ENVI 412 Hydrologic Losses and Radar Measurement

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ENVI 412Hydrologic Losses and Radar Measurement

• Dr. Philip B. Bedient
• Rice University

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Lake Energy Budget

• Qe energy used for evaporation
• Qh sensible heat
• Qq stored energy
• Qv advected energy
• QN net radiation absorbed by water body

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Lake Evaporation

• Function of wind speed, T, and      humidity
  gradient
•  Energy source - solar energy
• Mass transfer, energy budget,     and pan
  evaporation
• Penmans combined (1948)

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Mass Transfer
E es - ea (a bu) Where E evaporation
(cm/day) es Sat vapor pressure (T) ea
Vapor pres at fixed z u wind speed in
m/sec a,b constants
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Shallow Lake Evap (Kohler, 1955)
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Evaporation Pans

• Anemometer - wind
• Rain Gage - precip.
• Pan for water - evap
• Level measured daily
• Refilled as necessary

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Soil Moisture Cycle

• Autumn - rainfall recharge
• Winter - max soil storage
• Spring - some evap loss
• Summer - most depleted

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Surface Flow Distribution
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Hortons Infiltration Concept f(t) Rate of
water loss into soil
f fc (fo - fc) exp (-kt) fc final rate
value fo initial rate value K decay rate Can
integrate to get F(t) Vol of infiltration
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Hortons Eqn
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? index Method

• Assumes constant rate   over time of rainfall
• Volume above line is   DRO
• Volume below line is F(t)
• Trial and error computed

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Example of F Index
DRO
VOL Infiltration F(t)
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Example of F Index
Assume 4.9 in of DRO from a 560 acre Basin Set up
a general Eqn for F index 2(1.4 - F) 3(2.3-
F) 2(1.1- F) 3(0.7- F) 2(0.3- F) 4.9 Find
F by trial and error by assuming a value and
solving - try F 1.5 in/hr And it only accounts
for 2.4 in of DRO F 0.5 in/hr yields 9.0 in of
DRO - too much DRO Try F 1.0 in/hr or 2(.4)
3(1.3)2(.1) 4.9 inches
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Brays Bayou at Main St Bridge
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Stream Cross-Section for Q

• Measure v at 0.2 and 0.8 of depth
• Average v and multiply by DWD
• Sum up across stream to get total Q

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Typical Rating Curve for Stream

• Plot of z vs. Q
• Determined from stream    measurements of V
• Unique for each stream
• Changes with development
• Available for all USGS gages

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Standard Flood Alert System
Use measured rainfall Predict hydrologic
Response in x,y, and t Alert various
agencies and emergency mgrs Save lives and
damages
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Use of NEXRAD Rainfall for Hydrologic Prediction

• Dr. Baxter Vieux, University of Oklahoma
• National Severe Storm Laboratory

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NEXRAD Radar Data

• Recent Innovation
• Uses radar - NWS
• DPA every 5 minutes
• Accurate to 230 km
• Provides better spatial     detail than gages

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Radar Provides Visual Effects
Midnight 1 a.m.
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RadarGage Calibration October 17, 1994
Sims Bayou
Total Rainfall Radar (in.)
Brays Bayou
Total Rainfall measured at the Gage (in.)
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October, 1994 Calibration
Cumulative Rainfall (in.)
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Weather Radar Systems

• Recently deployed weather radar systems such as
  NEXRAD offer accurate and reliable precipitation
  estimation
• Increased sensitivity coupled with improved
  processing provides high-resolution radar data
  sets for a variety of applications.
• Provides another source of rainfall information
  in addition to rain gauges

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WSR-88D - NEXRAD

• The first operational WSR-88D was installed in
  May 1990 at Twin Lakes, OK
• 160 deployed nationwide and overseas.
• Is now being used for much more than weather
  forecasts.
• Most significant advancement in hydrology in last
  20 years!

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Users of Radar and Meteorological Data

• Real-time access to radar and other
  meteorological data is now provided to
• users outside of the NWS
• Nexrad has spawned a private sector
  meteorological services industry
• Now other users are beginning to experience the
  benefits within the hydrologic community

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Low Precision 16-level Image
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16-level precision image vs. 256-level data
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FAS2 will add 482 radar rain gauges over Brays
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T.S. Allison Storm TotalJune 8-9, 2001
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Prospects for Flood Modeling in Real-Time

• Forecasting urban streams that respond rapidly to
  heavy rainfall is difficult.
• Such forecasts can easily underpredict the river
  stage with little or no lead time
• Why have hydrologic models lagged the development
  of radar technology and meteorological science?
• How can we improve current hydrologic practice in
  order to forecast flood levels in real-time?