ENVI 412ENVI 412Hydrologic Losses and Hydrologic Losses and

Radar MeasurementRadar MeasurementDr. Philip B. BedientDr. Philip B. Bedient

Rice UniversityRice University

Qe = energy used for evaporation

Qh = sensible heat

Q = stored energy

Qv = advected energy

QN = net radiation absorbed by water body

Lake Energy BudgetLake Energy Budget

• Function of wind speed, T, and

     humidity gradient

•  Energy source - solar energy

• Mass transfer, energy budget,

    and pan evaporation

• Penman’s combined (1948)

Lake EvaporationLake Evaporation

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

Mass TransferMass Transfer

Shallow Lake Evap Shallow Lake Evap (Kohler, 1955(Kohler, 1955))

Evaporation PansEvaporation Pans

• Anemometer - wind

• Rain Gage - precip.

• Pan for water - evap

• Level measured daily

• Refilled as necessary

Soil Moisture CycleSoil Moisture Cycle• Autumn - rainfall recharge

• Winter - max soil storage

• Spring - some evap loss

• Summer - most depleted

Surface Flow Surface Flow DistributionDistribution

Horton’s Infiltration ConceptHorton’s Infiltration Conceptf(t) = Rate of water loss into soilf(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

Horton’s EqnHorton’s Eqn

index Methodindex Method• Assumes constant rate

  over time of rainfall

• Volume above line is

  DRO

• Volume below line is F(t)

• Trial and error computed

Example of Example of IndexIndex

DRO

VOL Infiltration F(t)

Example of Example of IndexIndexAssume 4.9 in of DRO from a 560 acre BasinSet up a general Eqn for indexindex

2(1.4 - +3(0.7-

Find by trial and error by assuming a value and solving - try = 1.5 in/hrAnd it only accounts for 2.4 in of DRO0.5 in/hr yields 9.0 in of DRO - too much DRO

Try 1.0 in/hr or 2(.4) +3(1.3)+2(.1) = 4.9 inches

Brays Bayou at Main Brays Bayou at Main St BridgeSt Bridge

• Measure v at 0.2 and 0.8 of depth

• Average v and multiply by W*D

• Sum up across stream to get total Q

Stream Cross-Section for QStream Cross-Section for Q

• Plot of z vs. Q

• Determined from stream

   measurements of V

• Unique for each stream

• Changes with development

• Available for all USGS gages

Typical Rating Curve for StreamTypical Rating Curve for Stream

Standard Flood Alert SystemStandard Flood Alert SystemUse measured rainfall

Predict hydrologic Response in x,y, and t

Alert various agenciesand emergency mgrs

Save lives and damages

Use of NEXRAD Rainfall for Use of NEXRAD Rainfall for Hydrologic PredictionHydrologic Prediction

Dr. Baxter Vieux, University of OklahomaDr. Baxter Vieux, University of Oklahoma

National Severe Storm LaboratoryNational Severe Storm Laboratory

• Recent Innovation

• Uses radar - NWS

• DPA every 5 minutes

• Accurate to 230 km

• Provides better spatial

    detail than gages

NEXRAD Radar DataNEXRAD Radar Data

Radar Provides Visual EffectsRadar Provides Visual Effects

Midnight 1 a.m.

y = 0.8991x - 0.0382

R2 = 0.9033

0

2

4

6

8

10

12

0 2 4 6 8 10 12

Gauge (in.)

Radar (in.)

Brays Bayou

Sims Bayou

Radar–Gage CalibrationRadar–Gage Calibration October 17, 1994 October 17, 1994

Tot

al R

ainf

all R

adar

(in

.)

Total Rainfall measured at the Gage (in.)

Rice Blvd. and Brays Bayou

02468

1012

0 10 20 30 40 50

Time (hr.)

Gauge DataRadar Data

Cum

ulat

ive

Rai

nfal

l (

in.)

October, 1994 CalibrationOctober, 1994 Calibration

Weather Radar SystemsWeather Radar Systems

Recently deployed weather radar systems such Recently deployed weather radar systems such as NEXRAD offer accurate and reliable as NEXRAD offer accurate and reliable precipitation estimation precipitation estimation

Increased sensitivity coupled with improved Increased sensitivity coupled with improved processing provides high-resolution radar data processing provides high-resolution radar data sets for a variety of applications. sets for a variety of applications.

Provides another source of rainfall information in Provides another source of rainfall information in addition to rain gaugesaddition to rain gauges

WSR-88D - NEXRADWSR-88D - NEXRAD The first operational WSR-88D was The first operational WSR-88D was

installed in May 1990 at Twin Lakes, OKinstalled in May 1990 at Twin Lakes, OK 160 + deployed nationwide and overseas.160 + deployed nationwide and overseas. Is now being used for much more than Is now being used for much more than

weather forecasts. weather forecasts. Most significant advancement in hydrology Most significant advancement in hydrology

in last 20 years!in last 20 years!

Users of Radar and Users of Radar and Meteorological DataMeteorological Data

Real-time access to radar and other Real-time access to radar and other meteorological data is now provided to meteorological data is now provided to

users outside of the NWSusers outside of the NWS

Nexrad has spawned a private sector Nexrad has spawned a private sector meteorological services industrymeteorological services industry

Now other users are beginning to Now other users are beginning to experience the benefits within the experience the benefits within the hydrologic communityhydrologic community

Low Precision 16-level Image

16-level precision image vs. 256-level data

FAS2 will add 482 radar rain gauges over Brays

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∂

∂

T.S. Allison Storm TotalJune 8-9, 2001June 8-9, 2001

Bayous

Counties

Highways

Drainage

TMCÊÚStorm Total (in)

0.01 - 0.250.25 - 0.50.5 - 11 - 22 - 44 - 66 - 88 - 1010 - 1212 - 1414 - 1616 - 1818 - 2020 - 2222 - 25> 25

ÊÚ

.-,45

.-,10

.-,59

N

0 5 10 Miles

26.6 in

Prospects for Flood Modeling Prospects for Flood Modeling in Real-Timein 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?