1. Hydrologic Analysis (Bedient chapter 2)
Learning Objectives
Be able to describe watershed characteristics that impact runoff hydrographs (p96-100)
Be able to use the Rational Method to predict peak flow (p )
Identify baseflow and direct runoff components of a hydrograph (p )
Use time-area methods to calculate a hydrograph from rainfall excess (p )
Determine the unit hydrograph for a watershed (p )
Use the S-Curve method to construct a unit hydrograph of different duration (p )
Use the unit hydrograph to calculate a storm hydrograph (p )

2. Rational Method Qp = C i A C = Runoff coefficient
i = intensity of rainfall of chosen frequency based on time of concentration
tc = equilibrium time for rainfall occurring at the most remote portion of a watershed to contribute flow at the outlet (based on wave travel time)
A = Area of watershed

3. Figure 2-1c

4. Figure 2-3

5. Time-Area Example A1 A2 A3 A4 Area (Ac) 100 200 300 Time to gage (hr)A1 A2 A3 A4
Area (Ac)
100
200
300
Time to gage (hr) 1 2 3 4
Hour 1 2 3
Excess precipitation (in/hr)
0.5

6. A D hour unit hydrograph is the characteristic response of a given watershed to a unit volume (e.g. 1 in or cm) of effective water input (usually rain) applied at a constant rate for D hours
Runoff (mm/hr)
Runoff and Flow
Flow (m3/s)
Time

7. Determining the Unit Hydrograph

8. Hydrologic Response from the unit Hydrograph
Excess Precipitation
Precipitation
Infiltration Capacity
Excess Precipitation
Time

9. Limitations
Linearity is violated when deeper water flows faster. Linearity generally taken to hold for large watersheds (> 100 km2)
Rainfall is seldom uniform in space
Effective input is very uncertain and depends on antecedent conditions

10. Figure E2-4

11. Figure 2-9