Understanding Soil Erosion & Weathering: Interplay of Processes & Factors - Prof. T. Dunne, Study notes of Environmental Science

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Dunne and more Study notes Environmental Science in PDF only on Docsity! 1 ESM 203: Erosion of Continental Surfaces Jeff Dozier & Tom Dunne Fall 2007 Soils erode: Weathering has severely weakened the rock material as the soil i s produced • Soils are granular, mobile, and subject to gravity on hillslopes • They can be moved by several processes • They enter rivers and are transported long distances downstream • The conditions that mobilize soils (erosion) can be strongly affected by management • They can also be natural (beyond human control) Fate of soil • The – existence – amount (depth) – physical, chemical, and biological) condition • of soil profiles are subject to changes in – environmental conditions – land management (including socioeconomic conditions --- rapacity, poverty, disease, efuel policy, etc) Soil profiles are differentiated into horizons that have various functions (root space, water and nutrient storage) that sustain the biosphere,including us Ap: Decaying plant matter Ao: Mineral horizon with some organic matter A1: Leached – most organic and clay and dissolved material removed B: Accumulation of clays, oxides, and solutes leached from upper horizons C: Unconsolidated, earthy, disturbed but little or no bioturbation D: Parent material with little or no weathering Chemicals are migrating through the soil as solids (e.g. C&N-rich humus), and solutes (e.g. nitrates, cations) That migration and storage processes can be altered to the detriment of soil functions (see ESM 202, while we retreat to the safety of physical processes) 2 Profile of soil and weathering bedrock Soil depth is the product of the mass balance of soil formation and removal over time (T) Primary rock Soil Erosion (E) Weathering (W) z W and E are expressed as rates per unit area (kg m -2 yr-1) [ ]dtEW ? 1 z T 0b ?= -- [ ]dt T 0∫• means “history” Mass balance of soil formation over time A ‘wave’ of erosion chases the ‘wave’ of decomposition into the bedrock Reflect on a three cases: • (1) Craton in wet tropical climate (Amazon basin, W Kenya, Australia): – Stable craton, low gradients – thick forest vegetation (erosion rate (E) low) – high T, P, primary production, rate of weathering (W). ‘ – Long time. – Result is old, deep soil, leached of nutrients by high rainfall and acid production from vegetation. 5 Mass wasting: Natural landslides/debris slides under forest SF Bay Japanese Alps Cascades, WA Mass wasting triggered by weakening of tree roots by wildfire (or logging): Landslides/debris slides Active orogens (mountain ranges): Review description of rocks, topography, and erosion processes in Lectures 11 & 12 Erosion processes Steep Low- gradient Topography Climate/vegetation/landuse Subhumid, thin veg. Surface runoff Humid, thick veg. Subsurface flow Rock slides Soil slides Rain-flow wash Biogenic soil creep 6 Long, steep hillslopes along actively downcutting rivers à very large natural landslides/rockslides Earth’s largest sediment supplies to rivers are from mountain ranges such as Himalayas and Andes Active and recently active volcanoes also supply large amounts of sediment to rivers Infiltration capacity lowered from >100 mm/hr (forest floor) to 1 mm/hr by eruption of silty volcanic ash, Mt St Helens 1980 Current and geologically recent glaciations eroded large amounts of rock from the northern continents and mountain ranges and deposited the mechanically weak debris in surrounding regions 7 Example ‘exceptional’ sediment source: Loess Plateau in the Yellow R. basin. Erosion and stabilization of wind blown silt deposits (loess) in Shaanxi Prov.,China World Bank and CSIRO Anthropogenically accelerated erosion • Most forms of resource use accelerate soil erosion • Effect varies dramatically from place to place • Especially in proportion to natural rates • Magnitude of impact and utility of conservation therefore debated Sediment release by mining on a steep, mechanically weak, wet,convergent plate margin, Papua New Guinea Keystone Center, Colo http://www.keystone.org/spp/env-oktedi.html 10 USLE applied to Kenya on basis of local data for estimation of the impact of charcoal production “Tolerable” Soil Loss (T) • Estimate T from independent evidence • Then set T = A= RKCLSP in USLE • Given the factors you can’t change, manipulate C and P to try keeping A = T • Though widely applied in the US, this concept of T is very vaguely defined and poorly defended from empirical information. • USLE widely used in planning Example of use of USLE as a planning tool • How much increase in soil erosion and associated chemical water pollution should we expect if marginal croplands that have been retired for decades to reduce soil erosion are cultivated again to produce grain for ethanol? The Erosion Conundrum • Soil erosion is difficult to quantify and especially to predict. • Because of this, there is a major controversy over whether it is being accurately estimated or exaggerated (see Watershed Analysis and the Trimble et al. articles in the course reader). • The controversy has major policy implications about the degree to which soil conservation should be subsidized. 11 The Universal Soil Loss Equation (USLE) is a planning tool, rather than an accurate predictor Instructions for application and estimation of parameters in: T. Dunne and L.B. Leopold (1978) Water in Environmental Planning, W.H. Freeman Co., San Francisco, 808 pp. Wischmeier , W.H. and D.D. Smith (1978) predicting rainfall erosion losses: a guide to conservation planning; US Dept. Agric. Agriculture Handbook 537. Renard, K.G. et al. (1997) Predicting soil erosion by water: a guide to conservation planning with the Revised Universal Soil Erosion Equation (RUSLE); US Dept. Agric. Agriculture Handbook 703, 404 pp. http://bioengr.ag.utk.edu/rusle2/