1. Runoff

2. Lecture Goals
Identify factors such as precipitation intensity, watershed slope, watershed roughness, cover, soil type, and pre-existing water content affecting runoff volume and peak flow conditions.
Understand runoff volume, peak flow, and hydrograph shape resulting from design or actual storms over compact watersheds drained by natural channels and constructed diversions.

3. Hydrologic Cycle

4. Runoff Runoff generated by precipitation has three components:
Surface Runoff – water that flows over the soil surface.
Interflow – water that infiltrates and reemerges as a surface seep or ephemeral spring.
Subsurface or Groundwater Runoff – Runoff that originates from springs, seeps, and other areas where the water table intersects the land surface.

5. Runoff SURFACE RUNOFF SURFACE INFILTRATION RUNOFF INTERFLOW
GROUNDWATER
GROUNDWATER
AQUICLUDE
REGIONAL
GROUNDWATER

6. Factors Affecting Runoff

7. Factors Affecting Runoff
Rainfall
Duration
Intensity
Areal Distribution
Watershed
Size
Shape
Orientation
Topography
Geology
Surface Culture

8. Runoff Water Balance ET = evapotranspiration (cm/h)
f = infiltration rate (cm/h)
P = precipitation (cm)
Qi = interflow (cm)
Qg = groundwater runoff (cm)
Qs = surface runoff (cm)
qd = geological water loss (cm)
qg = groundwater recharge (cm)
Sd = surface detention (cm)
Si = interception (cm)
Sg = groundwater storage (cm)
Sr = surface retention (cm)
Ss = soil moisture storage (cm)
t = time interval (h) P ET Si Sr f Qs
ground qg Qi Ss qd Sg Qg

9. Runoff Equations Surface Runoff: Interflow: Groundwater flow:
ET = evapotranspiration (cm/h)
f = infiltration rate (cm/h)
P = precipitation (cm)
Qi = interflow (cm)
Qg = groundwater runoff (cm)
Qs = surface runoff (cm)
qd = geological water loss (cm)
qg = groundwater recharge (cm)
Sd = surface detention (cm)
Si = interception (cm)
Sg = groundwater storage (cm)
Sr = surface retention (cm)
Ss = soil moisture storage (cm)
t = time interval (h)

10. Runoff Water Balance Qs = P – Si – Sr - ft Qi = (f-ET) t – Ss – qg
ground qg Qi
Qi = (f-ET) t – Ss – qg Ss qd Sg Qg
Qg = qg – Sg - qd

11. Runoff Equations Surface Runoff: Interflow: Groundwater flow:
Qs = P – Si – Sr - ft
Interflow:
Qi = (f-ET) t – Ss – qg
Groundwater flow:
Qg = qg – Sg - qd
ET = evapotranspiration (cm/h)
f = infiltration rate (cm/h)
P = precipitation (cm)
Qi = interflow (cm)
Qg = groundwater runoff (cm)
Qs = surface runoff (cm)
qd = geological water loss (cm)
qg = groundwater recharge (cm)
Sd = surface detention (cm)
Si = interception (cm)
Sg = groundwater storage (cm)
Sr = surface retention (cm)
Ss = soil moisture storage (cm)
t = time interval (h)

12. Surface Runoff Interception INTERCEPTION SURFACE DETENTION
INFILTRATION

13. Interception
Rainfall stored on the ground surface (leaf surfaces, roof shingles…) and never reaches the soil surface.
Interception water is returned to the atmosphere via evaporation.
Interception is a function of the type and density of vegetation or surface cover, volume and intensity of precipitation, and wind speed.
Forest: up to 13 mm (0.5 inches)
Prairie: up to 7 mm (0.25 inches)
Corn: up to 0.5 mm
Typical assumption: 0.5 to 2.5 mm (up to 0.1 inches)
Interception both stores (retention) and detains (detention) rainfall.
Often estimate is based on vegetation type, subtract from rainfall.

14. Surface Retention (Depression Storage)
Rainfall stored on the ground surface (surface roughness, depressions, tire tracks,…)
Function of: management, time of year, land slope
Parking lot: not 0 mm, but close
Tilled soil: up to 25 mm
Typical assumption: 15 to 25 mm (up to 1 inch)
Water in retention storage either evaporates of infiltrates into the soil.
Surfaces both store (retention) and detain (detention) rainfall.
Estimate based on landuse type, subtract from rainfall.

15. Other Terms
Infiltration: movement of water into the soil (across soil surface boundary)
Horton’s equation
Green-Ampt Equation
Permeability  hydraulic conductivity: movement of water through the soil
Use Darcy’s Law

16. Hydrograph
Hydrograph: represents flowrate (volume per unit time – water moving past a given point per time) vs. time
Area under hydrograph = cfs x sec = volume (ft3)
Can be used to verify runoff volume estimations

17. USGS Hydrographs

18. USGS Hydrographs

19. Hydrograph Properties
Hydrograph does not always start as soon as rainfall starts – Lag Time: time from center of mass of rainfall excess to peak runoff flow
Rising Limb or Concentration Curve: hydrograph increases as water from various points in the watershed “concentrate” at the outlet
Time to peak: time to maximum flow rate
Falling Limb or Recession Curve: storm has stopped
Falling limb always longer (slower) than rising limb
Total Runoff Volume: area under the hydrograph
Time of Concentration: time for water to flow from the farthest point in a watershed to the outlet.

21. Runoff Comparison Preurban lag time from peak of
Rainfall in
inches per hour
Preurban streamflow
Urban streamflow
Urban lag time from peak of
storm to peak of stream discharge
Preurban lag time from peak of
After Marsh, 1998

22. Runoff Calculations Total volume of runoff Maximum rate of runoff
Ponds
Storage structures
Maximum rate of runoff
Culverts
Open channels
Conveyance systems