Project 0-4193 Regional Characteristics of Unit Hydrographs

1. 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

2. 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 . . .

3. 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)

4. 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.)

5. GUGAS SHAPE PARAMETER NRCS

6. Undeveloped watershed DUH is more symmetrical and peaky than developed watershed DUH and the NRCS DUH. COMPARISON OFGUH TO NRCS DIMENSIONLESS UH

7. 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

8. Comparison of GUHs

9. Comparison of GUHs

10. ESTIMATION OF TIME TO PEAK FROM TIME OF CONCENTRATION

11. TWO METHODS FOR TIME TO PEAK

12. 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!

13. 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

15. 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.

16. Process Model