DECLARATION 2- PUBLICATIONS
4.1 Nested Catchment Monitoring, Materials and Methods
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CHAPTER FOUR
4 METHODOLOGY
Chapter 4 reports on the methodologies used to achieve the objectives of the study. Initially a nested catchment monitoring layout is specified which takes into consideration scale issues from local, field to catchment levels. Materials and methods used to aid in collection of observed data at plot, field and catchment scales are also given. The various laboratory procedures and analysis that were used to sample nutrients, sediments and isotopes is elaborated. The development of a modified ACRU-NPS model is presented with the envisioned incorporation of the connectivity concept into the model. This makes it possible to study hydrological connectivity between land segments and the linked control structures (in this case buffers, wetlands and dams). This approach takes into account the runoff, NO3, P and SS exchanges between the land segments and river channel together with their fate on entering and leaving buffers, wetlands and dams.
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Figure 4.1: The nested Mkabela Catchment showing instrumentation and sampling points at the Wartburg research site (Lorentz et al., 2011).
The automatic recording meteorological weather station installed in the headwaters of Mkabela research catchment shown in Figure 4.2 comprises the following instruments:
• CR 200 Campbell Scientific data logger
• RM Young wind sentry anemometer - model 03101
• Vaisala Temperature/ RH probe - HMP 50-L
• Texas Electronic Rain gauge - TES25 mm-L
• Apogee Silicon Pyranometer sensor.
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Figure 4.2: Downloading weather data from the automatic weather station located near Wartburg, Mkabela Catchment, KwaZulu-Natal.
Other additional materials and methods used to aid in the collection of observed data at plot, field and catchment scales included:
Metallic flow isolators, collector troughs and baffle tanks for runoff plots.
Tipping buckets, event data loggers and manual counters used in runoff plots.
Watermark sensors for automatic recording of soil water tensions at various depths in soil horizons.
Pressure transducers for river gauging.
Constructed H-Flumes and ISCO samplers for discharge measurement and WQ sampling, respectively.
Liquid-Water Isotope Laser Analyser for determination of δ18O and δ2H isotopes in water samples collected from various locations in the catchment.
Manta-2 WQ (water quality) instrument with ion, pH and EC probes for in-situ water quality detection.
Geophysical surveys using ERT (Electrical Resistivity Tomography) techniques.
Laboratory methods for sediment and water quality analysis.
4.1.1 Local scale
Local scale monitoring was conducted on two runoff plots (RP1 and RP2) and included the measurement of discharge rates of overland flow on an event basis in the sugar cane fields located in the upper part of the catchment. 1/10th of the overland flow from each plot was
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routed into a container for subsequent sampling of sediments and nutrients (Figure 4.3 left).
Borehole samples were also collected and tested for nutrients (NO3 and P) and suspended solids (SS) (Figure 4.3 right).
Figure 4.3: Downloading data from runoff plot (left) and drawing groundwater from borehole using a bailer (right).
Two-D surveys conducted using ERT equipment were also obtained to assist in the hydrological characterization of the catchment and were used in a reconnaissance fashion to assist in the interpretation of hydrological processes (Figure 4.4). The surveys were performed during late winter (dry season, March–October/November) period of 2009/2010 as soil moisture contents were at their minimum and therefore variability in electrical resistivity measurements were assumed to have been minimally affected.
Figure 4.4: ABEM Terrameter used for 2-D Electrical Resistivity Tomography (ERT) (ABEM, 2005)
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Field scale observations were made at an H-flume where discharge was measured and samples automatically extracted for sediment and nutrient analysis. The upper flume consists of a constructed H-Flume with an ISCO sampler triggered by a specified flow volume, recorded by measuring the depth of flow using a pressure transducer. Multiple field or small catchment monitoring was accomplished downstream in a similar H-flume with the exception that the depth of flow in a stilling basin was measured via a float and pulley apparatus (Figure 4.5).
Figure 4.5: Taking in-situ readings using Manta-2 WQ instrument (left) and downloading data from CR 200 data logger (right) at the Lower H-flume.
Monitoring of the water levels and subsequent discharge was based on the principle of a piezometer, where the water level in interconnected columns would always be the same thus making it possible to monitor the water levels in the approach channel by recording the water levels in the stilling well through the use of a pressure transducer (Flume 1), float (Flume 2) and data logger mechanism (both). In Flume 2, the floater in the stilling well oscillates with the rise and fall of the water level in the approach channel and such movements are translated into rotational movements through a pulley system in a shaft encoder, which is linked to a CR 200 data logger. In Flume 1, the height recorded (m) by the pressure transducer results from dividing the pressure measured by the transducer (N/m2) with specific weight of water (N/m3) through equations entered into CR 200 data logger, hence converting pressure (N/m2) into height (m).
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The Mkabela Catchment H-flumes were equipped with ISCO samplers, with capacities of 24 sampling bottles of 500 ml each, and controlled by a CR 200 data logger. The number of samples and the sampling rate of the ISCO sampler were varied by the conditional parameters in the CR 200 data logger shown in Table 4.1. The sampling strategy in the Mkabela Catchment was to take infrequent samples during steady flows (low flows) and frequent samples during rapidly changing flows (events).
Table 4.1: Conditional parameters for the CR 200 data logger at the Mkabela H-Flumes Parameter Flume 1 Flume 2 Description
Vhf 100 1000 Cumulative flow volume for changing flow i.e. high flow volume threshold (m3)
Vlf 300 6000 Cumulative flow volume for constant flow
i.e. low flow volume threshold (m3) Delta HR 0.002 0.01 Elevation change (mm) for recording Q Delta HS 0.10 0.10 Depth (m) for change in sampling flow
volume calculation.
Delta T 5 5 Time interval for depth of flow recording (s)
Maximum samples 24 24 Maximum number of samples to be taken
Sampling head 3 3 Suction head (m)
Suction line 7 7 Total length of the suction line (m)
A1(0) 0 0 Polynomial variable
A1(1) 0.004 0.0013 Polynomial variable
A1(2) 0.59 1.747 Polynomial variable
A1(3) 0.012 0.062 Polynomial variable
A1(4) 0.71 0.2996 Polynomial variable
With reference to Table 4.1, Delta HS is used to establish if the flow is constant (low flows) or changing (high flows). If the change in flow rate is such that Delta HS is less than 0.10 m, then a sample will be taken after Vlf m3 of flow has passed the H-flume. Alternatively, if the change in flow is such that Delta HS is greater than 0.10 m, as in runoff events, then the samples will be taken after Vhf m3 of flow. The parameters Vhf, Vlf and Delta HS were derived through a calibration process, by simulating observed flow data while checking the sampling scheme of the data logger before adjusting the appropriate variables (Vhf, Vlf and
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Delta HS). An example of the computer program written for the ISCO sampler that was used at Flume 1 is presented in Appendix A.
4.1.3 Catchment scale
Manual sampling and flow depth observations were periodically made at selected flow controls such as road crossings, wetlands, dams and bridges (Figure 4.6). These grab sampling stations were located at different positions in the catchment. This was done after every week during summer events (frequently) and less frequently after every fortnight during winter.
Figure 4.6: Dam 1 (left) and Bridge 1 (right) located at the middle sub-catchment of Mkabela where grab sampling was done.
In order to get an indication of the sources of water, sediments and nutrients from the headwaters to the catchment outlet, sampling events were conducted such that the complete stream network was sampled within a 2 hour period in certain instances. This sampling campaign called for comprehensive sampling throughout the catchment and during times when negligible precipitation had occurred in the preceding several days.
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