2. Watershed Simulation Flashcards
What is the watershed?
Area of land that contributes runoff to a common point of interest. (ie. an outlet)
What is delineation?
a contour map or GIS
What are the watershed processes? (6)
precipitation, interpolation, infiltration, depression storage, evapotranspiration, and surface run off.
From an engineering perspective the most important subsystem is……
the watershed system
What are some model constituents?
state variables, parameters, boundary conditions, initial conditions, system response.
What are state variables?
the state of a system at a particular time and location. (eg. mean volume of water in storage)
What are parameters?
Numerical measures of the properties of a real system,. eg. basin lag time, hydraulic conductivity, infiltration rate, surface permeability
What are boundary conditions?
Values of the system input - forces that act on the system to cause a change. Eg. precipitation, a driving force that creates a response.
What are initial conditions?
known value with which the computations start. (eg runoff before start time, initial soil moisture content
What is a system response?
runoff to the outlet
What does a watershed simulation do?
It describes how a watershed responds to precipitation falling on it or to upstream water flowing into it. Watershed modelling may include: rainfall-runoff, snowmelt, channel and reservoir routing, landuse.
- What are the functions of the watershed?
CD WAS. Collection, Storage, Discharge, Attenuation, and Water balance.
- What is the collection function?
How the watershed interacts with the storm timing, movement, and areal extent. Precipitation intensity, duration and frequency, are also important in deterring the basic nature of the storm hydrograph.
What is the storage function?
Understanding watershed hydrology is based on the fact that the greatest percentage of water on the watershed is in storage and, as a consequence, storage greatly influences the hydrological processes that move the water between the several storage locations; it also helps determine water quality.
What are the 2 types of runoff?
Storm flow and base flow
- What are the 6 types of storage?
Depression, Channel, Retention, Detention, Groundwater, Vegetation
What is the discharge function?
the discharge function is profoundly affected by the collection and storage of water. In addition, the storm and annual hydrographs contain individual physical and chemical signatures for each watershed. In spite of that individuality, there are similarities among hydrographs regionally and these, along with climate and geomorphology, define hydrographic areas.
What are some implications to the Storage function?
-Infiltration, Storm flow,
What is the variable source area?
As water drains from the watershed during a runoff-causing event, the area that contributes runoff to the stream changes. eg. only the bottom half of a bowl contributing to the runoff. This is a concept is a cornerstone of watershed management strategy in that it defines a hydrologically sensitive zone, sometimes referred to as a buffer zone or riparian buffer.
What is the repairian/buffer zone?
part of the Runoff that is always running
True or False: Floods (and droughts) are not normally characteristics of watershed hydrology and riverine ecology
False, they are. Flood control structural measures modify functions of the watershed such as attenuation and flushing
The storm hydrograph is dominated by:
weather and by the interactions between characteristics of the watershed and the runoff-causing event
the annual hydrograph is dominated by:
climate and the volume and areal distribution of groundwater storage on the watershed
The relationship between storm runoff and base flow may take different shapes dependent upon
infiltration capacity and precipitation intensity and between the total volumes of precipitation and soil moisture deficit.
The time of concentration (Tc) may be changed with
change in land use, especially the dramatic change in infiltration capacity associated with urbanization. In such a case, the Tc becomes very short and the storm hydrograph is quite flashy, with the rapid runoff conveying surface pollutants and sediment downstream.
What is the Hyetograph?
a graphical representation between rainfall intensity and time. Area under the line gives the total rainfall that occurred.
What is the hydrograph?
used to analyze surface runoff. It is a graph of discharge in a river or stream vs time.
What is the attenuation function?
is perhaps the simplest of all the functions, for it is a self-evident consequence of the changes that occur in the storm hydrograph as the runoff moves downstream.
Since storms are most intense over small areas, a flood peak generated on a very smallwatershed becomes lower and loweras it moves downstream, gatheringless and less runoff per unitunit area* from the larger and larger watersheds
When is the start of the hydrological year?
October 1st
What are the causes for a storm hydrograph being Lower and Later on Larger watersheds?
1) storms exhibit greater intensities over small areas.
2) storms are less likely to cover large watersheds, and
3) takes longer for runoff to gather at the outlet on larger watersheds
For these reasons runoff on a small watershed is “flashier” than from a large shed
What is the Water Balance?
displayed on a climatograph. Don’t all happen at the same time and vary over a year.
What is the equation for intensity?
depth/duration
A storm is characterized by:
1) Depth ( or average intensity)
2) Duration
What is the Return Period?
The AVERAGE time between the storms with the SAME DURATION and the SAME or GREATER INTENSITY
What is the difference between inductive and deductive modelling?
Inductive looks at creating a model based on observed results. It tries to fit a model(eq.) that best describe the observed data. eg. Hydrograph
Deductive looks at obtaining based on a predefined selected model or equation. eg. Kinematic wave method
What are some assumptions of the UH (unit Hydrograph)
1) The runoff is assumed to be due to direct runoff.
2) The rainfall is uniformly distributed in time (runoff must be of short duration).
3) The rainfall is uniformly distributed in space (the area must not be too large).
4) The watershed response is linear (linear superposition may be used).
5) The watershed characteristics do not change with time.
6) The unit volume under the UH is equal to 1.0.
Clark’s unit hydrograph explicitly represents 2 processes in transforming excess precipitation to runoff. These processes are
1) Translation- movement to the outlet
2) Attenuation- reduction of magnitude as the excess is stored.
The Soil Conservation Service (SCS) Curve Number (CN) estimates..
excess precipitation as a function of cumulative precipitation, soil cover, landuse, and antecedent moisture. CN ranges from 100(water bodies) to 30(permeable soils like sand). Urbanization would have a high CN value.
What are the two types of Stream Routing?
1) Hydraulic Methods: (St. Venant equations) -kinematic wave method
2) Hydrologic Methods: (Storage equations) - kinematic storage(muskingum) method
The Muskingum Method assumes:
that storage is related to discharge
What does HEC-HMS stand for and what is it?
Hydraulic Engineering Center - Hydrologic Modeling System
it is a computer program that analyzes the watershed of a system in order to predict the hydrologic response of the system to a defined rainfall event.
What are some watershed subdivisions?
Geophysical boundaries, geopolitical boundaries, and engineering boundaries(damage centres, control structures)
