3.4.5 Erosion and sediment transport
Changes in water balance terms affect many geomorphic processes including erosion, slope stability, channel change, and sediment transport (Rumsby and Macklin, 1994). There are also indirect consequences of geomorphic change for water quality (Dennis et al., 2003). Furthermore, hydromorphology is an influential factor in freshwater habitats.
All studies on soil erosion have suggested that increased rainfall amounts and intensities will lead to greater rates of erosion unless protection measures are taken. Soil erosion rates are expected to change in response to changes in climate for a variety of reasons. The most direct is the change in the erosive power of rainfall. Other reasons include:
- changes in plant canopy caused by shifts in plant biomass production associated with moisture regime;
- changes in litter cover on the ground caused by changes in plant residue decomposition rates driven by temperature, in moisture-dependent soil microbial activity, and in plant biomass production rates;
- changes in soil moisture due to shifting precipitation regimes and evapotranspiration rates, which changes infiltration and runoff ratios;
- soil erodibility changes due to a decrease in soil organic matter concentrations (which lead to a soil structure that is more susceptible to erosion) and to increased runoff (due to increased soil surface sealing and crusting);
- a shift in winter precipitation from non-erosive snow to erosive rainfall due to increasing winter temperatures;
- melting of permafrost, which induces an erodible soil state from a previously non-erodible one;
- shifts in land use made necessary to accommodate new climatic regimes.
Nearing (2001) used output from two GCMs (HadCM3 and the Canadian Centre for Climate Modelling and Analysis CGCM1) and relationships between monthly precipitation and rainfall erosivity (the power of rain to cause soil erosion) to assess potential changes in rainfall erosivity in the USA. The predicted changes were significant, and in many cases very large, but results between models differed both in magnitude and regional distributions. Zhang et al. (2005) used HadCM3 to assess potential changes in rainfall erosivity in the Huanghe River Basin of China. Increases in rainfall erosivity by as much as 11 to 22% by the year 2050 were projected across the region.
Michael et al. (2005) projected potential increases in erosion of the order of 20 to 60% over the next five decades for two sites in Saxony, Germany. These results are arguably based on significant simplifications with regard to the array of interactions involved in this type of assessment (e.g., biomass production with changing climate). Pruski and Nearing (2002a) simulated erosion for the 21st century at eight locations in the USA using the HadCM3 GCM, and taking into account the primary physical and biological mechanisms affecting erosion. The simulated cropping systems were maize and wheat. The results indicated a complex set of interactions between the several factors that affect the erosion process. Overall, where precipitation increases were projected, estimated erosion increased by 15 to 100%. Where precipitation decreases were projected, the results were more complex due largely to interactions between plant biomass, runoff, and erosion, and either increases or decreases in overall erosion could occur.
A significant potential impact of climate change on soil erosion and sediment generation is associated with the change from snowfall to rainfall. The potential impact may be particularly important in northern climates. Warmer winter temperatures would bring an increasing amount of winter precipitation as rain instead of snow, and erosion by storm runoff would increase. The results described above which use a process-based approach incorporated the effect of a shift from snow to rain due to warming, but the studies did not delineate this specific effect from the general results. Changes in soil surface conditions, such as surface roughness, sealing and crusting, may change with shifts in climate, and hence affect erosion rates.
Zhang and Nearing (2005) evaluated the potential impacts of climate change on soil erosion in central Oklahoma. Monthly projections were used from the HadCM3 GCM, using the SRES A2 and B2 scenarios and GGa1 (a scenario in which greenhouse gases increase by 1%/yr), for the periods 1950 to 1999 and 2070 to 2099. While the HadCM3-projected mean annual precipitation during 2070 to 2099 at El Reno, Oklahoma, decreased by 13.6%, 7.2%, and 6.2% for A2, B2, and GGa1, respectively, the predicted erosion (except for the no-till conservation practice scenario) increased by 18-30% for A2, remained similar for B2, and increased by 67-82% for GGa1. The greater increases in erosion in the GGa1 scenario was attributed to greater variability in monthly precipitation and an increased frequency of large storms in the model simulation. Results indicated that no-till (or conservation tillage) systems can be effective in reducing soil erosion under projected climates.
A more complex, but potentially dominant, factor is the potential for shifts in land use necessary to accommodate a new climatic regime (O’Neal et al., 2005). As farmers adapt cropping systems, the susceptibility of the soil to erosive forces will change. Farmer adaptation may range from shifts in planting, cultivation and harvest dates, to changes in crop type (Southworth et al., 2000; Pfeifer and Habeck, 2002). Modelling results for the upper Midwest U.S. suggest that erosion will increase as a function of future land-use changes, largely because of a general shift away from wheat and maize towards soybean production. For ten out of eleven regions in the study area, predicted runoff increased from +10% to +310%, and soil loss increased from +33% to +274%, in 2040–2059 relative to 1990–1999 (O’Neal et al., 2005). Other land-use scenarios would lead to different results. For example, improved conservation practices can greatly reduce erosion rates (Souchere et al., 2005), while clear-cutting a forest during a ‘slash-and-burn’ operation has a huge negative impact on susceptibility to runoff and erosion.
Little work has been done on the expected impacts of climate change on sediment loads in rivers and streams. Bouraoui et al. (2004) showed, for southern Finland, that the observed increase in precipitation and temperature was responsible for a decrease in snow cover and increase in winter runoff, which resulted in an increase in modelled suspended sediment loads. Kostaschuk et al. (2002) measured suspended sediment loads associated with tropical cyclones in Fiji, which generated very high (around 5% by volume) concentrations of sediment in the measured flows. The authors hypothesized that an increase in intensity of tropical cyclones brought about by a change in El Niño patterns could increase associated sediment loads in Fiji and across the South Pacific.
In terms of the implications of climate change for soil conservation efforts, a significant realisation from recent scientific efforts is that conservation measures must be targeted at the extreme events more than ever before (Soil and Water Conservation Society, 2003). Intense rainfall events contribute a disproportionate amount of erosion relative to the total rainfall contribution, and this effect will only be exacerbated in the future if the frequency of such storms increases.