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MidtermReview

Introduction

This midterm review includes the very minimum as to what should be known for the midterm.

 

Included:

  • Lectures 1-9
  • Lab 1-4

Formulas needed to be known:

  • D = V/A
  • Area of a circle/square/rectangle
    • A = πr²
  • Volume of a cylinder/prism  - by rearranging D = V/A -> V = D*A
  • Circumference of a circle

 

Lecture 1

Watershed characteristics

  • A number of factors affect the way water and sediment moves from upland areas to the stream and from there to its terminus

Basic parameters

Explored in lab 1; read chapter 1: getting to know your stream

  • Area [ha or km²]
  • Shape, topography and slope
  • Elevation (max, min, mean, outlet) [m]
  • Land use (forest, open, agriculture, urban, etc.)
  • Stream discharge (max, min, mean, etc) [L/s or m³/s]
  • Soil characteristics (type, depth)
  • Road network

Drainage density

R_(d) = total channel length / basin area

 

Stream ordering schemes

Issue with stream ordering

  • streams grow and shrink throughout the season (changing stream order throughout the year);
  • map might not be large enough to show all streams (affecting stream order based on map resolution)

Horton

Stream is ordered according to the highest entering order. Streams order is promoted when a stream combines with another stream of the same order.

Strahler

For a stream to be promoted to a higher order, it needs to combine with a stream of the same order.

  • Stream of order 1 + stream of order 1 = 2
  • Stream of order 1 + stream of order 2 = 2
  • Stream of order 2 + stream of order 2 = 3

Shreve

Stream orders are simply added together:

  • Stream of order 1 + stream of order 1 = 2
  • Stream of order 1 + stream of order 2 = 3
  • Stream of order 2 + stream of order 2 = 4

Stream patterns

Determined by

  • Climate
  • Topography
  • Geology
  • Vegetation
  • Human intervention

Stream types

  • Influent / effluent
  • Perennial / intermittent / ephemeral
  • Bedrock controlled / alluvial
  • Headwater / middle-order / lowland
  • Stable / aggrading / degrading
  • Regulated / natural
  • Channelized / non-channelized

Hydrological cycles

Should be able to draw a watersheds water cycle

  • Precipitation forms
  • The accumulation / decumulation of stores
    • Glaciers / soil water / snowpack

Energy and mass balance

Definitely on the exam!!!

Water Balance Equation

  • Q = P - ET + ΔS
  • Q = P - ET
    • Q = stream outflow
    • P = precipitation
    • ET = evapotranspiration
    • S = storage

Energy Balance Equation

Day: Q_(E) = S + D - aK + L⤓ - L⤒ ± L_(h) ± S_(h) ± C_(h) Night: Q_(E) = L⤓ - L⤒ ± L_(h) ± S_(h) ± C_(h)

Terms:

  • Direct solar radiation (S)
  • Diffuse radiation (D)
  • Incoming shortwave radiation / Insolation (K) =  S + D
  • Albedo (a)
  • Outgoing shortwave radiation (aK, K⤒)
  • Net shortwave radiation (K*)
  • Incoming Longwave Radiation (L⤓)
  • Outgoing Longwave Radiation (L⤒)
  • Net Radiant Energy (R)
  • Latent Heat Flux (L_(h))

Conductive heat flux (C_(h)) Local Energy Balance

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Precipitation process

Atmospheric pressure and relative humidity

Formation of precipitation requires a 4-stage process:

  1. Saturation of air by cooling to reach dew point temperature
  2. Condensation of water vapor to form ice crystals or droplets
  3. Growth of droplets or crystals into raindrops, snowflakes, or hailstones, to a size large enough to fall to the ground before evaporating
  4. Continuous input of water vapor to sustain the process (depending on vertical wind and humidity)

 

Relative humidity = Actual water vapor pressure / maximum water vapor pressure at a specific air temperature X 100

 

Uplifting mechanisms

Precipitation originates from different uplifting mechanisms:

  1. Convective
    • Results from strong heating of air near ground (unstable) that expands, becomes lighter, and rises
    • Causes adiabatic cooling
    • Cells typically 10s of km apart
    • Precipitation is intense, heavy, and short-lived
    • Associated with thunderstorms and hail
    • Highly patched rainfall patterns; might not be detected by rain gauge
  2. Convergent (frontal or non-frontal)
    • Frontal:
      • Warm front
        • Warm air is less dense than cold air and easily slides up gradually pushing the cold air out of the way.
        • In BC, comes from the ocean
        • precipitation Covers large areas, is not intense, and longer lasting
      • Cold front
        • Cold air is denser than the warm air mass it replaces. As the cold air moves into the warm air, it forces it to rise quickly.
        • Results in deeper clouds and heavier precipitation than in warm fronts
        • Precipitation covers a smaller area and is more intense
  3. Orographic
    • Air moving horizontally hits a topographic barrier
    • Hills and mountains deflect air upwards

Errors in ground-based measurements

  • Wind: increased wind results in decrease in catch
  • Evaporation and wetting: Handling traces is problematic
  • Splashback: Gauges are usually mounted 1.5 - 2 m above ground the minimize splash but mounting closer to ground reduces wind effects
  • Location of gauge: observe angle and distance to obstructions

Spatial interpolation

Process of using points with known values to estimate values at other points

 

Soil water potential

Water Potential (ψ_(T)): Total potential energy of water:

ψ_(T) = ψ_(g) + ψ_(p) + ψ_(m)

ψ_(g) = gravitational potential; ψ_(p) = pressure potential; ψ_(m) = matric potential

  • Gravitation potential: Amount of work done or energy released when mass is moved vertically. Energy depends on the gravitational force field relative to some reference level
  • Pressure potential: hydrostatic pressure of the water column
  • Negative pressure due to capillary and adsorptive forces

Pressure potential occurs in the saturated zone (below water table) and matric potential occurs in the unsaturated zone (above water table); therefore, both cannot coexist. So the equation is either:

ψ_(T) = ψ_(g) + ψ_(p) ψ_(T) = ψ_(g) + ψ_(m)

_(Difference between wells and piezometer)

  • Piezometers provide information about vertical and lateral movement of water between two specific points
  • Wells provide information about lateral movement of water with no correspondence to specific points but the entire water table

 

Zero Flux Plane

A theoretical plane that separates two zones. One of upward movement, the other of downwards movement.

Must be able to draw and write a paragraph on the "seasonality" of soil water flux. Read "Hydrology Principles and Processes"

 

Chapter 6

Fundamentals of infiltration

Infiltration curve starts high at the beginning, as soil saturates, soil infiltration rate slows down and decays.

Type of flow Flow Process Soil Features Homogenous matrix flow Straight down Permeable soils due to homogenous texture and aggregation Homogeneous matrix flow and fingering Straight down but some obstructions Spatially heterogeneous soil due to varying texture or aggregation, macropores, or water repellency Macropore flow with low interaction Down, some horizontal flow Macropores in a low permeable or saturated soil matrix Macropore flow with high interaction Down, lots of horizontal flow Macropores in a permeable soil matrix

Factors that influence infiltration

  • Forested sites are usually porous, open at the soil surface, with an extensive macropore system caused by old root activity, burrowing animals, and earthworms
  • Initial and final infiltration rates can be orders of magnitude higher than those of agricultural soils with the same/similar textures
  • Deep forested soils in humid climates can have infiltration capacities far in excess of any expected rainfall intensity
  • Surface runoff rarely occurs under these conditions unless soil is shallow and water table is at the surface

 

Lecture 7

Definitions of groundwater

  • Groundwater: water that occurs in saturated zones beneath the soil surface

Types of aquifers / definitions

Should know the different types of aquifers, where they are found, etc.

Groundwater storage and movement

Drawing equipotential lines

  • Equipotential lines (dashed) extending horizontally above water table = elevated head
  • Below the water table, the equipotential lines are curvilinear; reflecting the sum of elevation and pressure potentials

 

When it comes to groundwater:

mid to Higher elevations in the mountains are always called the recharge zones

  • Higher up, higher precipitation on watersheds
    • more chances of snow
    • contributes to the recharge of the ground water
  • Responsible for recharge of the groundwater; especially in the aquifers

 

low to mid elevations in the mountains are the discharge zones

  • As we go down in elevation, the closer is the groundwater to the surface
    • Lakes / ponds / bogs being formed

 

Beneath soil, bedrock; Beneath bedrock; possibly pressurized confined aquifers.

 

Evapotranspiration

Components of evapotranspiration

  • Evaporation
  • Interception
  • Transpiration

 

Fundamentals of evapotranspiration

Importance of evapotranspiration

  • Water availability = precipitation - evapotranspiration (Q = P - ET)
  • Plants grow through the process of transpiration
  • Efficient irrigation requires knowledge of crop water use
  • Significant influence on yield of water-supply resevoire
  • Key to estimate antecedent soil moisture conditions prior to a storm

 

Importance of interception

  • Substantial part of the water balance (up to 35% loss in closed-canopy stands)
  • Lowers the intensity of precipitation (up to a certain threshold)
  • protects the soil surface from raindrop energy and thus reduces erosion

 

Requirements for evaporation

  • Energy flux to the surface
  • Flow of liquid water to the surface
  • Flow of water vapor away from the surface

I.e., energy, water, unsaturated air, wind

 

Actual and potential evapotranspiration

Actual evapotranspiration (ET)

  • Actual rate of evapotranspiration determined by net radiation, interception, advection, turbulent transport, leaf area, stomatal resistance, vegetation type, and water availability

Potential evapotranspiration (PET)

  • Rate at which evapotranspiration would occur from a large area completely and uniformly covered with growing vegetation, having access to an unlimited supply of soil water

 

Factors that influence evapotranspiration

  • Meterology (radiation, wind, temerature, humidity, precipitation type
  • Topography: energy (aspect, elevation), soil moisture
  • Seasonal and daily cycles: leaves on or off, photosynthetic activity
  • Vegetation: leaf area, species
  • Soil properties: water holding capacity, etc.

 

North aspect are "hydrologically more productive" than other aspects as they receive less energy

  • Less chances to lose water

 

 

Runoff Processes

Subsurface flow

"A runoff generating mechanism operating in most upland terrains"

Two mechanisms:

  • Micropore (matrix) flow
  • Macropore flow

 

Overland flow

  • Infiltration-excess runoff (hortonian)
    • Most likely in arid and semiarid landscapes where vegetation densities and infiltration rates are low or in disturbed areas
  • Saturation-excess runoff (dunne)
    • Most likely in riparian zones

 

Variable source area:

  • Area actively involved in producing quickflow will vary from less than 1% of the basin during small storms to more than 50% in large storms

 

Factors that affect runoff

  • Surface and bedrock topography
  • Soil depth and compaction
  • Vegetation type & abundance
  • Antecedent soil moisture
  • Precipitation form, intensity, duration, distribution
  • Flow pathway: matrix vs macropores
  • Spatial distribution and occurrence of geomorphic features
  • Watershed characteristics: circularity, drainage density, slope, etc.