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Hydrologic cycle

Hydrologic cycle. Free water distribution (not hydrated minerals) 97% in oceans 2% in ice Melting would raise sealevel by 2% (about 80 m) Greenland alone would raise sealevel ~7 m 1% in ground water 0.01% in streams and lakes 0.001% in atmosphere. Some terminology.

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Hydrologic cycle

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  1. Hydrologic cycle • Free water distribution (not hydrated minerals) • 97% in oceans • 2% in ice • Melting would raise sealevel by 2% (about 80 m) • Greenland alone would raise sealevel ~7 m • 1% in ground water • 0.01% in streams and lakes • 0.001% in atmosphere

  2. Some terminology • Reservoirs = location of water (e.g. lake, ocean, river etc.) • Flux = motion of water between reservoirs • Units = mass per area per time • Hydrologic cycle = closed loop of the flux of water

  3. Flux = can be motion of any material (e.g. water, solutes, students coming into room) • Steady state system: • One that has invariant concentrations through time • Input = output

  4. Box model of hydrologic cycle • Three reservoir box model • Fluxes and abundances of water • Are all fluxes present between boxes?

  5. A more complicated (complete?) box model

  6. A natural system: Suwannee River What are the values water and mass for each box? Abundance in reservoir What are values for arrows? Fluxes

  7. IPCC Global Carbon Cycle Perturbation Perturbation • Black – fluxes and reservoirs - pre 1750 • Red – Anthropogenic induced fluxes • Includes weathering – but limited to silicate minerals Solomon et al., (eds) IPCC report 2007

  8. Residence Time • Average time that material is in reservoir • Only systems in steady state • Definition: t= A/J Where: A = abundance (not concentration) of material (units of mass) J = flux (in or out of reservoir) of material (units of mass/time)

  9. Example: • What is t of students if 6 students/hr enter room with 6 students? • t = 6 students/6 students/hr = 1 hour

  10. Transient conditions • System not in steady state called transient • Best defined by “Response time” • The amount of time for mass to change to certain value • Typically doubling or halving. • Sometimes considered “e-folding time” • Amount of time for exponentially growing quantity to increase by a factor of e. • Exponential decay = time to decrease by a factor of 1/e

  11. Water chemistry and the hydrologic cycle • So far just considered water as a cycle • Water not pure – so could also consider elemental cycling within the water cycle • E.g. C transport largely hydrologic (atmosphere well mixed) • Reservoirs are solid, liquid and gas

  12. Water chemistry and the hydrologic cycle • Rain • Starting point – what controls composition? • Streams & Groundwater • Water/rock interactions – greatest amount of alteration • Meteoric vs non-meteoric water • Oceans – constant salinity, constant composition for some solutes

  13. Sublimation Precipitation Recirculated seawater/MOR Fluxes in hydrologic cycle – this figure is for water. How would dissolved mass be included in this?

  14. Chemical (and Isotopic) composition of water • Natural water always in contact with soluble material – air, sediments, rocks, organic matter • Consequence – no natural water is “pure”

  15. Importance • Dissolution of gases (e.g., CO2) • Dissolution of solid phases – porosity • Precipitation of solid phases – cements • Coupled with hydrologic cycle - controls flux of material • Next – short discussion of some controls on solute composition

  16. Rain water chemistry Na+ concentrations • What might be the most likely source for Na and Cl? • How could you test to see if this hypothesis is true? • What are implications if this is true, e.g. what and where are other sources? Cl- concentrations

  17. Ca Concentration Sources of Ca other than seaspray

  18. Relative concentrations, Rainfall Note – total concentrations differ between samples Pollution – H2SO4 Gypsum dust Close to ocean composition but still modified SO4 matches pH – H2SO4 SO4 matches Ca SO4 marine influence – dimethyl sulfide

  19. Fractionation factor, Fc • Determine amount of dissolved mass from oceans • Where: • C is conservative dissolved component, Cl is chloride composition of sample or seawater

  20. Temporal variations • During storm • Rain starts salty, becomes fresher during storm • O and H isotopes also change during storm • Snow melt initially saltier & lower pH • change in melting temperature

  21. Other atmospheric sources • Rainfall is not the only mechanism to deposit material from atmosphere to land surface • Aerosol – suspension of fine solid or liquid in gas (e.g. atmosphere) • Examples – smoke, haze over oceans, air pollution, smog

  22. Dry deposition – aerosols • Dissolution of gases and aerosols by vegetation and wet surfaces • Sedimentation of large aerosols by gravity • Occult deposition • More general term - Dry deposition plus deposition from fog • Dry and Occult deposition difficult to measure

  23. Atmospheric deposition of material called “Throughfall” • Sum of solutes from precipitation, occult deposition, and dry deposition • A working definition • Data Available • National Atmospheric Deposition Program • http://nadp.sws.uiuc.edu/

  24. Rain water chemistry Na+ concentrations • What might be the most likely source for Na and Cl? • How could you test to see if this hypothesis is true? • What are implications if this is true, e.g. what and where are other sources? Cl- concentrations

  25. Ca Concentration Sources of Ca other than seaspray

  26. Relative concentrations, Rainfall Note – total concentrations differ between samples Pollution – H2SO4 Gypsum dust Close to ocean composition but still modified SO4 matches pH – H2SO4 SO4 matches Ca SO4 marine influence – dimethyl sulfide

  27. Compositional changes resulting from throughfall – NE US Open box – throughfall composition Closed box – incident precipitation composition

  28. Surface and Groundwater • Atmospheric deposition leads to surface and ground water • Variety of processes alter/move this water: • Evaporation • Transpiration (vegetative induced evaporation) • Evapotranspiration

  29. Movement across/through land surface • Overland flow – heavy flow on land surface • Interflow – flow through soil zone • Percolate into ground water

  30. Conceptualization of water flow Important to consider how each of these flow paths alter chemical compositions of water Through- fall

  31. Examples of changing chemistry • Plants • Provide solutes, neutralize acidity, extract N and P species • Soil/minerals • Dissolve providing solutes • Evaporation • Increase overall solute concentrations • Elevated concentrations lead to precipitation • Salts/cements

  32. Stream Hydrology • Baseflow • ground water source to streams • Allow streams to flow even in droughts • Augmentations of baseflow • Interflow, overland flow, direct precipitation • Result in flooding • Chemical variations in time • caused by variations in compositions of sources

  33. Bank storage • Flooding causes hydraulic head of stream to be greater than hydraulic head of ground water • Baseflow direction reversed • Water flows from stream to ground water • Hyporheic flow • Exchange of water with stream bed and stagnant areas of stream • Nutrient spiraling – chemical changes in composition because changing reservoir

  34. Stream compositions • Generally little change downstream • Short residence time in stream • Little contact with solids • Changes usually biologically mediated • Nutrients (N, P, Si) uptake and release (Nutrient spiraling) • Pollutants • Chemistry changes with discharge • Chemistry changes with exchange of GW and SW

  35. Diel Stream varations • Solar radiation changes • Nutrient and DO change • SpC, pH and Ca change • All sub-aqueous plant mediated

  36. Ground water • Unconfined example • Porosity – fraction of total solid that is void • Porosity filled w/ water or water + gas • Vadose zone – zone with gas plus water (unsaturated – can be confusing term) • Phreatic zone – all water (saturated zone) • Water table – separates vadose and phreatic zone

  37. Groundwater flow • Flow through rocks controlled by permeability • Water flows from high areas to low areas • Head gradients • Water table mimics land topography • Flow rate depends on gradient and permeability

  38. Confined aquifers • Regions with (semi) impermeable rocks • Confining unit • Confined aquifers have upper boundary in contact with confining unit • Water above confining unit is perched • Level water will rise is pieziometric surface • Hydrostatic head

  39. Effects of confined aquifers Perched aquifers, springs, water table mimic topography GW withdrawal lowers head

  40. Other types of water • Meteoric water – rain, surface, ground water • Water buried with sediments in lakes and oceans • Formation waters • Pore waters • Interstitial water/fluids • Typically old – greatly altered in composition

  41. Other water sources • Dehydration of hydrated mineral phases • Clays, amphiboles, zeolites • Metamorphic water • Water from origin of earth – mantle water • Juvenile water • Both small volumetrically; important geological consequences

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