Test 2 Flashcards
Transpiration
-Water moves from soils through roots, stems, branches to leaves
-Leaves have stomata, which are openings to the atmosphere. They contain H2O so that CO2 can dissolve in water and be used by the plant
-Transpiration is a physical process not a metabolic process (vapour pressure deficit between stomata and air drives the process)
Evapotranspiration
Combination of evaporation and transpiration
Critical parameter in long term water budgets
-Evaporation and Transpiration (ET).
-In NS, annual amounts are in range of 300-600 mm/yr.
Potential Evapotranspiration (PET)
Maximum rate of water loss from land surface if water is unlimiting.
Reference Crop Evapotranspiration (ETo)
Maximum rate of water loss from land surface with specific vegetation if water is unlimiting.
Actual Evapotranspiration (AET or ETa)
Actual rate of water loss from land surface with specific vegetation accounting for water availability/use
Lake Evaporation (EL)
Rate of water loss from an open water surface
Latent Heat Exchange
heat loss associated
with phase change
Sensible Heat Exchange
heat exchange related to a change in temperature of a substance
Direct Measurement Approaches: Lysimeters
-Micro or mesoscale systems containing a defined volume/surface area of soil and plants.
-Weighed at regular intervals to measure water loss
Net Radiation
Net input of radiation at land/water surface
Combination Methods: Penman Combination Method
-Penman (1948) combined mass-transfer and energy balances to derive an expression that does not require surface temperatures, assumes:
-no heat exchange with the ground
-no water advected energy
-no change in heat storage
Combination Methods: Priestley-Taylor Method
-Priestly and Taylor (1972) found that the aerodynamic term in the Penman Combination equation is approximately 30% of the energy term.
Thornthwaite’s
Method
Predicts monthly potential evapotranspiration based solely on air temperature
Saturation Overland Flow
Direct runoff
Return Flow
Subsurface flow intersects with land surface
Baseflow
Groundwater
Runoff Predictions
-When we design and size stormwater management systems we are typically interested in managing Surface Runoff
-Runoff = Effective Precipitation = Precipitation - Abstractions
-Abstractions = interception, infiltration, depression storage
Types of runoff predictions
-The design of a stormwater management system could require different types of runoff predictions:
* Peak runoff rates
* Total runoff volume
* Complete hydrograph
Time of Concentration
Time for wave to travel from most hydraulically distant point to catchment outlet
Lag time
Time between centroid of rainfall and peak of the response hydrograph
PEAK RUNOFF MODEL: Rational Equation
-The Rational Equation is the most widely used method for estimating peak flow rates from relatively small, urban watersheds.
-Simple equation which relates peak flow rates to rainfall intensity, watershed area, and a Runoff Coefficient, which characterizes the physical nature of the watershed.
Application of the Rational Method to predict a peak flow requires three main assumptions:
- The entire catchment area is contributing to flow at the watershed outlet. This means that the duration of the storm must be equal to or greater than the time of concentration of the watershed
- Rainfall is distributed uniformly over the entire watershed
- All catchment losses can be incorporated into one empirical coefficient.
TIME OF CONCENTRATION
-The Time of Concentration of a catchment represents the travel time of a wave to move from the hydraulically most distant point in the catchment to the outlet
-In urban hydrology, we also refer to Time of Concentration as Inlet Times
-Once the “time of concentration” of the watershed has been reached, we can assume an equilibrium condition in which the runoff rate is equal to rate of effective rainfall.
Storm Duration Approach
-We select a storm duration that is equal to the time of concentration of the watershed, and obtain a rainfall intensity, i, for a design return period from an IDF curve.
-This means that we will be modeling the worst case scenario…the highest rainfall intensity that lasts long enough for the whole watershed to contribute
at once
Types of flow regimes along the hydraulic length
-Sheet flow
-Shallow concentrated flow
-Channel flow
Sheet Flow
-Runoff that occurs as a continuous sheet of water flowing over the land surface
Shallow Concentrated Flow
-Channelization of overland flows into small rills and channels
Channel Flow
-Would exist in larger watersheds where overland flows eventually are routed through larger defined stream channels
-Sheet Flow water velocities would be the lowest, while channel flow velocities would be the highest.
Most equations express time of concentration as a function of 2 or more of the following parameters:
-Slope
-Length of overland flow
-Rainfall intensity
-Parameter that describes the catchment surface
SCS Theory
-The SCS has developed an approach to estimating Tc that attempts to represent the different flow regimes (sheet flow, shallow concentrated flow, channel flow) that could exist along the drainage pathway:
-Total Tc = Tcsheet + Tcshallow concentrated + Tcchannel
The runoff coefficient depends on basin characteristics such as:
-groundcover
-soil type
-land use
-slope
-rainfall intensity
Unit Hydrographs
-Unit Hydrograph methods are widely used in the field of Engineering Hydrology for translating rainfall excess
amounts into catchment outlet flow rates
-A Unit Hydrograph (UH) represents the characteristic
flow rate response of a watershed to 1 cm of rainfall
excess occurring over a defined time step (i.e. 10 min, 1 hr, etc)
Convolution
-Numerous short duration storm hydrographs are lagged and added to together to build a full hydrograph for a storm
Hydrograph Assumptions
-Runoff response to a unit of rainfall excess over a specified time period.
-We treat it as linear scalar.
-Time base remains the same (major assumption).
-Two methods: deconvolution, equations
Routing
-Prediction of translation and attenuation of inflow hydrographs through surface water impoundments and channels.
Routing Applications in Engineering Hydrology
-Designing/Sizing:
*Detention and retention ponds for stormwater management
*Hydroelectric power reservoirs
*Municipal water supply reservoirs
-Flood modeling in river systems
-Runoff routing
