NWS-COMET Hydrometeorology Course 15 - 30 June, 1999

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NWS-COMET Hydrometeorology Course 15 - 30 June,
1999

• Hydrology Primer

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Dennis L. Johnson, Asst. ProfessorMichigan
Technological UniversityDepartment of Civil
Env. Engineering(906) 487 - 3613 (phone)(906)
487 - 2943 (fax)dennisj_at_mtu.edu
(email)http//www.civil.mtu.edu/dennisj/
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Where is MTU?
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Topographic Relief of Great Lakes Region
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Some Lake Superior Facts

• Surface Area 31,700 square miles
• Land Drainage Area 49,300 square miles
• Lake Superior covers 39 of the total basin
  area!!
• More than 200 rivers enter with 1000s of
  streams.
• Max. depth 1,330 feet
• Volume 2,900 cubic miles
• More than half of all the great lakes and 10
  of earths flowing surface fresh waters!!

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Usual Houghton
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• Purpose of the Hydrometeorology Course
• Increase the participants knowledge and
  understanding of the interaction between
  meteorology and hydrology in watersheds
• Increase participants understanding of the
  functional aspects of watersheds
• Enhance the participants knowledge of the
  capabilities, limitations, and applications of
  new hydrometeorological observing systems
• Improve the participants ability to identify
  significant mesoscale meteorological events and
  to produce Quantitative Precipitation Forecasts
• Increase participants understanding of the
  effectiveness of the NWS forecast and warning
  methodologies and plan future enhancements and
• Build awareness of the need for close ties
  between RFC's and WFO's.

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• Purpose of the PRIMER
• Provide an introduction between participants
  establish backgrounds.
• Introduce participants to basic terminology and
  concepts of hydrologic forecasting that will be
  used throughout the hydrology portion of the
  COMET Hydromet course. The primer introduces
  these concepts and specific detail will be
  provided in week 3.
• Establish the course objectives as per the
  expectations of the participants.
• Establish hydrologic concerns in the various
  participants' regions.

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In the end, it is intended that participants will
understand the hydrologic forecast process, the
assumptions in the process, and the
responsibilities associated with interpreting and
issuing the forecast.
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Mission of NOAA's NWSHydrologic Services Program

• To provide river and flood forecasts and warnings
  for protection of life and property
• Provide basic hydrologic forecast information for
  the nation's economic and environmental well
  being.

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Modernized NWS

• It is essential to emphasize the complementary
  aspects of operational hydrology and meteorology
  in the modernized NWS, while recognizing the
  uniqueness of RFC and WFO operations.

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New or Improved Products

• ...the production of a variety of hydrologic
  forecast products for an increased number of
  river locations across the country, including
  ESP-based products

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What is ESP?
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What is ESP?

• Ensemble Streamflow Production (ESP)
• Inputs the current moisture level of soil and the
  precipitation from previous years into a model
  which produces the diagram seen above.
• For example, the moisture content of today would
  be inputted, along with the precipitation that
  occurred over the next week, but 50 years ago.
• This would then be repeated for 49 years ago, 48,
  etc., and then an average discharge based on
  history can be determined.

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NWSOffice of Hydrology
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Hydrology

• an earth science. It encompasses the
  occurrence, distribution, movement, and
  properties of the waters of the earth and their
  environmental relationships." (Viessman, Knapp,
  Lewis, Harbaugh, 1977 - Introduction to
  Hydrology, Harper Row Publishers, New York)

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Hydrometeorology

• an interdisciplinary science involving the
  study and analysis of the interrelationships
  between the atmospheric and land phases of water
  as it moves through the hydrologic cycle."
  (Hydrometeorological Service Operations for the
  1990's, Office of Hydrology, National Weather
  Service, NOAA, 1996).

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Hydrometeorology - Links
Hydrology
Engineering/Fluid
Mechanics
Ø

In-depth hydrologic
Hydrometeorology
analysis
Interdisciplinary
Ø

Execution of complex
Orientation
Meteorology
hydrologic models.
Thermodynamics/atmospheric
Adjustment of
Ø

Assimilation/use of
physics orientation
model parameters, and
WSR-88D based
the derivation of
precip. estimates
Ø

In-depth meteorological
hydrologic forecasts for

Production and/or
Ø
analysis
all
time scales
use of
QPF's and
Ø

Weather forecast and
Ø

Applied hydrologic
other
hydromet.
warning operations
research
forecasts
Ø

Climatological forecasting
Ø

Development and
Ø

Use of RFC guidance
Ø

Applied meteorological
calibration of
(e.g. flash flood) in
and climatological
hydrologic models
hydrologic warning
research.
Ø

Development of
operations
Ø

Development and calibration
hydrologic applications
Ø

Use of soil moisture
of meteorological models
procedures.
states from
Ø

Development of
hydrologic model in
meteorological applications
atmospheric model
and procedures.

Applied
Ø
hydrometeorological
research.
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A Basic Review of Fluid Properties
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Units Properties of Water

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Common Unit Conversions
Area Volume Runoff Volume
Discharge Power
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Area

• 1 acre 43,560 ft2
• 1 mi2 640 acres
• 1 hectare 100m x 100m 2.471 acres 10,000 m2
• 1 km2 0.386 mi2

Area Volume Runoff Volume
Discharge Power
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Volume

• 1 acre-foot 1 ac-ft 1 acre of water x 1 foot
  deep 43,560 x 1 43,560 ft3
• 1 ac-inch 1 acre x 1 inch deep 43,560 x 1/12
  3,630 ft3
• 1 ft3 7.48 gallons
• 1 gallon H2O 8.34 lbs.

Area Volume Runoff Volume
Discharge Power
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Runoff Volume

• 1-inch of runoff over 1 square mile
• 1/12 feet x 1 mi2 x 640 acres/mi2 x 43,560
  ft2/mi2 2,323,200 ft3

Area Volume Runoff Volume
Discharge Power
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Discharge

• 1 cfs 1 cubic foot per second
• 1 cfs x 7.48 gal/ft3 x 3600 sec/hr x 24 hrs/day
  646,272 gpd 0.646 MGD
• 1 cfs x 3600 sec/hr x 24 hrs/day 86,400 cfs/day
• 86,400 cfs/day x 1 ac-ft/43,560 ft3 1.983
  ac-ft/day ( 2 ac-ft/day)
• 1.983 ac-ft/day x 12 inches/ft x 1 day/24 hrs
  0.992 ac-in/hr
• 1 ac-in/hr x 43,560 ft3/ac-ft x 1 hr/3600 sec x 1
  ft/12 inches 1.008 cfs

Area Volume Runoff Volume
Discharge Power
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Power

• 1 hp 550 ftlb/sec 0.7547 kilowatts

Area Volume Runoff Volume
Discharge Power
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Hydrologic Cycle
Topics Precipitation Evaporation Transpiration Sto
rage-surface Infiltration Storage -
Subsurface Runoff Water Movement Streamflow Storag
e-Reservoirs
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Precipitation

• ... primary "input" for the hydrologic cycle (or
  hydrologic budget).
• The patterns of the precipitation are affected
  by large scale global patterns, mesoscale
  patterns, "regional" patterns, and
  micro-climates.
• Knowing and understanding the general,
  regional, and local precipitation patterns
  greatly aids forecasters in determining QPF
  values.
• In addition to the quantity of precipitation,
  the spatial and temporal distributions of the
  precipitation have considerable effects on the
  hydrologic response.

Precipitation -Snow Evaporation Transpiration St
orage-surface Infiltration Storage -
Subsurface Runoff Water Movement Streamflow Storag
e-Reservoirs
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Snow

• ... nature of the modeling efforts that are
  required.
• response mechanisms of snow are at a much
  slower time scale than for most of the other
  forms of precipitation.
• The melt takes place and the runoff is "lagged"
  due to the physical travel processes.
• Items to consider in the snowmelt process are
  the current "state" of the pack and the snow
  water equivalent of the snow pack., as well as
  the melt potential of the current climate
  conditions.
• A rain-on-snow event may produce very high
  runoff rates and is often a difficult situation
  to predict due to the integral nature of the
  runoff and melt processes. The timing of these
  events is often very difficult to predict due to
  the inherent "lag" in the responses.

Precipitation -Snow Evaporation Transpiration St
orage-surface Infiltration Storage -
Subsurface Runoff Water Movement Streamflow Storag
e-Reservoirs
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Evaporation

• Evaporation is a process that allows water to
  change from its liquid phase to a vapor.
• Hydrologists are mostly interested in the
  evaporation from the free water surface of open
  water or subsurface water exposed via the
  capillary action however, precipitation that is
  intercepted by the vegetative canopy may also be
  evaporated and may be a significant amount in
  terms of the overall hydrologic budget.
• Factors that affect evaporation are
  temperature, humidity and vapor pressure,
  radiation, and wind speed.
• A number of equations are used to estimate
  evaporation. There are also a number of
  published tables and maps providing regional
  estimates of annual evaporation.

Precipitation Evaporation Transpiration Storage-su
rface Infiltration Storage - Subsurface Runoff Wat
er Movement Streamflow Storage-Reservoirs
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Transpiration

• Water may also pass to the atmosphere by being
  "taken up" by plants and passed on through the
  plant surfaces.
• Transpiration varies greatly between plants or
  crops, climates, and seasons.
• Evaporation and transpiration are often
  combined in a term - evapotranspiration.
• In many areas of the country and during certain
  seasons evapotranspiration is a major component
  of the hydrologic budget and a major concern in
  water supply and yield estimates.

Precipitation Evaporation Transpiration Storage-su
rface Infiltration Storage - Subsurface Runoff Wat
er Movement Streamflow Storage-Reservoirs
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Storage - Surface

• ... Storage - Surface is used to describe the
  precipitation that reaches the ground surface
  however, is not available for runoff or
  infiltration.
• It is instead, held in small quantities on the
  surface in areas, such as the leafy matter and
  small depressions.
• In general, surface storage is small and only
  temporary in terms of the overall hydrologic
  budget however, it may have an effect on a
  storm response as it is effectively "filled"
  early on a storm event.

Precipitation Evaporation Transpiration Storage-su
rface Infiltration Storage - Subsurface Runoff Wat
er Movement Streamflow Storage-Reservoirs
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Infiltration

• Soils, depending on current conditions, have a
  capacity or ability to infiltrate precipitation,
  allowing water to move from the surface to the
  subsurface.
• ... "physically based -gt soil porosity, depth
  of soil column, saturation levels, and soil
  moisture.
• The infiltration capacity of the soil column is
  usually expressed in terms of length per time
  (i.e. inches per hour).
• As more water infiltrates, the infiltration
  generally decreases, thus the amount of water
  that can be infiltrated during the latter stages
  of a precipitation event is less than that at the
  beginning of the event.

Precipitation Evaporation Transpiration Storage-su
rface Infiltration -Subsurface Storage -
Subsurface Runoff Water Movement Streamflow Storag
e-Reservoirs
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Infiltration cont.

• Storms that have high intensity levels may also
  cause excess precipitation because the intensity
  (inches per hour) may exceed the current
  infiltration capacity (inches per hour).
• periods of low rainfall or no rainfall will
  allow the soil to "recover" and increase the
  capacity to infiltrate water.
• Infiltrated water replenishes soil moisture and
  groundwater reservoirs. Infiltrated water may
  also resurface to become surface flow.
• attempt to account for infiltration by
  estimating excess precipitation (the difference
  between precipitation and excess being considered
  infiltration), for example, the Soil Conservation
  Service (SCS) runoff curve number method

Precipitation Evaporation Transpiration Storage-su
rface Infiltration -Subsurface Storage -
Subsurface Runoff Water Movement Streamflow Storag
e-Reservoirs
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Subsurface Flow
Precipitation Evaporation Transpiration Storage-su
rface Infiltration -Subsurface Storage -
Subsurface Runoff Water Movement Streamflow Storag
e-Reservoirs

• water may move via several paths.
• subsurface flow can be evaporated if there is a
  well maintained transfer mechanism to the
  surface. This is particularly true for areas of
  high ground water table (the free water surface
  of the groundwater) which is within the limits of
  the capillary action or transport abilities.
• Vegetation may also transpire or use the water.
  
• The subsurface flow may also continue to move
  with the groundwater table as a subsurface
  reservoir, which the natural system uses during
  periods of low precipitation.

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Storage - Subsurface

• The infiltrated water may continue downward in
  the vertical, may move through subsurface layers
  in a horizontal fashion, or a combination of the
  two directions.
• Movement through the subsurface system is much
  slower than the surface and thus there are
  storage delays. The water may also reach an
  aquifer, where it may be stored for a very long
  period of time.
• In the NWS River Forecast System (RFS), the
  subsurface storage is represented by imaginary
  zones or "tanks". These tanks release the stored
  water at a given or calibrated rate. The
  released water from the subsurface zones is added
  to the surface runoff for convolution with the
  unit hydrograph.

Precipitation Evaporation Transpiration Storage-su
rface Infiltration Storage - Subsurface Runoff Wat
er Movement Streamflow Storage-Reservoirs
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Runoff

• runoff will be used to collectively describe
  the precipitation that is not directly
  infiltrated into the groundwater system.
• is generally characterized by overland, gully
  and rill, swale, and channel flows.
• is that portion of a precipitation event that
  "quickly" reaches the stream system. The term
  "quickly" is used with caution as there may be
  great variability in response times for various
  flow mechanisms.
• Runoff producing events are usually thought of
  as those that saturate the soil column or occur
  during a period when the soil is already
  saturated. Thus infiltration is halted or
  limited and excess precipitation occurs. This
  may also occur when the intensity rate of the
  precipitation is greater than the infiltration
  capacity.

Precipitation Evaporation Transpiration Storage-su
rface Infiltration Storage - Subsurface Runoff Wat
er Movement Streamflow Storage-Reservoirs
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Overland Flow
Precipitation Evaporation Transpiration Storage-su
rface Infiltration Storage - Subsurface Runoff Wat
er Movement -Overland flow -Gullies and
Rills -Swales -Channel Flow -Stream
Channels Streamflow Storage-Reservoirs

• Overland flow or surface flow is that
  precipitation that either fails to penetrate into
  the soil or that resurfaces at a later point due
  to subsurface conditions.
• often referred to as "sheet" flow.
• for the purposes of this discussion, overland
  flow (sheet and surface flow, as well) is
  considered to be the flow that has not had a
  chance to collect and begin to form gullies,
  rills, swales

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Overland Flow (cont.)

• will eventually reach defined channels and the
  stream system.
• may also be infiltrated if it reaches an area
  that has the infiltration capacity to do so.
• Overland flow distances are rather limited in
  length - National Engineering Handbook (1972) -
  overland flow will concentrate into gullies in
  less than 1000 feet.
• Other (Seybert, Kibler, and White 1993)
  recommend a distance of 100 feet or less.

Precipitation Evaporation Transpiration Storage-su
rface Infiltration Storage - Subsurface Runoff Wat
er Movement -Overland flow -Gullies and
Rills -Swales -Channel Flow -Stream
Channels Streamflow Storage-Reservoirs
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Gullies Rills
Precipitation Evaporation Transpiration Storage-su
rface Infiltration Storage - Subsurface Runoff Wat
er Movement -Overland flow -Gullies and
Rills -Swales -Channel Flow -Stream
Channels Streamflow Storage-Reservoirs

• ... sheet flow or overland flow will soon
  concentrate into gullies and rills in the process
  of flowing towards the stream network. The
  location of these gullies and rills may vary from
  storm to storm, depending on storm patterns,
  intensities, current soil and land use
  conditions.

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Swales

• swales are of a more constant or permanent
  nature.
• do not vary in location from storm to storm.
• Swales are a natural part of the landscape or
  topography that are often more apparent than
  gullies and rills.
• Flow conditions and behaviors in swales are
  very close to that which is seen in channels.

Precipitation Evaporation Transpiration Storage-su
rface Infiltration Storage - Subsurface Runoff Wat
er Movement -Overland flow -Gullies and
Rills -Swales -Channel Flow -Stream
Channels Streamflow Storage-Reservoirs
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Channel Flow

• Excess precipitation ultimately reaches the
  stream channel system.
• the stream system is generally more defined, it
  is by no means a constant or permanent entity.
• The stream bed is constantly changing and
  evolving via aggredation and degradation.
• Stream channels convey the waters of the basin
  to the outlet and into the next basin.
• attenuation of the runoff hydrograph takes
  place.
• Stream channel properties (flow properties)
  also vary with the magnitude of the flow.

Precipitation Evaporation Transpiration Storage-su
rface Infiltration Storage - Subsurface Runoff Wat
er Movement -Overland flow -Gullies and
Rills -Swales -Channel Flow -Stream
Channels Streamflow Storage-Reservoirs
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Stream Channels

• Channels are commonly broken into main channel
  areas and overbank areas.
• overbank areas are often referred to as
  floodplains.
• Stream gaging stations are used to determine
  flows based on elevations in the channel and/or
  floodplain.
• Bank full is often thought of as flood stage
  although more rigorous definitions are more
  applicable as they pertain to human activity and
  potential loss of life and property.
• It is worth noting that the 2-year return
  interval flow is often thought of as "bank-full".

Precipitation Evaporation Transpiration Storage-su
rface Infiltration Storage - Subsurface Runoff Wat
er Movement -Overland flow -Gullies and
Rills -Swales -Channel Flow -Stream
Channels Streamflow Storage-Reservoirs
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Streamflow

• in the public eye -gt the most important aspect
  of flooding and hydrology.
• flooding from streams and rivers have the
  greatest potential to impact human property and
  lives although overland flow flooding,
  mudslides, and landslides are often just as
  devastating.
• Subsurface flow also enters the stream
  although in some instances and regions, stream
  channels lose water to the groundwater table -
  regardless, this must be accounted for in the
  modeling of the stream channel.
• Channels also offer a storage mechanism and the
  resulting effect is most often an attenuation of
  the flood hydrograph.

Precipitation Evaporation Transpiration Storage-su
rface Infiltration Storage - Subsurface Runoff Wat
er Movement Streamflow Storage-Reservoirs
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Storage - Reservoirs

• Lakes, reservoirs, structures, etc. are given
  a separate category in the discussion of the
  hydrologic cycle due to the potential impact on
  forecasting procedures and outcomes.
• provide a substantial storage mechanism and
  depending on the intended purpose of the
  structure will have varying impacts on the final
  hydrograph, as well as flooding levels.
• This effect can vary greatly depending on the
  type of reservoir, the outlet configuration, and
  the purpose of the reservoir.

Precipitation Evaporation Transpiration Storage-su
rface Infiltration Storage - Subsurface Runoff Wat
er Movement Streamflow Storage-Reservoirs
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Storage - Reservoirs (cont.)

• Flood control dams are used to attenuate and
  store potentially destructive runoff events.
• Other structures may adverse effects. For
  example, bridges may cause additional "backwater"
  effects and enhance the level of flooding
  upstream of the bridge.
• a catastrophic failure of a structure often has
  devastating effects on loss of life and property.
  

Precipitation Evaporation Transpiration Storage-su
rface Infiltration Storage - Subsurface Runoff Wat
er Movement Streamflow Storage-Reservoirs
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NWS - Forecast Terminology
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Hydrology Terminology
Topics Watershed Stream flow Reservoirs Channel Pr
ecipitation Snow Runoff Infiltration Unit
hydrograph Timing Flooding Flow Grade lines Land
Use Frequency
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Hydrology Terminology
Watershed -drainage area -drainage basin
-sub-basin -sub-area Streamflow Routing Reservo
irs Channel Precipitation Snow Runoff Infiltration
Unit hydrograph Timing Flooding Flow Grade
lines Land Use Frequency

• A watershed is an area of land that drains to a
  single outlet and is separated from other
  watersheds by a divide.
• Every watershed has a drainage area.
• Related terms drainage basin, sub-basin,
  sub-area.

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Hydrology Terminology
Watershed Streamflow -cross-section area
-Mannings n Routing Reservoirs Channel Precipit
ation Snow Runoff Infiltration Unit
hydrograph Timing Flooding Flow Grade lines Land
Use Frequency

• Streamflow is the movement of water through a
  channel.
• The cross-sectional area of a stream is the
  region bounded by the walls of the stream and the
  water surface. The cross-sectional area is
  illustrated below.
• See also Mannings n.

Cross-sectional Area
Stream Flow
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Hydrology Terminology
Watershed Streamflow -cross-section area
-Mannings n Routing Reservoirs Channel Precipit
ation Snow Runoff Infiltration Unit
hydrograph Timing Flooding Flow Grade lines Land
Use Frequency

• Mannings n is a measure of the roughness of a
  surface, and in streamflow it is the roughness of
  the channel bottom and its sides.

Diagram 2 will have a higher Mannings n
because it has rougher surface due to the jagged
bottom and pebbles.
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Hydrology Terminology
Watershed Streamflow Routing -Hydrologic
-Hydraulic Reservoirs Channel Precipitation Snow R
unoff Infiltration Unit hydrograph Timing Flooding
Flow Grade lines Land Use Frequency

• Routing is used to account for storage and
  translation effects.

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Hydrology Terminology
Watershed Streamflow Routing -Hydrologic
-Hydraulic Reservoirs Channel Precipitation Snow R
unoff Infiltration Unit hydrograph Timing Flooding
Flow Grade lines Land Use Frequency
Generalized effect of routing
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Hydrologic Routing
Watershed Streamflow Routing -Hydrologic
-Hydraulic Reservoirs Channel Precipitation Snow R
unoff Infiltration Unit hydrograph Timing Flooding
Flow Grade lines Land Use Frequency

• Hydrologic routing is the more simple of the two
  techniques.
• Based on the continuity equation which says
• Inflow - Outflow Change in Storage - or -
• A second relationship is also required which
  relates storage to discharge. This relationship
  is usually assumed, empirical, or analytical in
  nature.
• Two types of hydrologic routing, River and
  Reservoir Routing.

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Hydraulic Routing
Watershed Streamflow Routing -Hydrologic
-Hydraulic Reservoirs Channel Precipitation Snow R
unoff Infiltration Unit hydrograph Timing Flooding
Flow Grade lines Land Use Frequency

• Hydraulic routing is more complex and generally
  considered more accurate than hydrologic routing.
• Based on the simultaneous solution of the
  continuity equation and the momentum equation,
  commonly called the St. Venant equations.

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Hydrology Terminology
Watershed Streamflow Routing Reservoirs
-Storage -routing Channel Precipitation Snow R
unoff Infiltration Unit hydrograph Timing Flooding
Flow Grade lines Land Use Frequency

• Reservoir storage attenuates the flow and delays
  the impact of flood waters. Reservoirs are
  generally used for flood control, drinking water
  supply, hydropower, and recreation.

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Hydrology Terminology
Watershed Streamflow Routing Reservoirs
-Storage -routing Channel Precipitation Snow R
unoff Infiltration Unit hydrograph Timing Flooding
Flow Grade lines Land Use Frequency

• Reservoir routing is generally easier to perform
  than river routing because storage-discharge
  relations for pipes, weirs, and spillways are
  single-valued functions independent of flow.
• Storage indication method or Puls Method
• Other Methods Runge-Kutta Method

58
Hydrology Terminology
Watershed Streamflow Routing Reservoirs Channel
-Muskingum -Muskingum-Cunge -dynamic Precipitati
on Snow Runoff Infiltration Unit
hydrograph Timing Flooding Flow Grade lines Land
Use Frequency

• Channel routing can be broken into hydrologic
  and hydraulic methods.
• Hydrologic routing again uses the storage or
  continuity equation
• This formula subtracts the average outflow from
  an average inflow to determine the change in
  storage over a given time period.

59
Hydrology Terminology
Watershed Streamflow Routing Reservoirs Channel
-Muskingum -Muskingum-Cunge -dynamic Precipitati
on Snow Runoff Infiltration Unit
hydrograph Timing Flooding Flow Grade lines Land
Use Frequency

• Common methods of hydrologic routing
• Lag K
• Tatum
• Mod-Puls
• Kinematic Wave
• Muskingum
• Muskingum-Cunge

60
Hydrology Terminology
Watershed Streamflow Routing Reservoirs Channel
-Muskingum -Muskingum-Cunge -dynamic Precipitati
on Snow Runoff Infiltration Unit
hydrograph Timing Flooding Flow Grade lines Land
Use Frequency

• Hydraulic river routing includes solving the
  continuity equation and the momentum equation
  simultaneously.
• Dynamic routing is an example of this.
• DAMBRK FLDWAV, as well as, UNET are dynamic
  routing models

61
Hydrology Terminology
Watershed Streamflow Routing Reservoirs Channel Pr
ecipitation -excess -intensity
-patterns Snow Runoff Infiltration Unit
hydrograph Timing Flooding Flow Grade lines Land
Use Frequency

• Precipitation is water that falls to the earth in
  the form of rain, snow, hail or sleet.
• Excess precipitation is the precipitation that is
  not infiltrated into the soil and becomes
  available as a rapid runoff component in the
  hydrologic response of a basin.

62
Hydrology Terminology
Watershed Streamflow Routing Reservoirs Channel Pr
ecipitation -excess -intensity
-patterns Snow Runoff Infiltration Unit
hydrograph Timing Flooding Flow Grade lines Land
Use Frequency

• The intensity of the precipitation is the rate at
  which it is raining, and is measured in
  length/time. A radar picture of rainfall
  intensity can be seen below.

63
Hydrology Terminology
Watershed Streamflow Routing Reservoirs Channel Pr
ecipitation -excess -intensity
-patterns Snow Runoff Infiltration Unit
hydrograph Timing Flooding Flow Grade lines Land
Use Frequency

• Precipitation can fall in many different
  patterns, which influences the hydrologic
  response.
• For example, a storm may be
• Uniform over the entire watershed
• A storm may move up the watershed
• A storm may move down the watershed
• A storm may only rain on a portion of
  the watershed.

64
Hydrology Terminology

• Snowfall is a form of precipitation that comes
  down in white or translucent ice crystals.
• Snowmelt is the excess water produced by the
  melting of snow. This leads to flooding
  possibilities in the spring when temperatures
  begin to rise. There is generally a delay in the
  snowmelt response of a basin due to the melting
  process and travel times.
• Snowpack is the amount of annual accumulation at
  higher elevations.

Watershed Streamflow Routing Reservoirs Channel Pr
ecipitation Snow -snowfall -snowmelt
-snowpack Runoff Infiltration Unit
hydrograph Timing Flooding Flow Grade lines Land
Use Frequency
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Hydrology Terminology
Watershed Streamflow Routing Reservoirs Channel Pr
ecipitation Snow Runoff -overland flow
-sub-surface flow -baseflow Infiltration Unit
hydrograph Timing Flooding Flow Grade lines Land
Use Frequency

• Runoff is the excess precipitation and is often
  considered a fast response.
• Overland flow is the flow of water across the
  land surface.
• Sub-surface flow is the flow of water through the
  soil layers to the stream.
• Baseflow is the flow in a channel due to ground
  water or subsurface supplies. The baseflow is
  generally increased by precipitation events that
  produce enough infiltration.

66
Hydrology Terminology
Watershed Streamflow Routing Reservoirs Channel Pr
ecipitation Snow Runoff Infiltration Unit
hydrograph Timing Flooding Flow Grade lines Land
Use Frequency

• Infiltration is the movement of water from the
  surface into the soil.
• The rate of infiltration is based on a number of
  factors, including but not limited to
• soil types
• current conditions
• precipitation intensity
• The are many methods to estimate infiltration
  and/or excess precipitation. To name a few
• f index
• Hortons
• Green-Ampt
• SCS - curve number
• Continuous simulations (SAC-SMA)

67
Hydrology Terminology
Watershed Streamflow Routing Reservoirs Channel Pr
ecipitation Snow Runoff Infiltration Unit
hydrograph -derived -synthetic Timing Floodi
ng Flow Grade lines Land Use Frequency

• The unit hydrograph is the hydrograph for 1 unit
  of runoff in a given specified time or duration
  of runoff.

68
Hydrology Terminology
Watershed Streamflow Routing Reservoirs Channel Pr
ecipitation Snow Runoff Infiltration Unit
hydrograph -derived -synthetic Timing Floodi
ng Flow Grade lines Land Use Frequency

• The unit hydrograph is a transfer mechanism
  for transforming excess precipitation into
  streamflow.

69
Derived Unit Hydrograph

• Rules of Thumb
• the storm should be fairly uniform in nature
  and the excess precipitation should be equally as
  uniform throughout the basin. This may require
  the initial conditions throughout the basin to be
  spatially similar.
• Second, the storm should be relatively constant
  in time, meaning that there should be no breaks
  or periods of no precipitation.
• Finally, the storm should produce at least an
  inch of excess precipitation (the area under the
  hydrograph after correcting for baseflow).

Watershed Streamflow Routing Reservoirs Channel Pr
ecipitation Snow Runoff Infiltration Unit
hydrograph -derived -synthetic Timing Floodi
ng Flow Grade lines Land Use Frequency
70
Synthetic Unit Hydrograph

• SCS
• Snyder
• Clark - (time-area)

Watershed Streamflow Routing Reservoirs Channel Pr
ecipitation Snow Runoff Infiltration Unit
hydrograph -derived -synthetic Timing Floodi
ng Flow Grade lines Land Use Frequency
71
Hydrology Terminology
Watershed Streamflow Routing Reservoirs Channel Pr
ecipitation Snow Runoff Infiltration Unit
hydrograph Timing -lag time -time of
concentration -duration Flooding Flow Grade
lines Land Use Frequency

• Lag Time is the time from the center of mass of
  the rainfall to the peak of the unit hydrograph.
  
• Time of concentration is the time at which
  outflow from a basin is equal to the inflow. It
  is often considered the longest travel time from
  any point in the watershed.
• Duration is the time span of the rainfall.

72
Hydrology Terminology
Watershed Streamflow Routing Reservoirs Channel Pr
ecipitation Snow Runoff Infiltration Unit
hydrograph Timing Flooding -bank-full Flow Grad
e lines Land Use Frequency

• Flooding is the main concern of forecasters.
• Bank-full flooding is often thought of as the
  two-year return flow or Q2.
• The effects of flooding can drastically effect an
  ecosystem, which can be seen in the next two
  pictures.

73
Hydrology Terminology
Watershed Streamflow Routing Reservoirs Channel Pr
ecipitation Snow Runoff Infiltration Unit
hydrograph Timing Flooding -bank-full Flow Grad
e lines Land Use Frequency
Before
After
74
Hydrology Terminology
Watershed Streamflow Routing Reservoirs Channel Pr
ecipitation Snow Runoff Infiltration Unit
hydrograph Timing Flooding Flow -quantity
-timing -velocity -wave speed Grade
lines Land Use Frequency

• The flow and its effect on the environment and
  the human population depends on quantity, timing,
  velocity, and wave speed.
• The quantity of the flow is the volume of water,
  while the peak flow is generally of greatest
  interest.
• The timing of the flow is based on when a storm
  event occurs. If it occurs when a river is
  already close to flood stage, it will have a
  greater impact than if it occurred over a river
  that was relatively low. The time to peak, time
  of concentration, lag time, response time, and
  duration are all of great concern.

75
Hydrology Terminology
Watershed Streamflow Routing Reservoirs Channel Pr
ecipitation Snow Runoff Infiltration Unit
hydrograph Timing Flooding Flow -quantity
-timing -velocity -wave speed Grade
lines Land Use Frequency

• The velocity of the flow is based on the slope of
  the stream bottom. The greater the slope the
  greater the potential velocity of the flow.
• The wave speed is the velocity of the flood
  wave down the channel. The speed of this wave
  affects how quickly the downstream area will
  effected.

76
Hydrology Terminology

• The energy grade line represents the depth of the
  water surface and the velocity component of the
  Bernoulli equation.
• The hydraulic grade line represents the depth of
  the water surface.

Watershed Streamflow Routing Reservoirs Channel Pr
ecipitation Snow Runoff Infiltration Unit
hydrograph Timing Flooding Flow Grade lines
-EGL -HGL Land Use Frequency
77
Hydrology Terminology
Watershed Streamflow Routing Reservoirs Channel Pr
ecipitation Snow Runoff Infiltration Unit
hydrograph Timing Flooding Flow Grade lines Land
Use -land cover -urbanization -karst
-slope Frequency

• Land Use is a major contributor to runoff
  behavior.
• If the land is covered by trees, it will behave
  differently than if it was a pasture or a meadow.
• Urbanization also changes runoff patterns by the
  increase in artificial materials which decrease
  infiltration and increase flow response time.

78
Hydrology Terminology
Watershed Streamflow Routing Reservoirs Channel Pr
ecipitation Snow Runoff Infiltration Unit
hydrograph Timing Flooding Flow Grade lines Land
Use -land cover -urbanization -karst
-slope Frequency

• Karst hydrology is caused by pores and holes in
  limestone formations. This increases the
  infiltration into the limestone, reducing the
  runoff potential.

• The slope changes the speed of runoff and
  therefore effects collection times.

79
Hydrology Terminology
Watershed Streamflow Routing Reservoirs Channel Pr
ecipitation Snow Runoff Infiltration Unit
hydrograph Timing Flooding Flow Grade lines Land
Use Frequency -return period -probability

• The frequency of a storm event is described by
  its return period. For example a two year storm
  event has a 1 in 2 chance of occurring in any
  given year.
• The probability is also affected by the return
  period. Thus the probability of a 2 year storm
  occurring is 50. The probability of a 100-year
  event occurring is 1/100 or 1

80
Fluid Concepts
Topics Energy Head Momentum Open Channel
81
Energy or Energy Head
Energy Head -Elevation Head -Velocity Head
-Total Head Momentum Open Channel

• Elevation head
• Velocity head
• Total head

82
Energy or Energy Head

• The total energy of water moving through a
  channel is expressed in total head in feet of
  water.
• This is simply the sum of the the elevation above
  a datum (elevation head), the pressure head and
  the velocity head.
• The elevation head is the vertical distance from
  a datum to a point in the stream.
• The velocity head is expressed by

Energy Head -Elevation Head -Velocity Head
-Total Head Momentum Open Channel
83
Energy Head
Graphical depiction of elevation head, velocity
head, and total head. Total head is the sum of
velocity head, depth and elevation head.
Energy Head -Elevation Head -Velocity Head
-Total Head Momentum Open Channel
Energy Grade Line
headloss
Hydraulic Grade Line
Velocity head
(water surface)
Depth1
Channel Bottom
Elevation Head
Depth2
Datum
84
Momentum Equation
Energy Head Momentum -Equation -Forces Open
Channel
Hydrostatic Forces Friction Forces
Weight External Forces
85
Hydrostatic Forces

• Hydrostatic Forces are the forces placed on a
  control volume by the surrounding water.
• The strength of the force is based on depth and
  can be seen in the following relationship

Energy Head Momentum -Equation -Forces Open
Channel
Hydrostatic Forces
Control Volume
Hydrostatic Forces Friction Forces
Weight External Forces
86
Friction Forces
Energy Head Momentum -Equation -Forces Open
Channel
The friction force on a control volume is due to
the water passing the channel bottom and depends
on the roughness of the channel.
Control Volume

Friction Force
Hydrostatic Forces Friction Forces
Weight External Forces
87
Weight
The weight of a control volume is due to the
gravitational pull on the its mass.
Energy Head Momentum -Equation -Forces Open
Channel
Weight mg
Control Volume
Weight
Hydrostatic Forces Friction Forces
Weight External Forces
88
External Forces
External Forces (Fd) the forces created by a
control volume striking a stationary object.
External Forces can be explained by the following
equation
Energy Head Momentum -Equation -Forces Open
Channel
Fd1/2CdrAv2
Hydrostatic Forces Friction Forces
Weight External Forces
89
Steady vs. Unsteady Flow

• Fluid properties including velocity, pressure,
  temperature, density, and viscosity vary in time
  and space.
• A fluid it termed steady if the depth of flow
  does not change or can be assumed constant during
  a specific time interval.
• Flow is considered unsteady if the depth changes
  with time.

Energy Head Momentum Open Channel -Steady -vs-
Unsteady -Uniform -vs- Nonuniform
-Supercritical -vs- subcritical -Equations
90
Uniform and Nonuniform Flow

• Uniform Flow is an equilibrium flow such that the
  slope of the total energy equals the bottom
  slope.
• Nonuniform Flow is a flow of water through a
  channel that gradually changes with distance.

Energy Head Momentum Open Channel -Steady -vs-
Unsteady -Uniform -vs- Nonuniform
-Supercritical -vs- subcritical -Equations
91
Super -vs.- Sub Critical
Energy Head Momentum Open Channel
-Steady-vs.-Unsteady -Uniform-vs. Nonuniform
-Sub/Supercritical -Equations
92
Critical flow a demonstration
If a stone is dropped into a body of water, with
no velocity, the waves formed by the water are
fairly circular. This is similar to sub-critical
flow.
Energy Head Momentum Open Channel
-Steady-vs.-Unsteady -Uniform-vs. Nonuniform
-Sub/Supercritical -Equations
No velocity
93
Critical flow a demonstration
Now, if a velocity is added to the body of water,
the waves become unsymmetrical, increasing to the
downstream side. This happens as the velocity
approaches critical flow. Notice that the wave
still moves upstream, though slower than the
downstream wave.
Energy Head Momentum Open Channel
-Steady-vs.-Unsteady -Uniform-vs. Nonuniform
-Sub/Supercritical -Equations
Small velocity
94
Critical flow a demonstration
Energy Head Momentum Open Channel
-Steady-vs.-Unsteady -Uniform-vs. Nonuniform
-Sub/Supercritical -Equations
Now if a large velocity is added to the body of
water, the wave patterns only go in one
direction. This represents the point when flow
has gone beyond critical, into the supercritical
region.
Large velocity
95
Froude number
The Froude number is a numerical value that
describes the type of flow present (critical,
supercritical, subcritical), and is represented
by the following equation for a rectangular
channel
Energy Head Momentum Open Channel
-Steady-vs.-Unsteady -Uniform-vs. Nonuniform
-Sub/Supercritical -Froude number -Equations
NF Froude number v mean velocity of flow g
acceleration of gravity dm mean (hydraulic)
depth
96
Froude number
The generalized formula for the Froude number is
as follows
Energy Head Momentum Open Channel
-Steady-vs.-Unsteady -Uniform-vs. Nonuniform
-Sub/Supercritical -Froude number -Equations
Fr Froude number Q Flow rate in the channel T
Time A Area of the channel
97
Froude number - mean depth
Energy Head Momentum Open Channel
-Steady-vs.-Unsteady -Uniform-vs. Nonuniform
-Sub/Supercritical -Froude number -Equations

• Mean depth is a ratio of the width of the free
  water surface to the cross-sectional area of the
  channel.

98
Froude number

• The Froude number can then be used to quantify
  the type of flow.
• If the Froude number is less than 1.0, the flow
  is subcritical. The flow would would be
  characterized as tranquil.
• If the Froude number is equal to 1.0, the flow is
  critical.
• If the Froude number is greater than 1.0, the
  flow is supercritical and would be characterized
  as rapid flowing. This type of flow has a high
  velocity which can be potential damaging.

Energy Head Momentum Open Channel
-Steady-vs.-Unsteady -Uniform-vs. Nonuniform
-Sub/Supercritical -Froude number -Equations
99
Super-vs.-Subcritical

• Critical depth can also be determined by
  constructing a Specific Energy Curve.
• The critical depth is the point on the curve with
  the lowest specific energy.
• Any depth greater than critical depth is
  subcritical flow and any depth less than is
  supercritical flow.

Energy Head Momentum Open Channel
-Steady-vs.-Unsteady -Uniform-vs. Nonuniform
-Sub/Supercritical -Equations
100
Super-vs.-Subcritical
101
Open Channel Equations
Energy Head Momentum Open Channel -Steady -vs-
Unsteady -Uniform -vs- Nonuniform
-Supercritical -vs- subcritical
-Equations Chezy Manning Bernoulli St. Venant

• Chezy Equation
• Mannings Equation
• Bernoulli Equation
• St. Venant Equations

102
Chezy Equation
Energy Head Momentum Open Channel -Chezy
Equation -Mannings -Bernoulli -St. Venant

• In 1769, the French engineer Antoine Chezy
  developed the first uniform-flow formula.

• The formula was derived based on two assumptions.
  First, Chezy assumed that the force resisting
  the flow per unit area of the stream bed is
  proportional to the square of the velocity (KV2),
  with K being a proportionality constant.

103
Chezy Equation
Energy Head Momentum Open Channel -Chezy
Equation -Mannings -Bernoulli -St. Venant

• The second assumption was that the channel was
  undergoing uniform flow.
• The difficulty with this formula is determining
  the value of C, which is the Chezy resistance
  factor. There are three different formulas for
  determining C, the G.K. Formula, the Bazin
  Formula, and the Powell Formula.

104
Chezy Equation
Energy Head Momentum Open Channel -Chezy
Equation -Mannings -Bernoulli -St. Venant

• Later on, when Manning's equation was developed
  in 1889, a relationship between Mannings n and
  Chezys C was established.
• Finally in 1933, the Manning equation was
  suggested for international use rather than
  Chezys Equation.

105
Mannings Equation
Energy Head Momentum Open Channel -Chezy
Equation -Mannings -Bernoulli -St. Venant

• In 1889 Robert Manning, an Irish engineer,
  presented the following formula to solve open
  channel flow.

V mean velocity in fps R hydraulic radius in
feet S the slope of the energy line n
coefficient of roughness
The hydraulic radius (R) is a ratio of the water
area to the wetted perimeter.
106
Mannings Equation
Energy Head Momentum Open Channel -Chezy
Equation -Mannings -Bernoulli -St. Venant

• This formula was later adapted to obtain a flow
  measurement. This is done by multiplying both
  sides by the area.

• Mannings equation is the most widely used of all
  uniform-flow formulas for open channel flow,
  because of its simplicity and satisfactory
  results it produces in real-world applications.

107
Mannings Equation
Energy Head Momentum Open Channel -Chezy
Equation -Mannings -Bernoulli -St. Venant

• Note that the equation expressed in the previous
  slide was the English version of Mannings
  equation.
• There is also a metric version of Mannings
  equation, which replaces the 1.49 with 1. This
  is done because of unit conversions.
• The metric equation is

108
Bernoulli Equation
Energy Head Momentum Open Channel -Chezy
Equation -Mannings -Bernoulli -St. Venant

• The Bernoulli equation is developed from the
  following equation

This equation states that the elevation (z) plus
the depth (y) plus the velocity head (V12/2g) is
a constant. The difference being the headlosses
- hL
109
Bernoulli Equation

• This equation was then adapted by making a few
  assumptions.
• First, the head loss due to friction is equal to
  zero. This means the channel is perfectly
  frictionless surface.
• Second, that alpha1 is equal to alpha2 which is
  equal to 1. The alphas are in the original
  equation to account for a non-uniform velocity
  distribution. In this case we will assume a
  uniform distribution which produces the following
  equation

Energy Head Momentum Open Channel -Chezy
Equation -Mannings -Bernoulli -St. Venant
110
Bernoulli Equation
Energy Head Momentum Open Channel -Chezy
Equation -Mannings -Bernoulli -St. Venant
A simplified version of the formula is given
below
111
Bernoulli Equation
Energy Head Momentum Open Channel -Chezy
Equation -Mannings -Bernoulli -St. Venant

• Some comments on the Bernoulli equation
• Energy only
• Headloss in terms of energy
• Cannot calculate forces
• Limited Effect in rapidly varying flow

112
St. Venant Equations
Energy Head Momentum Open Channel -Chezy
Equation -Mannings -Bernoulli -St. Venant
The two equations used in modeling are the
continuity equation and the momentum equation.
Continuity equation
Momentum Equation
113
St. Venant Equations
The Momentum Equation can often be simplified
based on the conditions of the model.
Energy Head Momentum Open Channel -Chezy
Equation -Mannings -Bernoulli -St. Venant
Unsteady -Nonuniform
Steady - Nonuniform
Diffusion or noninertial
Kinematic
114
Simulating the Hydrologic Response
Model Types Precipitation Losses Modeling
Losses Model Components
115
Model Types
Model Types Precipitation Losses Modeling
Losses Model Components

• Empirical
• Lumped
• Distributed

116
Precipitation

• magnitude, intensity, location, patterns, and
  future estimates of the precipitation.
• In lumped models, the precipitation is input in
  the form of average values over the basin. These
  average values are often referred to as mean
  aerial precipitation (MAP) values.
• MAP's are estimated either from 1)
  precipitation gage data or 2) NEXRAD
  precipitation fields.

Model Types Precipitation -Thiessen
-Isohyetal -Nexrad Losses Modeling Losses Model
Components
117
Precipitation (cont.)

• If precipitation gage data is used, then the
  MAP's are usually calculated by a weighting
  scheme.
• a gage (or set of gages) has influence over an
  area and the amount of rain having been recorded
  at a particular gage (or set of gages) is
  assigned to an area.
• Thiessen method and the isohyetal method are
  two of the more popular methods.

Model Types Precipitation -Thiessen
-Isohyetal -Nexrad Losses Modeling Losses Model
Components
118
Thiessen
Model Types Precipitation -Thiessen
-Isohyetal -Nexrad Losses Modeling Losses Model
Components

• Thiessen method is a method for areally weighting
  rainfall through graphical means.

119
Isohyetal
Model Types Precipitation -Thiessen
-Isohyetal -Nexrad Losses Modeling Losses Model
Components

• Isohyetal method is a method for areally
  weighting rainfall using contours of equal
  rainfall (isohyets).

120
NEXRAD
Model Types Precipitation -Thiessen
-Isohyetal -Nexrad Losses Modeling Losses Model
Components

• Nexrad is a method of areally weighting rainfall
  using satellite imaging of the intensity of the
  rain during a storm.

121
Losses

• modeled in order to account for the destiny of
  the precipitation that falls and the potential of
  the precipitation to affect the hydrograph.
• losses include interception, evapotranspiration,
  depression storage, and infiltration.
• Interception is that precipitation that is
  caught by the vegetative canopy and does not
  reach the ground for eventual infiltration or
  runoff.
• Evapotranspiration is a combination of
  evaporation and transpiration and was previously
  discussed.
• Depression storage is that precipitation that
  reaches the ground, yet, as the name suggests, is
  stored in small surface depressions and is
  generally satisfied during the early portion of a
  storm event.

Model Types Precipitation Losses Modeling
Losses Model Components
122
Modeling Losses

• simplistic methods such as a constant loss
  method may be used.
• A constant loss approach assumes that the soil
  can constantly infiltrate the same amount of
  precipitation throughout the storm event. The
  obvious weaknesses are the neglecting of spatial
  variability, temporal variability, and recovery
  potential.
• Other methods include exponential decays (the
  infiltration rate decays exponentially),
  empirical methods, and physically based methods.
• There are also combinations of these methods.
  For example, empirical coefficients may be
  combined with a more physically based equation.
  (SAC-SMA for example)

Model Types Precipitation Losses Modeling Losses
-SAC-SMA Model Components
123
Simulating Watershed ResponseInfiltration
Long Term vs.- Short Infiltration Evapotranspirat
ion Unit Hydrograph Timing Routing
Infiltration or losses - this section describes
the action of the precipitation infiltrating into
the ground. It also covers the concept of
initial abstraction, as it is generally
considered necessary to satisfy the initial
abstraction before the infiltration process
begins.
124
Simulating Watershed ResponseInfiltration
Long Term vs.- Short Infiltration Evapotranspirat
ion Unit Hydrograph Timing Routing
Initial Abstraction - It is generally assumed
that the initial abstractions must be satisfied
before any direct storm runoff may begin. The
initial abstraction is often thought of as a
lumped sum (depth). Viessman (1968) found that
0.1 inches was reasonable for small urban
watersheds. Would forested rural watersheds be
more or less?
125
Simulating Watershed ResponseInfiltration
Long Term vs.- Short Infiltration Evapotranspirat
ion Unit Hydrograph Timing Routing
Forested rural watersheds would probably have a
higher initial abstraction. The Soil Conservation
Service (SCS) now the NRCS uses a percentage of
the ultimate infiltration holding capacity of the
soil - i.e. 20 of the maximum soil retention
capacity.
126
Simulating Watershed ResponseInfiltration
Long Term vs.- Short Infiltration Evapotranspirat
ion Unit Hydrograph Timing Routing
Infiltration is a natural process that we attempt
to mimic using mathematical processes. Some of
the mathematical process or simulation methods
are conceptual while others are more physically
based.
127
Simulating Watershed ResponseInfiltration
Constant Infiltration Rate A constant
infiltration rate is the most simple of the
methods. It is often referred to as a phi-index
or f-index. In some modeling situations it is
used in a conservative mode. The saturated soil
conductivity may be used for the infiltration
rate. The obvious weakness is the inability to
model changes in infiltration rate. The phi-index
may also be estimated from individual storm
events by looking at the runoff hydrograph.
Long Term vs.- Short Infiltration Evapotranspirat
ion Unit Hydrograph Timing Routing
128
Simulating Watershed ResponseInfiltration
Long Term vs.- Short Infiltration Evapotranspirat
ion Unit Hydrograph Timing Routing
Constant Percentage Method Another very
simplistic approach - this method assumes that
the watershed is capable of infiltrating or
using a value that is proportional to rainfall
intensity. The constant percentage rate can be
calibrated for a basin by again considering
several storms and calculating the percentage by
129
Constant Percentage Example
Long Term vs.- Short Infiltration Evapotranspirat
ion Unit Hydrograph Timing Routing
2
77.5 infiltrates
1
0
130
Simulating Watershed ResponseInfiltration
Long Term vs.- Short Infiltration Evapotranspirat
ion Unit Hydrograph Timing Routing
Exponential Decay This is purely a mathematical
function - of the following form
131
Simulating Watershed ResponseInfiltration
Long Term vs.- Short Infiltration Evapotranspirat
ion Unit Hydrograph Timing Routing
Exponential Decay
Effect of fo or fc
132
Simulating Watershed ResponseInfiltration
Long Term vs.- Short Infiltration Evapotranspirat
ion Unit Hydrograph Timing Routing
Exponential Decay
Effect of K
133
Simulating Watershed ResponseInfiltration
Long Term vs.- Short Infiltration Evapotranspirat
ion Unit Hydrograph Timing Routing
SCS Curve Number
Soil Conservation Service is an empirical method
of estimating EXCESS PRECIPITATION We can imply
that P - Pe F
134
SCS (NRCS) Runoff Curve Number

• The basic relationships used to develop the curve
  number runoff prediction technique are described
  here as background for subsequent discussion.
  The technique originates with the assumption that
  the following relationship describes the water
  balance of a storm event.

where F is the actual retention on the watershed,
Q is the actual direct storm runoff, S is the
potential maximum retention, and P is the
potential maximum runoff
135
More Modifications

• At this point in the development, SCS redefines S
  to be the potential maximum retention
• SCS defines Ia in terms of S as Ia 0.2S
• and since the retention, F, equals effective
  precipitation minus runoff F (P-Ia) - Q
• Substituting gives the familiar SCS
  rainfall-runoff

136
Estimating S

• The difficult part of applying t