DECLARATION 2- PUBLICATIONS
5.2 Plot Scale Geophysics, Soil Water and Nutrient Dynamics
5.2.2 Soil water and nutrient dynamics
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Figure 5.16: Transect W5 located immediately upstream of Flume 2 (Lorentz et al., 2011).
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Figure 5.17: Soil water tension variation: Watermark Nest 1, from 17th – 25th Nov’11.
Interestingly all 3 watermarks responded at the same time on 21st November after an additional rainfall of 14.4 mm was added to the antecedent moisture content (AMC) in the soil. This meant that hydrological connectivity was complete all the way from the shallowest watermark (250 mm-depth) to the deepest watermark (1000 mm-depth). Soil moisture had infiltrated completely through the soil layers and connected the shallowest to the deepest watermark. A further 21.3 mm rainfall was added to the soil column after the event of 23rd November 2011, resulting in watermarks 2 (400 mm-depth) and 3 (1000 mm-depth) being fully saturated (negative soil tension). Watermark 1 (250 mm-depth) was just below the water table and exhibited a soil tension of 0.
Similarly dissolved nutrient migration in the soil column (i.e. NO3 and soluble-P movement) is expected to follow the same trend as soil water. Initially the leached NO3 and soluble-P should percolate through fissures or preferential flow lines in the soil, and would be expected to reach the shallowest watermark first and the deepest watermark last. Soil moisture and nutrient dynamics would be expected to behave in the same manner when discharge is observed at the flumes. It is only when complete connectivity of soil moisture has been achieved within the catchment that the flow volumes in the flume would be expected to
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increase. It is also expected at this juncture that there would be elevated NO3 and soluble-P loads when higher peak discharges are observed at the flumes.
Figure 5.18 shows the hydrologic responses to the additional soil water moisture during the summer rainfall events of 9th Feb- 17th Mar 2009 at Nest 3 and Nest 4. Only two watermarks at Nest 3 were operational; they were positioned at a depth of 250 mm and 1000 mm. At Nest 4, three watermarks were located at depths of 200 mm, 400 mm and 1000 mm. From the ERT survey, these two nests were located away from the waterway with watermark Nest 4 being the furthest, where sandy soils were present. ERT images in Figure 5.14 had shown these sandy soils to be very dry. The thin layer of sand in Nest 3 holds very little water which is reflected by very high soil water tensions of up to 30000 mm (Figure 5.18 top). The deepest watermark at 1000 mm-depth was a bit wet with soil water tensions as low as ~5000 mm.
Watermark Nest 4 is located adjacent to the bitumen road where it receives runoff from the impervious surface of the road during rainfall events. Figure 5.18 bottom shows lower soil water tensions of up to ~4000 mm. Unlike the previous case, all the watermarks in Nest 4 responded at the same time to rainfall events of 18.6 mm and 51 mm on the 11th and 28th February 2009, respectively. This could be attributed to the ease with which sandy soils saturate allowing water to infiltrate quickly because of the bigger pore sizes in the soil particles.
Immediately after the rainfall event of 28th February 2009, the sandy soils began to dry up at a faster rate resulting in increased soil water tensions (Figure 5.18 bottom). All the watermarks in Nest 4 started drying immediately after the rains stopped. The shallowest watermark in Nest 3 (200 mm-depth) however did not dry up. Such behaviour has implications for the migration of NO3 and soluble-P in the catchment. This illustrates that with heavy rains sustained for longer periods such soils will allow for the movement of pollutants dissolved in surface water to the sub-surface since hydrological connectivity will be much facilitated.
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Figure 5.18: Soil water tension variation: Nest 3 & 4, 9Feb-17 Mar’09 (summer). Figure 5.19: Soil water tension variation: Nest 5 & 6, 25-28 Jul’11(winter).
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The 53 mm rainfall event of 26th July 2011 occurring in winter showed watermarks at Nest 5 were more responsive than those at Nest 6 to the movement of moisture in the soil column.
Since the watermark at Nest 5 was closer to the waterway than that at Nest 6, the soil water tension was lower at Nest 5 (>500 mm) than at Nest 6 (>1000 mm) before the rainfall event (Figure 5.19). This means that it was drier at Nest 6 than at Nest 5.
After the rainfall event, all the three watermarks at Nest 5 were responsive to the infiltrated rain water in the soil column. It was however different at Nest 6 where only watermarks at depths of 300 mm and 600 mm responded to the rainfall event. The deepest watermark (1000 mm-depth) at Nest 6 did not respond to the 53 mm rainfall event as it remained dry.
This could be attributed to its distant location from the waterway compared to that at Nest 5.
If additional soil moisture was added during a rainfall event, it would be much easier for runoff to occur at Nest 5 than it would at Nest 6. The riparian zone responded much faster to rainfall events due to its high antecedent wetness and presence of shallow soils. The occurrence of this hydrologic connectivity is essential before flushing of solutes and nutrients downslope through the riparian zone to the stream.
Figure 5.20 shows the soil water tension dynamics obtained from the installed watermarks for Nest 2 (closer to a waterway and the presence of clay soils) and Nest 4 (furthest from the waterway and presence of sandy soils, but adjacent to the impermeable bitumen road) for the period of 23rd -26th January 2010. Initially soil water responses from all the watermarks at both of these stations showed the intermediate soil layer (400 mm-depth) to be the driest. It was also noted that the upper soil layer (200 mm-depth) at both nests was the wettest, confirming the presence of antecedent soil moisture from previous rainfall events. Between 23rd and 25th January 2010, the upper soil layer for both nests began to steadily dry with the layer at Nest 4 drying at a faster rate because of its sandy nature. The intermediate (400 mm- depth) and deepest (800 mm-depth) soil layers at Nest 4 were kept at lower soil water tensions (~2000 mm) compared to Nest 2 (~3000 mm) at the start. This can be explained by the presence of sandy soils at Nest 4 and it being closer to the bitumen road where even lower rainfall intensities and runoff would be directed to this area.
Nest 2 was located at a place of consolidated clay with lower electrical resistivity as earlier confirmed from the ERT survey. There were responses to a 40 mm rainfall event in the soil profile on 26th January 2010 at all three depths. The shallowest (200 mm-depth) and the
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intermediate (400 mm-depth) watermarks responded instantly to the rainfall event and approached the soil saturation point. The deepest watermark (1000 mm-depth) responded later but was far from reaching the soil saturation point as much of the rainfall did not percolate to this depth. Hence, most of the water that was available at this instant in the nearby Flume 1 together with the dissolved NPS pollutants, would most probably originate from the subsurface at depths of 200 mm and 400 mm.
The ERT survey shown in Section 5.2.1 above confirmed the existence of a very dry area containing sandy soils where watermark Nest 4 was located. Because of this sandy soil type, there were instantaneous soil moisture responses at the watermarks from the 40 mm rainfall event of 26th January 2010 for all the soil profiles (i.e. 200 mm, 400 mm and 800 mm-depths) as shown in (Figure 5.20). The soil moisture at these depths however did not approach the soil saturation point and probably no surface or subsurface runoff was generated immediately. In fact, after this rainfall event, there was immediate drying of the shallowest soil layer. The intermediate soil layer dried up gradually but the deepest soil layer remained relatively wet. This further confirms the presence of sandy soils at this particular location.
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Figure 5.20: Nest 2 (closer to waterway and presence of clay soils) and Nest 4 (furthest from the waterway and presence of sandy soils, but adjacent to the impermeable bitumen road).
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CHAPTER SIX
6 DEVELOPMENT OF THE ACRU-NPS MODEL
During this research modifications were made to the ACRU-NPS model on the basis of knowledge gained from studying the connectivity influences on nutrient and sediment migration in the Mkabela Catchment. These modifications enhanced the model’s ability to not only simulate N and P dynamics in a catchment but also the pathways and fate of nutrients (N and P) and sediments through farm dams, wetlands and riparian buffer strips.