1. “The Unit Hydrograph Model for Hydrograph Separation”
Ninghu Su
Landcare Research, New Zealand Ltd.
Environment International, 21, #5,

2. Article Summary Introduction to Hydrographs
Nash Instantaneous Unit Hydrograph (IUH) Model
Parameter Equation Development
3 Time Points for Parameter Estimation
Time of Concentration from Slope-Channel Relations
Recession Limb Used for Baseflow Calculation
Estimate Baseflow for Rising Limb (63%)

3. Key Hydrograph Features
Initial Loss – rainfall that does not affect hydrograph
Lag Time of Total Storm Hydrograph
Lag Time of Baseflow Hydrograph
Point A – Flow at Time = 0 hr
Point B – Time when Discharge Increases
Time of Duration for Effective Rainfall
Time of Concentration = Last of Overland Flow Flows
Point C – Time when Baseflow Begins
Point D – Time when Overland Flow Ceases

4. Hydrograph Calculations
Slope-Channel Empirical Equation for Time of Concentration
Time of Rainfall – Lag Time is Time of Effective Rainfall
To Draw in Points C & D
Rising & Recession Limbs are Determined