Study Guide for Exam 2

Exam will cover class presentations, class discussions, and Chapter 5

  1. Define albedo, sensible heat, latent heat, time of concentration.
    • ”Albedo is the ratio of reflected to incident electromagnetic radiation power. It is a unitless measure indicative of a surface's or body's diffuse reflectivity. The word is derived from albus, a Latin word for 'white'.”
    • ”Sensible heat is heat energy that is transported by a body that has a temperature higher than its surroundings via conduction, convection, or both.”
    • ”Latent heat is the amount of energy in the form of heat released or absorbed by a substance during a change of state.”
    • time of concentration: Time for water to flow from hydraulically most distant point on the watershed to the point of interest [SurfaceHydrology_3-19-07.PPT:21]
  2. Define all parameters of Horton's infiltration model.[SurfaceHydrology_2-16-07.PPT#11]
    • f_p = f_c +(f_0-f_c)e^{-kt}
    • fp = potential infiltration rate
    • fc = final (asymptotic) infiltration rate
    • fo = initial infiltration rate
    • k = constant
  3. Define unit hydrograph, S-curve, hydrograph's base length, excess rainfall, direct runoff.
    • A unit hydrograph is used to more easily represent the effect rainfall has on a particular basin. It is a hypothetical unit response of the watershed to a unit input of rainfall.
    • S-curve: ”…the response to a storm of infinite duration and intensity 1/Δt.”[Chin#429]
    • hydrograph's base length: [This could refer to the duration of excess rainfall resulting in the hydrograph, or it could refer to the base time of the hydrograph TB (how long runoff continues)]
    • excess rainfall: ”…the incident rainfall minus the abstractions…infiltration and depression storage…”[Chin#404]
    • direct runoff: “The flow resulting from surface runoff…”[Chin#405]
  4. What is hydrologic routing? How is it applied? How can one determine K and X coefficients.
    • “Hydrologic-routing models are based on the simultaneous solution of the continuity equations and a second equation which usually expresses the storage volume within a channel reach as a function of inflow and outflow.”[Chin#458] {dS/dt} = I(t) - O(t)
    • [Applications include modified Puls and Muskingum methods.]
    • “If measured inflow and outflow hydrographs are available for the channel reaches, then K and X can be estimated by selecting the values of these parameters that give[] the best fit between the measured and predicted outflow hydrographs.”[Chin#465]
    • “In the absence of measured data, K is usually taken as the estimated mean travel time etween the inflow and outflow section and X is taken as 0.2.”[Chin#466]
  5. Discuss, in detail, the following types of evapotranspiration:
    1. Evaporation from open water bodies.[SurfaceHydrology_3-23-07.PPT#10-11]
      • Evaporation from open water is the simplest to understand and study:
        • Ample supply of water (no water limitation).
        • Easier to measure as a change in water volume.
        • Direct phase change from liquid to vapor phase.
      • Applies for the following:
        • Oceans and seas
        • Lakes, reservoirs, ponds, rivers, streams
        • Detention storage (roads, puddles, roof tops, gutters)
        • Intercepted water (on vegetation leaves)
      • Open water evaporation affected by:
        • Surface area, depth and temperature conditions of open water body:
          • Deeper, larger and colder water bodies will have lower evaporation rate as compared to shallow, small, warm waters.
        • Location of water body and fetch conditions:
          • Well protected bodies with low fetch (upwind area) and wind speeds reduce evaporation.
        • Dissolved material in the water body:
          • More dissolved constituents (salts) reduces evaporation.
      • Open water evaporation can be measured via an evaporation pan with adjustments.
    2. Evaporation from bare soil.[SurfaceHydrology_3-23-07.PPT#12-14]
      • Evaporation from bare soils and other earth materials is more complex:
        • Soil water content varies with depth in soil profile.
        • Soil properties can dictate transport.
        • Interaction with water table must be accounted for.
        • Surface condition (roughness, shading, albedo) important.
      • Applies for the following:
        • Evaporation from natural soils.
        • Evaporation from porous geological materials.
      • Evaporation from soil is considered to occur in two separate stages.
        1. Stage I: Soil surface at or near saturation
          • Evaporation is controlled by the heat input and turbulent transport (winds) at surface.
          • No soil water content control.
          • Evaporation occurs near maximum rate.
        2. Stage II: Soil surface drying
          • Upper soil layer drying out, water limitation.
          • Transport of water vapor through soil becomes critical.
    3. Transpiration from vegetation.
      • Transpiration is a complex process to understand, study and measure:
        • Influenced by atmospheric conditions such as humidity, temperature, CO2 concentration, wind speed
        • Depends on the physiology of a plant species (tree, grass, shrub) and its adaptations to water availability
        • Extracts water over the entire root zone which can extend vertically and laterally in vicinity of plant
      • Transpiration is limited by:
        • Energy availability (solar radiation for photosynthesis)
        • Water availability (soil moisture for plant uptake)
        • Turbulent transport (wind speed near leaf surfaces)
      • Atmospheric CO2 fixed into organic carbon through photosynthesis using solar energy at leaf surfaces
      • CO2 enters leaf via stomata (pore spaces).
      • As a by-product of photosynthesis, water is produced inside a leaf
      • Water evaporation occurs in stomata and leaves the leaf through open pores
      • Upward water flux pulls water (with nutrients) up from root zone
      • Stomata are critical components of transpiration and determine water loss
      • Rooting and Branching strategies [effect transpiration]
  6. What are the basic assumptions behind the rational method?[SurfaceHydrology_3-28-07.PPT#1]
    • Entire area contributing to runoff
    • Rainfall distributed uniformly over catchment
    • All losses incorporate in runoff coefficient C
    • Implies ie =Ci
    • Storm duration equals or exceeds time of concentration
    • Maximum peak when storm duration equals time of concentration
    • Rational method assumes peak run-off rate occurs when rainfall intensity (I) lasts >= Tc [SurfaceHydrology_3-19-07.PPT:21]
  7. What are the basi[c] features of the TR-55 method?[SurfaceHydrology_3-28-07.PPT#4]
    • Simple to use
    • Graphical
    • Computer Program
    • Assumptions in SCS method must apply
    • Best for relatively small and homogeneous watersheds
  8. Describe, in detail, the two major surface runoff mechanisms. [SurfaceHydrology_2-19-07.PPT#3,5] In which situation is each mechanism dominant?
    • Horton overland flow may occur in localized areas within the catchment because infiltration capacity varies considerably within a catchment depending on soil types and vegetation cover
      • Sheet flow occurs when depression storage is exhausted
      • Horton runoff common in non-vegetated areas:
        • semi-arid regions
        • compacted soil
        • paved urban area
    • Dunne Runoff-Variable Source Area
      • Saturated areas are typically limited to the close vicinity of the stream. They expand during the storm resulting in larger rate of runoff generation
      • Because this runoff-producing zone occupies only a small portion of the watershed, even small changes can cause important differences in the volume and rate of storm runoff
  9. What is subsurface storm flow?[SurfaceHydrology_2-19-07.PPT#3,5]
    • Before the onset of a storm, the water table declines gently toward the stream providing base flow (BF). During the storm, the water table near the stream rises rapidly and increases the volume of groundwater flow.
  10. What are the main differences between runoff mechanisms?[SurfaceHydrology_2-19-07.PPT#2-3,10]
    • Horton (Infiltration Excess): Overland flow occurs when rainfall intensity exceeds infiltration capacity.
    • Dunne (Saturation Excess): If the water table near the stream rises to the ground surface, groundwater seeps out from the ground surface and generates overland flow.
    • Subsurface storm flow generates lower volume of runoff than Horton overland flow. Most of the rain is stored in the sediments and is released slowly to supply steady base flow. Hydrograph peaks lag rainfall by a few hours to one day, even for small catchments, and the shape of the hydrograph is broader than that of Horton overland flow. Flow velocity of SSSF is several orders of magnitude smaller than that of Horton overland flow.
    • Hydrographs of saturation overland flow have much higher peaks and shorter lag times than SSSF. The peak rate of runoff generation is less than that of Horton overland flow, because only a portion of the drainage basin is contributing saturation overland flow. Flow velocity is somewhat smaller than that of Horton overland flow, because saturation overland flow takes place on gentle vegetated surface.
  11. Describe the USDA Soil Texture Triangle. [SurfaceHydrology_2-16-07.PPT#8] What is it used for?
    • [Use to find curve number?]
  12. Describe types of rainfall abstraction and how they are estimated. [SurfaceHydrology_2-16-07.PPT#2-7]
    • Interception [Table based on ecology]
    • Depression Storage [Table based on surface type]
    • Infiltration…depends on soil type and surface cover
    • Runoff [Remainder of rainfall]
  13. Describe the NRCS (SCS) Curve Number method.[SurfaceHydrology_2-21-07.PPT#2-10]
    • Soil Conservation Service is an empirical method of estimating EXCESS PRECIPITATION
    • We can assume that precipitation minus excess precipitation = infiltration/retention :
      • P – Q - Ia = F … Q={(P-0.2S)^2}/{(P+0.8S)}
    • CN = {{1000}/{10+0.0394S}}
    • The Soil Conservation Service has classified over 8,500 soil series into four hydrologic groups according to their infiltration characteristics, and the proper group is determined for the soil series found.
    • Once the hydrologic soil group has been determined, the curve number of the site is determined by cross-referencing land use and hydrologic condition to the soil group
  14. What is the difference between a point source and a non-point source of contamination?
    • A point source of pollution is a single identifiable localized source of air, water, thermal, noise or light pollution.
    • Nonpoint source pollution (NPS) does not come from a single source like point source pollution. It comes from many unidentifiable sources with no specific solution to rectify the problem, making it difficult to regulate. An example of NPS pollution would be urban runnoff of items like oil, fertilizers, and lawn chemicals.
  15. Describe the step used to determine the spacing of street gutter inlets.[SurfaceHydrology_3-21-07.PPT#8]
    • L=A_contributing/w_contributing
  16. Carefully review all problems discussed in class and problems in homework assignments [4-7] (several times). [This will be about 80% of the exam.]

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