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Infiltration, Streamflow and Groundwater

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1. Chapter 11 Infiltration, Streamflow and Groundwater

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

4. Watershed Hydrology

5. Watershed Delineation

8. Subsurface Capture

9. 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

10. 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?

11. Runoff Efficiency

12. 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

13. 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

14. 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

15. 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.

16. 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.

17. 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

18. Pond Storage

19. 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!

20. 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

22. 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

23. Infiltration Capacity

25. 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

26. Wetting Front Infiltration

27. Infiltration Capacity

29. 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

30. 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

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

38. 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

39. 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.

40. Biogeochemical Cycling

41. 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.

42. 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.

43. 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.

44. 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.

45. 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.

46. 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.

47. Subsurface Hydrology

48. 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

51. Confined Aquifers A confined aquifer is isolated from above and below by aquitards. Most of its flow comes from recharge at outcrops in the updip direction. Confined aquifers have a potentiometric surface instead of a water table. Sometimes the potentiometric surface rises above the ground surface, in this case the wells flow naturally and are called artesian.

53. SRS Aquifer Tests: Southwest Pad

55. We need to use Darcy’s Law: Q = A K G Assume a hydraulic conductivity of K = 0.003 ft/s Using the contour lines, we estimate a hydraulic gradient: Between the 100 ft and 60 ft contours, the head drop is 40 feet The distance between these contour lines is approximately six miles or 32,000 feet (using the map scale). The hydraulic gradient is G = 40 ft / 32,000 ft = 0.00125. To get the area, consider points A and B on the potentiometric map. They are on the same contour, so water is flowing perpendicular to the line between A and B in a southeasterly direction. The aquifer thickness is b = 600 feet The aquifer width is w = 25 miles between points A and B The area is A = b w = (600 ft) x (132,000 ft) The flow through the aquifer is: Q = K A G = (0.003 ft/s) x (600 ft) x (132,000 ft) x (0.00125) Q = 300 ft3/s = 192 mgd

56. This is enough water to provide domestic supply for approximately 1.3 million people (assuming a per capita use of 150 gallons/day). If you go back to the schematic for hydraulic conductivities shown in Chapter 9, you can see that the range of conductivities for carbonate rocks is huge. The flow estimated above could easily be 10 times greater.

58. Find the hydraulic gradient, G = ?h / L ?h is the water surface change between contours We can use the 80 and 60 foot contours to get a change in head of 20 feet L is the distance between contour lines. The average distance between these two contours is approximately 1/2 mile (from the scale at the bottom) or 2640 feet. Therefore the gradient is: G = ?h / L = 20 ft / 2640 ft = 0.00758 ft/ft

59. Find the ground-water flux: q = K G q = (1.5 x 10-3 ft/s) x (0.00758 ft/ft) q = 1 ft/day This has the units of a velocity Flux is often called the darcian velocity. It is equivalent to the average velocity calculated as if water moved through the entire aquifer Rather than just through the pores of the aquifer as it actually moves.

60. Find the total flow, Q = q A q is from the previous step A is the cross-sectional area of flow equal to the length of the valley between A and B (approximately 2.5 miles or 13200 feet) times the average depth of the aquifer (100 feet). Q = (0.0015 ft/s) x (0.00758) x (1,320,000 ft2) Q = 15 ft3/s This aquifer flow is discharging to the river. Therefore, flow in the river next to point B should be at least 15 cfs greater than adjacent to point A Keep in mind that aquifer water is entering the river from the other side as well.

61. Seven Landscape Factors that Drive Channel Morphology Geology and Soils Topography Vegetation: riparian and upslope Climate Flows Sediment loading Woody Debris

62. Geology and Soils The parent geology of a basin determines the type of sediment available to the channel system. Highly weathered granite produces poor gravel Channels in weathered granite tend to be sandy. Young basalt produces highly resistant, long-lasting gravel. The parent material also is a factor in soil conditions. Soil layering, hydrologic characteristics of soil horizons, and depths of soil horizons are strong controls on the runoff generating processes in a basin.

63. Topography Channel slope (along with flow) drives the sediment transport capacity of a stream. Steep channels tend to have rocky and coarse substrate. Flat channels tend to have sand and fine sediment substrates. Valley side slopes affect sediment production from upslope activities and can also affect woody debris recruitment to the channel system. Valley confinement controls the amount of energy in the channel versus energy expended on the floodplain during high flows.

64. Vegetation: riparian and upslope The quantity and type of vegetation on the uplands determines the amount of surface runoff and erosion from the hillsides. It also affects the actual evapotranspiration with consequences for stream baseflows. Riparian vegetation provides bank stability, shade, and organic debris inputs to the channel.

65. Climate The characteristics and amount of rainfall in a basin, as well as the potential evapotranspiration in a basin, determine the amount of flow in a stream per unit area. They also affect the stream density in a basin.

66. Flows The temporal characteristics of flows and the total volume of flow, along with channel slope, are the dominant drivers of sediment movement, channel scour, and woody debris transport. They also affect the survival of fish during the low flow period, the flushing of fish from the channel during high flows, and the scour and transport of fish eggs. The amount and velocity of flow affects DO concentrations and water temperatures during the summer.

67. Sediment Loading The amount of sediment introduced to the stream affects whether a channel is aggrading, incising, or maintaining a constant level. The amount and type of sediment affects the occurrence of pool habitat and the amount of interstitial habitat in the channel bed material.

68. Woody Debris Woody debris acts as "scour elements" in channels, meaning that pools tend to form around large woody debris during high flow events. During baseflows, these pools are important habitat features for fish. Woody debris also provides cover for fish, and provides substrate for the growth of macroinvertebrates (fish food). Art Benke, an aquatic entomologist, has determined that woody debris is responsible for over half the macroinvertebrate production in blackwater rivers.

69. Quiz 11 Download from my website: www.hydrology.uga.edu/rasmussen Soils and Hydrology website