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ENVI 412 Hydrologic Losses and Radar Measurement

ENVI 412 Hydrologic Losses and Radar Measurement. Dr. Philip B. Bedient Rice University. Lake Energy Budget. Q e = energy used for evaporation Q h = sensible heat Q q = stored energy Q v = advected energy Q N = net radiation absorbed by water body. Lake Evaporation.

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ENVI 412 Hydrologic Losses and Radar Measurement

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  1. ENVI 412Hydrologic Losses and Radar Measurement Dr. Philip B. Bedient Rice University

  2. Lake Energy Budget • Qe = energy used for evaporation • Qh = sensible heat • Qq = stored energy • Qv = advected energy • QN = net radiation absorbed by water body

  3. Lake Evaporation • Function of wind speed, T, and      humidity gradient •  Energy source - solar energy • Mass transfer, energy budget,     and pan evaporation • Penman’s combined (1948)

  4. 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

  5. Shallow Lake Evap (Kohler, 1955)

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

  7. Soil Moisture Cycle • Autumn - rainfall recharge • Winter - max soil storage • Spring - some evap loss • Summer - most depleted

  8. Surface Flow Distribution

  9. Horton’s 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

  10. Horton’s Eqn

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

  12. Example of F Index DRO VOL Infiltration F(t)

  13. Example of F Index Assume 4.9 in of DRO from a 560 acre Basin Set up a general Eqn forF index 2(1.4 - F) + 3(2.3- F) +2(1.1- F) +3(0.7- F) + 2(0.3- F) = 4.9 Find Fby 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

  14. Brays Bayou at Main St Bridge

  15. Stream Cross-Section for Q • Measure v at 0.2 and 0.8 of depth • Average v and multiply by DW*D • Sum up across stream to get total Q

  16. 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

  17. 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

  18. Use of NEXRAD Rainfall for Hydrologic Prediction Dr. Baxter Vieux, University of Oklahoma National Severe Storm Laboratory

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

  20. Radar Provides Visual Effects Midnight 1 a.m.

  21. Radar–Gage Calibration October 17, 1994 Sims Bayou Total Rainfall Radar (in.) Brays Bayou Total Rainfall measured at the Gage (in.)

  22. October, 1994 Calibration Cumulative Rainfall (in.)

  23. 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

  24. 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!

  25. 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

  26. Low Precision 16-level Image

  27. 16-level precision image vs. 256-level data

  28. FAS2 will add 482 radar rain gauges over Brays

  29. T.S. Allison Storm TotalJune 8-9, 2001 26.6 in

  30. 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?

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