than for the others. The pattern for the cumulative deviation from for this borehole is different to that of HB676/1. The turning point for the waterlevels in was not July 1974, as was the case with HB676/1, even though a recovery can be noted. The real turning point was in June 1976, about two years before the rainfall measured at had its turning point at the end of the dry period in May The highest point for the cumulative deviation from plot was in December 1979, compared with the end of the wet period in September It would seem as if the borehole collapsed after May 1987, as the cumulative deviation from no longer reflects the patterns of the cumulative deviation from plots for the rain gauges. This can also be seen in the pattern of the waterlevel changes itself, as the seasonal changes are no longer as pronounced as before.

5 Waterlevel [mbc] Figure 5: Graph showing the rainfall at and compared with the waterlevels in. The cumulative deviation from is given for all three variables. Figure 6 shows the cumulative deviation from for the two rain gauges closest to and the cumulative deviation from for the waterlevels in the boreholes. The lowest point for the cumulative deviation from plot for was July 1973, about a year before the end of the dry period if judged from the plot of the data for. The highest point for the cumulative deviation plot for is December 1978, about a year after the onset of the second dry period Waterlevel [mbc] Figure 6: Graph showing the rainfall at and compared with the waterlevels in. The cumulative deviation from is given for all three variables. Figure 7 shows the cumulative deviation from for plotted against the same for and, as this borehole is closer to the lower weir and rain gauge. The turning point for

6 was in June 1976, which correlates with that of. The turning point for the cumulative deviation from for the waterlevels was October 1978 compared to September 1977 for. The turning point for was in July 1977, two months earlier Waterlevel [mbc] Figure 7: Graph showing the rainfall at and compared with the waterlevels in. The cumulative deviation from is given for all three variables. Figure 8 and Figure 9 shows the cumulative deviation from for the rain gauges measuring precipitation contributing to the flow in and respectively. The patterns seen in the cumulative deviation from for the weirs are much closer to that of the rain gauges than is the case for the boreholes. The lowest point for the cumulative deviation from plot for was June 1974, compared to May 1974 for the rain gauge. The end of the wet period was September 1977 according to, while the highest value for the cumulative deviation from for was in November 1978 about a year later. shows the same pattern for the end of the dry period, but the highest value for was October 1978 one month earlier than for.

7 Gauge plate reading [m] Figure 8: Rainfall for and compared with Gauge plate reading [m] Figure 9: Rainfall for, and compared with. Figure 1 shows the cumulative deviation from curves for the four boreholes and the two weirs. This was done to try and compare the different patterns for the measurements. The overall patterns are similar, but the precise timing may be different.

8 Waterlevels [mbc] Gauge plate readings [m] & Waterlevels [mbc] HB676/1 HB676/1 Figure 1: Graph showing the cumulative deviation from for the waterlevels in the boreholes and gauge plates readings at the weirs. 4. DISCUSSION Rainfall measurements for Zachariashoek only begin in 1969, which was the beginning of the dry period around the Gnangara Groundwater Mound, Perth (Yesertener, 28). It would thus not be possible to compare the whole pattern of wet and dry cycles found in Australia with that of Zachariashoek. The dry period in Zachariashoek seems to stretch from September 197 to May 1974, followed by a wet period from May 1974 till September Another dry period begins in September 1977 and lasts to May No clear pattern can be seen after May It is possible that the rainfall intensity was not high enough to bring out a clear pattern, even though high rainfall figures may have been recorded (Vermeulen, 213). Very bad flooding occurred shortly after the measurements at Zachariashoek was terminated (Prinsloo, 212). The time lags between the end on the dry period and the recovery in the boreholes seem to vary, with two months for HB676/1 to more than two years for. The decline in waterlevels after the onset of the dry period was about two years, again varying between boreholes. It would thus seem as if recovery in the waterlevels of the boreholes occurs faster than a decline in waterlevels after the onset of dry period. The effects of drought in the Zachariashoek area would thus only begin to show after a period of about two years because of the retention of water in the rock formations. There are a number of factors that can have an influence on the results, but that was not measured or recorded. These include changes in rainfall patterns which would include number of days for each event and rainfall intensity (measurements was done on a weekly basis). Another factor that could have had an effect was the direction from which wind and thus rainfall comes. Wind speed was measured in

9 Bakkerskloof, but not wind direction. The location of the rain gauges and boreholes would also influence the findings HB676/1 is on the south western slope of the valley, while HB 676/13 is on the north eastern slope; is at the confluence of a number of tributaries; proximity to the stream can also affect readings. The cumulative flow of tributaries contributing to the flow at caused this weir to reach its highest value faster than situated higher up in the valley. This shows that even in weirs it is important to have the best possible placement. 5. CONCLUSIONS AND RECOMMENDATIONS Waterlevels in boreholes and gauge plate readings of weirs are directly correlated to rainfall. Further statistical analysis will be needed to calculate the precise degree of correlation. Time delay in recovery from dry period for groundwater levels can be as little as two months, but delay with end of a wet spell can be as long as two years. This s that retention of groundwater in rock formation can delay the effects of drought. A longer data record would have made the wet/dry cycles more predictable, as it would seem from the short data set available that dry periods seem to vary between three and six years in this area, with two or three years of above average rainfall. Longer data sets would also have contributed to our understanding of climate change and enable risk management to prepare for the possible changes that will be possible. Even though this evaluation has shown a high degree of correlation between rainfall, streamflow, and groundwater levels in the area of Zachariashoek, there is a need to study this further. It may also prove worthwhile to rehabilitate the boreholes in the Zachariashoek area, to make it possible to collect data for a longer data series to be used in climate change studies and risk assessments and to collect baseline data in case the City of Cape Town should decide to use this area for one of their TMG wellfields. 6. REFERENCES Blight, J.J., Gush, M.B., Le Maitre, D., and Jewitt, G.P.W. (25) The Impact of Afforestation on Low Flows: Paired Catchment Data Revisited, SANCIAHS, 25. Google Earth (212). Accessed: 2 September 212. Lindley, A.J., Bosch, J.M. & van Wyk, D.B. (1988) Changes in water yield after fire in fynbos catchments. Water SA Vol. 14 No. 1 January Prinsloo, F. (212) Personal communication. van der Zel, D.W. (1974) Catchment Research at Zachariashoek. Forestry in South Africa, No. 15, June 1974, Government Printer, Pretoria. Vermaak, N., De Haast, H., and Brown, Z. (211) Mountain Catchment Studies revisited from a groundwater perspective, article written for the Young Water Professionals Conference 211, Pretoria. Vermeulen, D. (213) Personal communication.

10 Wicht, C.L. (1967) Forest hydrology research in the South African Republic, in Sopper, W.E. and Lull, H.W. (eds.), Proc. Int. Symp. on Forest Hydrology, Yesertener, C. (28) Assessment of the declining groundwater levels in the Gnangara Goundwater Mound, Hydrological Record Series: Report HG14 January 28, Department of Water, Government of Western Australia, Perth.