1. INTRODUCTION AND BACKGROUND
3.3 Impacts of Projected Hydro-Climatic Changes on Sediment and Nutrient
3.3.1 Impacts of Altered Precipitation Variability
Changes in climate variables, including precipitation (depth, duration and frequency), are expected to alter the transfer dynamics of sediment and nutrients worldwide (Jeppesen et al., 2011). Although not well understood over the full range of scales necessary for management, the relationships between precipitation variability, sediment delivery and nutrient transfer have been extensively studied (e.g. Viney and Sivapalan, 1996; McNamara and Cornish, 2004; Low, 2005; IPCC, 2007, Bates et al., 2008; Statham, 2011). In South Africa, for instance, an extensive climate change study was conducted in which the projected impacts of climate change on the water resources of the country were assessed through climate scenario development and impact modelling (Schulze et al., 2005). With respect to precipitation trends under climate change, some of the findings of this study were that increases in precipitation, increases in raindays and increases in rainfall event intensities may be expected for eastern and central parts of the country and the opposite is anticipated for western regions of the country (Hewitson et al., 2005).
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The statements made by Hewitson et al., (2005), therefore suggest that in countries such as South Africa, in which climate change related increases in precipitation and runoff are anticipated, an increase in the rate at which sediment and nutrients are generated and transferred across catchments located in eastern and central regions of the country may also be anticipated.
The timing and volume of runoff within a catchment is intrinsically linked to seasonal climate variability and change, particularly rainfall variability (Shrestha et al., 2012). Furthermore, the transfer of sediment and nutrients from upstream sources to downstream sinks has been shown to be highly sensitive to climatic factors (Donohue et al., 2005; Jeppesen et al., 2011).
Table 3.3, for instance, summarises the relationship between extreme climate change, rainfall variability and sediment and nutrient transfer. Table 3.4, on the other hand, provides a list of locally relevant, climate-sensitive biophysical processes and parameters that are anticipated to be impacted by climate change. Taking into consideration the sensitivity of runoff to rainfall variability, any changes in rainfall can, therefore, be expected to produce considerable changes in the dynamics of sediment and nutrient generation and transfer. For instance, heavy rainfall events (long/short duration, high intensity) result in increased erosion and the generation of high volumes of stormflow and runoff which subsequently leads to high sediment and nutrient generation and transport. Additionally, high intensity rainfall events can lead to the “flash” generation and rapid transport of stored sediment and nutrients through the sheer force of raindrops.
A lot of uncertainties abound in climate change predictive modelling, especially in rainfall- runoff modelling (Hewitson et al., 2005; Bates et al., 2008). However, these uncertainties are mitigated somewhat by the fact that the currently understood relationships between rainfall, runoff and non-point source pollution transport can be used as reliable benchmarks for future predictions (Christiansen et al., 2004). To complete the discussion regarding the impacts of hydroclimatic changes on sediment and nutrient transfer, the following section details the effects of temperature on these parameters.
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Table 3.3 Projected changes in precipitation trends and impacts on sediment and nutrient generation, transfer and deposition constructed from a survey of the literature.
i. Changes in climatic phenomena (derived from:
IPCC 2001; Hewitson et al., 2005; IPCC, 2007; Bates et al., 2008)
ii. Consequence on hydrologic processes
iii. Impact on sediment
iv. Impact on nutrients (N and P)
Heavy precipitation events with increased frequency (i.e. floods).
High stormflow and runoff volumes and increased streamflow (Jeppessen et al., 2011; Hewitson et al., 2005).
Increased transport of sediment from source generation areas (e.g.
landslides and soil slips) (Heathwaite and Dils, 2000; Fryirs et al., 2007).
Increased flushing of nutrients from source generation areas (EPA, 2009).
Increased water renewal of water bodies leading to washout of nutrients (Rosberg and Arheimer, 2007).
High intensity events. Rapid generation of stormflow and runoff (IPCC, 2001; 2007; Bates et al., 2008).
Increased soil
erodibility due to high raindrop energy (Mainstone et al., 2008).
Altered mixing patterns in stratified reservoirs (Tsujimura, 2004).
Increased variability of precipitation.
Increased inter-annual variability of runoff and streamflow (IPCC, 2001;
2007; Bates et al., 2008).
Increased variability in sediment transfer trends (Withers and Sharpley, 2007).
Increased variability in nutrient cycling trends, increased variability in the timing of nutrient delivery (Van Der Perk, 2006).
Reduced precipitation events with increased frequency (i.e. droughts).
Low volumes of stormflow and runoff and reduced streamflow (Hewitson et al., 2005).
Higher deposition of sediment in sink areas (e.g. floodplains and estuaries) (Fryirs et al., 2007).
Reduced dilution capacities, increased pollutant toxicity (e.g.
nitrate toxicity) (Carpenter et al., 2008).
Reduced transport of sediment from source to sinks (McKergow et al., 2006).
Reduced transport of nutrients from sources to sinks but increased storage of nutrients (Mainstone et al., 2008).
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Table 3.4 Selected climate-sensitive processes linked to climate change and their impacts on water quality*. The probable direction of change of these processes under climate change is indicated by the arrows**. Processes and trends in a particular row are not connected.
SEDIMENTS NUTRIENTS BIOCHEMICAL PROCESSES
Particle detachment/Erosion (↑) Fertilizer, manure and crop residue application (timing and rates) (↔)
Organic matter decomposition (release of N and P) (↑)
Overland flow (↑) Fixation (↑) Mercury mobilisation (caused by a build up of
benthic anoxic layers) (↑)
Instream sediment transport/Bed-load transport (↑) Ammonification (↔) Denitrification (e.g. in wetlands and riparian buffers) (↔)
Flow types (Laminar vs Turbulent) (↔) Mineralization (↑) Acidification (increased deposition of N and P and releases of H ions) (↑)
Attenuation of sediment conveyance (↓) Conversion of Organic N and P to Mineral N and P (↔)
Solubility of gases (high temp. reduces solubility of oxygen) (↓)
Sedimentation (↑) Wet and Dry deposition(↑) pH fluctuations (↔)
Particle settling (i.e. rates) (↓) Leaching (↑) N and P transformation (↑)
Suspension and Resuspension (influence on Turbidity and light pen.) (↑)
Hydraulic conductivity (as related to how easily nutrients move through soil) (↑)
Eutrophication (as the culmination of the impacts of NPS pollution of water resources) (↑)
Shear stress (influence on detachment) (↑) Subsurface flow (inclusive of g/water flow and recharge) (↑)
Formation of hypoxic zones (↑)
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* References: Novotny, 2003; Van der Perk, 2006; Heathwaite and Dils, 2000; Fryirs et al., 2007; Carmago et al., 2005; Withers and Jarvie, 2008;
Withers and Sharpley, 2007; Smithers and Schulze, 2004; McDowell and Wilcock, 2004; Deasy et al., 2007; IPCC, 2001; IPCC, 2007;
Bates et al., 2008; UNESCO, 2009.
** ↑ implies a highly probable increase, enhancement or exacerbation of the process under climate change.
↓
implies a highly probable decrease, retardation or impairment of the process under climate change.
↔
probable direction of change under climate change not known or not sufficiently documented in the literature to fully ascertain direction of change.
Deposition (↑) Surface runoff (excess N and P) (↑) Increased residence times (Increased growth of
algae and cyanobacteria) (↑) Riverbank scouring (↑) Dilution (capacity of stream to dilute nutrients) (↑) Solubilization of nutrients (↑) Phosphorus adsorption by very fine sediments (↑) Washout during extreme events (↑) Chelation (↔)
Transport of adsorbed nutrients (↑) Salinisation (↑) Mineralization-immobilization turnover (MIT)
(↑)
Lagging of downstream conveyance (↓) Volatilisation/Evaporation (↑) Heterotrophic nitrification (↑) Tributary transport capacities (↓) Effluent seepage(↑)
Breaching capacity of buffers (↓) Preferential flow via macropores (↑)
Selective size segregation (↓) Plant uptake (↑)
Sediment transport through overland flow (↑) SOM formation (contributing to N and P retention in soil) (↑)
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