No significant differences in nutrient levels upstream and downstream of the road stream crossings were observed. Dr Ramroop from the University of KwaZulu-Natal (Pietermaritzburg) for help with the statistical analysis of the data. 4d: The average PO43 concentrations for steep (A1 and A2) and gentle (B1 and B2) gradient path runoff, and stream water upstream and downstream of the stream.
4e: Mean TDO concentrations for steep (A1 and A2) and gentle (B1 and B2) road runoff gradients and stream water upstream and downstream of the stream.
- The Development of Forestry in South Africa
- Runoff and Sediment Production Associated with Unpaved Forest Roads
- Unpaved Forest Roads and Water Quality
- The Aim and Objectives of the Study
Forest roads have an impact on the environment and have received considerable research attention (Wemple et al., 1996). Unpaved forest roads have been cited as major sources of surface erosion causing water quality deterioration in forested areas (Sheridan and Noske, 2005; Coe, 2006). Forest roads are recognized as the most important and relatively constant source of surface erosion and water pollution in forested watersheds (Forsyth et al., 2006).
Unpaved forest roads are characterized by compacted surfaces with low permeability, which decrease hydraulic conductivity and water infiltration (Sidle et al., 2006).
The Best Management Practices to Minimize the Potential Impacts of Unpaved
The erosive potential of the water can be further minimized by increasing the road surface resistance (MacDonald and Coe, 2008). Graveling the unpaved forest roads increases the road surface's resistance to erosion (Forsyth et al., 2006), but is rarely cost-effective. The accumulated amount of runoff and the erosive force applied to the road surface are reduced by improving road drainage (MacDonald and Coe, 2008).
Road drainage can be improved through the construction and maintenance of road drainage structures, including drains and sewers (Ramos-Scharron and MacDonald, 2005).
Water Quality Analysis in Forested Catchments
Environmental Setting of the Study Area
- Location
- Landuse
- Geology and Soils of the New Hanover Area
- Climate
The bedrock geology of the study area consists mainly of Pietermaritzburg formation shales of the Ecca Group. The climate of the region is characterized by warm and humid summers, cool and dry winters, and foggy conditions. Therefore, the methods used to collect data to meet the objectives of the study will be briefly reviewed in the next chapter.
- Introduction
- Measurement of Runoff and Sediment Yield from Forest Roads
- Data collection
- Details of the Runoff Plots
- Rainfall and Runoff
- Water Quality
- Tree Breast Height Diameter Analysis
- Data Analysis Methods
According to Meritt et al., (2003) models differ in terms of complexity, processes considered and data required for model calibration and use. In order to determine the nature of the water quality from the plots, the quality parameters of the runoff water were analyzed. The comparisons allowed an assessment of the potential impacts of road runoff on local streams.
Upstream and downstream measurements allowed assessment of the impact of road runoff on the water quality of the stream system.
Introduction
Rainfall and runoff
Scatterplots for the regression of rainfall versus runoff for steep and gently sloping road surfaces are shown in Figures 4.2a-d. The coefficients of determination (R2) of the best-fit linear regression equations relating rainfall to runoff ranged from 0.14–0.32 for steep-slope road surfaces (Figure 4.2a) and 0.22–0. 43 for road surfaces with a gentle slope (Figure 4.2c). These results indicate that rainfall explains approximately 14% - 32% and 22% - 43% of the variation in runoff production for steep and gentle road segments, respectively.
The statistical F-tests at a significance level of 0.05 show that the regressions for rainfall and runoff are significant for all road diagrams (Table 4.1). 2a: Relationship between rainfall and runoff of the steep slope roads (a) A1, (b) A2 and (c) A1 and A2 combined. 2b: Relationship between rainfall and runoff of the steep slope roads (a) A1, (b) A2 and (c) A1 and A2 combined.
2c: Relationship between event rainfall and runoff from gently sloping road plots (a) B1, (b) B2, and (c) B1 and B2 combined. 2d: Relationship between event rainfall and runoff from gently sloping road plots (a) B1, (b) B2, and (c) B1 and B2 combined. The relationship between runoff depth and road gradient was determined with road gradient regression scatter plots for all plots (Figure 4.3).
The linear relationship between road slope and runoff depth indicates that runoff depths from road surfaces increase with increasing road slope. There were large differences in runoff produced from road surfaces of similar slope class, indicating differences in runoff generation.
Water Quality
4a: The average NH4+ concentrations for steep (A1 and A2) and light (B1 and B2) road runoff with slope and stream upstream and downstream of stream crossings C and D. 4b: The average NO2 concentrations for steep (A1 and A2) and gentle (B1 and B2) gradient runoff, and stream water upstream and downstream of stream crossings C and D. 4c: The average NO3 concentrations for steep (A1 and A2) and gentle (B1 and B2) gradient runoff, and streams upstream and downstream of stream crossings C and D.
4d: Average concentrations of PO43- for steep (A1 and A2) and gentle (B1 and B2) gradient road runoff, and runoff upstream and downstream of stream junctions C and D. 4e: Average TDO concentrations for slope (A1 and A2) and gentle flow (B1 and B2) of the gradient road, and stream water upstream and downstream of the junctions of streams C and D. 4f: Average levels of pH for steep (A1 and A2) and mild (B1 and B2) road flows, and stream water upstream and downstream of stream crossings C and D.
Independent t-tests (p≤ 0.05) were used to compare road runoff quality for steep and gently sloping roads, water quality upstream and downstream of stream crossings (Tables 4.3 and 4.4 ). A significant upward trend in NO3- concentrations was observed for road runoff during the study period (Figure 4.5). This suggests that stream water TDO concentrations remained stable during the sampling period.
The concentrations of TDO for stream water (Figure 4.8) remained higher than the concentrations for road discharge (Figure 4.9). The fluctuations of TDO concentrations in road runoff water were much larger than those in stream water.
- Introduction
- The Nature of Runoff from Forest Roads
- The Impact of Forest Road Runoff on Stream Water Quality
- The Breast Height Diameter of Trees in Relation to Road Runoff
Road runoff and stream water are classified according to the Aquamerck® (Germany) water quality classification guide (Table 5.1). The mean NH4+ concentrations suggest that the road runoff was moderately polluted and that the stream water was unpolluted. Road runoff and stream waters were classified as highly polluted and moderately polluted in terms of TDO, respectively.
PO43-mean concentrations for road runoff and stream water indicate that both were moderately polluted. As expected, the concentrations of NH4+, NO2- and NO3- were higher in the road runoff than in the watercourse (Table 4.1 and Figures 4.4 a-c). High temperatures were commonly measured for road runoff (Table 4.2) and can be attributed to absorption of light by road runoff sediments (Binkley and Brown, 1993).
Stevens (2001) states that increased nutrient concentrations of road runoff are the result of increased suspended sediments from erosion associated with rainfall events. Data presented from road runoff analysis have suggested that both sediment and nutrients are bound during runoff. This in turn suggests that the forest divisions themselves have a mitigating effect on road runoff.
The mitigating effects of forest compartments on road runoff were tested by measuring the tree BHD at six plots and six control plots as described in detail in Chapter 3. This suggests that road runoff from the drain outlet may only discharge a few meters from the drain.
The Impacts of Unpaved Forest Roads on Runoff and Water Quality
Elevated nutrient concentrations that occurred on the gentle slope surface B2 than other road parcels suggest that the road surface is not the only factor that determines the concentration of nutrients in the surface runoff. The leaf litter that is deposited and decays in the retention wells of the runoff fields can affect nutrient concentrations. Nutrient concentrations (ie ammonium, nitrites and nitrates) were measured in road runoff at higher concentrations than in streams for most of the sampling period.
These results suggest that stream characteristics, such as stream bank vegetation, may also influence stream nutrient concentrations. Unpaved forest roads produced nutrient concentrations higher than stream water nutrient loads during the sampling period. Road runoff nutrient concentrations that were higher than stream water would alter stream quality if the runoff were to flow into the stream.
Increased concentrations of other nutrients during high rainfall events suggested that rainfall affects stream nutrient concentrations. Although water quality for some of the road runoff was poor, using Merck's criteria, the stream water draining the property was not seriously degraded. Road runoff diversion resulted in increased tree breast height diameter for trees adjacent to gently sloping roads and in very close proximity to sewer outlets.
This suggested that the gradient determines the infiltration of redistributed runoff and thus the availability of water that can be used by trees. According to BHD data, and given that road runoff concentrations were significantly higher than stream water, diverting road runoff to adjacent forest plantation areas would reduce the potential for these loads to reach local streams.
Recommendations and the Potential for Future Research
The lower concentrations in streams likely represent both a reduction due to uptake into the compartment and dilution effects in the stream itself. It is suggested that forest managers devise measures to be used to ensure that gullies do not form at the outlets of the miter drains. While this best management practice is applied on the estate, it is important that forest managers consider other management practices, especially those aimed at reducing nutrient production from roads.
In the study, erosion of the road surface led to nutrient concentrations in road runoff. Although this study has contributed to the understanding of the impact of unpaved forest roads on runoff in forested watersheds, explicit research is needed that would help maximize the quality of observations. This will contribute to improved planning strategies for best management practices in the future and thereby reduce surface water erosion and discharge into watercourses.
R., 2004: Sediment production from forest roads in the Upper Oak Creek Watershed of the Oregon Coast Range, Unpublished MSc thesis, Forest Engineering Department, Oregon State University, Corvallis. P., 1997: Evaluation of the Effects of Forest Roads on Streamflow in Hard and Ware Creeks, Washington, Water Resources Series Technical Report No. R., 2006: Forest, Rangeland, and Watershed Stewardship, Colorado State University, Colorado. Sediment production and delivery from forest roads in the Sierra Nevada, California, unpublished MSc thesis, Department of Geography.
L., 2000: Surface water quality of the freshwater section of Elimbah Creek, Pumice Pass, Pumice Pass and Deception Bay Catchment Conference, Brisbane, 2000. E., 2006: Runoff, Loss of sediment and Water Quality from Forest Roads Queenland south Coastal Plain Pine Plantation, Forest Ecology and Management. S., 2008: Portable Laboratory Aqua-Test Designed for Field Environmental Drinking Water Analysis, Journal of Water Chemistry and Technology.
P., 2001: Effects of logging roads on flood flows in the Deschutes River, Washington, land surface processes and landforms.
