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Chapter 11

Chapter 11. Infiltration, Streamflow and Groundwater. Watershed: The area that contributes to a river or stream Watershed Divide: The boundary that separates two watersheds Usually a ridge or upland area. Watershed Hydrology. Watershed Delineation. Subsurface Capture.

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Chapter 11

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  1. Chapter 11 Infiltration, Streamflow and Groundwater

  2. Watershed: • The area that contributes to a river or stream • Watershed Divide: • The boundary that separates two watersheds • Usually a ridge or upland area

  3. Watershed Hydrology

  4. Watershed Delineation

  5. Subsurface Capture

  6. Water Budget Equation • Q = P - ET • Q is the mean annual streamflow • P is the mean annual precipitation, and • ET is the mean annual evapotranspiration

  7. Water Budgets - Some US Cities Location P ET Q Athens, GA 50 35 15 Seattle, WA 40 20 20 Olympic Mts, WA 120 20 100 Tucson, AZ 12 35 0 Where do you think the extra water in Tucson is coming from?

  8. Runoff Efficiency

  9. Water Budget Example • Say that we have: • P = 50" of rain in Athens • ET = 35" of plant and soil water loss • The streamflow depth is: • Q = 50 - 35 = 15” / yr

  10. We can convert depth per time (15”/year) to a volume per time (ft3/s), but how? • By multiplying by the watershed area, Q = A · D • This is because, if you add 1" to your bathtub, the volume is the area of the base times the depth • Think of it as spreading the water out over the watershed • The depth is over the whole watershed area. • The base is the watershed area

  11. For a mean annual streamflow depth of D = 15"/yr • For a A=10-mi2 watershed: • A = 10-mi2 x 640 acres / mi2 = 6,400 acres • Using Q = D A • Q = 96,000 acre-inches per year • Q = 8,000 acre-ft per year • Given, 1 cfs (ft3/s)  2 AF/day (acre-feet per day) • Q = (8,000 AF/yr) / (365 days/yr) = 22 AF/day • Q = (22 AF/day) / (1 cfs / 2 AF/day) = 11 cfs

  12. Acre-Foot: A volume of water • Equal to one foot of water that covers one acre of land • Lake Lanier holds 2,000,000 acre-feet of water • Georgia agriculture easily uses many times this much in one year • As do the Georgia pulp and paper mills.

  13. If I have a 100-acre golf course, and I put on 3" of water, how many acre-feet is this? • (100 acres) x (3") x (1 ft / 12") = 25 AF • Let's say I do this every week during the summer (20 weeks) • (25 AF/wk) x (20 wks/yr) = 500 AF/yr • How big of a pond do you need if the pond is 10 feet deep? • 500 acre-feet / 10 ft = 50 acres! • This only works if there is no inflow to the pond.

  14. Changes in Storage • S = I - O • S is the change in water storage • I are the hydrologic inputs, such as rain • O are the hydrologic outputs, such as streamflow and evapotranspiration

  15. Pond Storage

  16. I = 20 AF/wk, inflow to pond • O = 25 AF/wk, outflow from pond for irrigation • S = I - O = -5 AF/wk, change in pond storage • For 20 weeks of irrigation, we would only need a pond that held 100 AF • For a 10-foot deep pond this is only 10 acres instead of 50!

  17. Infiltration • Stormwater Budget: • P = F + I + O • P is the precipitation • F is the infiltration • I is the canopy interception • O is the overland flow

  18. Infiltration • Water moving from above the soil surface, into the soil. • Percolation • Water moving downward through the unsaturated zone. • Recharge • Water moving from the unsaturated zone to the saturated zone • Exfiltration • Water moving from below the soil surface to the surface

  19. Infiltration Capacity

  20. Reason why infiltration decreases during a rainstorm: • Soil wets up, filling all empty pores • Low permeability (restricting) layer below surface • When soil is bare, pores become clogged with eroded clay particles

  21. Wetting Front Infiltration

  22. Infiltration Capacity

  23. Reduction of infiltration rates with time.

  24. Methods for Increasing Infiltration • Surface mulching • Protects soil surface during rainstorm • Ponded water moves more slowly downslope • Soil humus increases aggregate formation - peds • Depression storage • Increases depth of ponding so higher gradient • Contour tilling decreasing downslope velocity • Soil liming, CaOH, CaCO3, CaSO4 • Increases aggregate formation - flocculation • Also increases base saturation • Can improve soil pH • No-till agriculture, planting w/o plowing • Maintains and improves soil structure • Increases soil organic matter

  25. Where Does Water in Rivers and Streams Come From? a. Pushed up from the center of the earth by pressure b. Pushed up through the earth by the winds on the oceans c. The earth eats salt water and uses the energy of the salt to pump water to springs d. Mostly overland flow from rainfall e. None of the above

  26. Subsurface Flow Components

  27. Map of saturated areas showing expansion during a rainstorm.

  28. Answer • Precipitation on channels, ponds, lakes: • The area covered by water in some watersheds is large, perhaps up to 20% • Precipitation on saturated areas near channels: • Following prolonged rainfall, the areas near streams become wet, and act just like the channel • Overland flow: • Also called sheet flow and surface runoff, it is water on the surface, flowing downhill, that is not in a channel • Subsurface flow: • Shallow and deep subsurface flow through soil and aquifers, usually discharging into or near channels

  29. Relationship of hillslope flow processes with land management concerns • Water Chemistry • Interaction, or lack thereof, between water and soils has a strong influence on the chemical composition of water entering streams and wetlands. • For instance, most microbial activity, nutrient cycling, and plant uptake occur in shallow soils. • The longer flow spends in this zone, the purer the water that leaves the hillslope. • It also influences the suitability of groundwater as a supply of drinking water.

  30. Biogeochemical Cycling

  31. Floods and Baseflows • Soil and vegetative conditions determine how rainfall moves to streams and thus dictate baseflows and flood peaks and volumes. • Land managers want to maximize infiltration and minimize overland flow to minimize flooding and maximize baseflows.

  32. Variation in Site Productivity and Irrigation Requirements • Soil moisture is a limiting factor for tree and crop growth in much of the U.S. • Some parts of the landscape grow trees or crop better because topographic and geologic conditions cause water to accumulate in those areas. • At the extreme, subsurface flow conditions may make an area too wet to grow many commercially valuable crops.

  33. Stormwater Management • The magnitude of hydrologic alteration caused by development depends on the degree to which soils are disturbed, vegetation is altered, and land is covered with pavement. • Appropriate design of stormwater management and treatment facilities depends on the ability to predict this change.

  34. Stream, Slope, and Wetland Geomorphology • Geologic conditions are a dominant control of hydrologic processes, but runoff patterns and characteristics in turn alter the landscape. • Landscapes are never in equilibrium, although some landscapes change much more rapidly than others. • Runoff patterns and groundwater flow in a basin determine the number and distribution of streams and wetlands as well as other landscape features.

  35. Hillslope Stability • The location and timing of landslides is largely driven by subsurface flow conditions. • For example, seepage areas on steep hillslopes are high landslide danger areas.

  36. Ground Water Hydrology • Ground water is the water held in pores in the subsurface • Ground water supplies the baseflow (flow during dry periods) to streams.

  37. Subsurface Hydrology

  38. A water table: • Separates the ground water under positive pressure (saturated zone or phreatic zone) from the water under negative pressure (unsaturated zone or vadose zone) • Above the water table is an unsaturated zone • Water pressures are negative • Soils hold water due to capillary forces. • Below the water table is the saturated zone • Water pressures are positive • Water flows freely into wells • A well or piezometer can be used to measure the location of the water table. • The water table is generally smooth, just like the land surface • Water tables rise in wet periods, fall in dry periods

  39. Perched aquifer: • A zone of saturation above an aquitard that prevents the water from moving downward. • Unconfined (or water table) aquifer: • A zone of saturation below the regional water table.

  40. An aquifer: • Moves significant quantities of water to a well • An aquitard: • Has some, but not much, ability to move water • An aquiclude: • Is almost impermeable

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