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Project 0-4193 Regional Characteristics of Unit Hydrographs. David B. Thompson, Texas Tech, RS George R. Herrmann, TxDOT, PD William H. Asquith, U.S. Geological Survey, Co-PI Xing Fang, Lamar University, Co-PI Theodore G. Cleveland, University of Houston, Co-PI. Background.
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Project 0-4193Regional Characteristics of Unit Hydrographs David B. Thompson, Texas Tech, RS George R. Herrmann, TxDOT, PD William H. Asquith, U.S. Geological Survey, Co-PI Xing Fang, Lamar University, Co-PI Theodore G. Cleveland, University of Houston, Co-PI
Background • Substantial progress on the “Texas” hydrograph method has been made over the past 7 years or so ... • Design storm depth and temporal distribution--DONE • Weakness in Curve Number method documented--DONE • Unit hydrographs--almost DONE • Time parameters--almost DONE • What is left? A temporal loss rate model=excess rainfall • Complete the loop: Rainfall --> Excess Rainfall --> Runoff • Motivation for 2-year extension request . . .
Primary 0-4193 Objectives • Is the NRCS dimensionless unit hydrograph representative of Texas hydrology? SORT OF (at times) • If not, can alternative methods be developed? certainly YES • Can an alternative loss model to NRCS curve number method be developed from the existing database and resulting UH? Can a loss model “tuned” to the UH procedures that will result from 0-4193 to date? (Requested modification)
0-4193 Approach • Common database (93 watersheds and 1,600 events) • Joint but INDEPENDENT data processing . . . • Assumption of differing LOSS-MODELS--A necessary step • Unit Hydrograph Computations • Gamma UH fit to Qp and Tp (USGS): Shape and Tp parameters • Rayleigh IUH fit by error minimization (Univ. of Houston) • Linear programming fit by error minimization (Lamar Univ.) • Traditional method (Texas Tech Univ.)
Undeveloped watershed DUH is more symmetrical and peaky than developed watershed DUH and the NRCS DUH. COMPARISON OFGUH TO NRCS DIMENSIONLESS UH
Time to Peak • Equation to estimate time to peak from main channel length, main channel slope, and development classification has been developed. • Measure of equation applicability • Measure of equation prediction accuracy. • Handy nomograph UNDEVELOPED WATERSHEDS
0-4193 Implications • Watershed development influences UH shape and time scale. • GUH of same order of NRCS GUH. • GUH shape can be predicted and uncertainty computed. • K = function(L,D) • GUH Time-to-Peak (Tp) can be predicted without Time-of-Concentration (Tc) and uncertainty computed. • Tp = function(L,S,D) • GUH Tp can be predicted with Tc and uncertainty computed. • Tp= function(Tc, D) • THE TWO TpMETHODS COMPLIMENT EACH OTHER!
Temporal Loss-Rate Functions • Potential candidate models • Initial loss, constant loss-rate, runoff coefficients • Green-Ampt, and other phenomenological models Work to Do • Modify existing analytical tools to accommodate search for optimal loss-rate function given a UH. • Apply to database to estimate parameters • Analyze results from parameter estimation • Synthesize and regionalize
Unit Hydrographs • A unit hydrograph (UH) is the hydrograph of runoff that results from a unit pulse of effective precipitation (runoff) distributed evenly over a watershed over a specific duration of time. • UHs are used for drainage designs for watersheds with drainage areas with a wide range of sizes.