1. Water Budget III: Stream Flow
P = Q + ET + G + ΔS

2. Why Measure Streamflow?
Water supply planning
How much water can we take out (without harming ecosystems we want to protect)
Flood protection
How much water will come down the channel if X storm happens? Who’ll be flooded?
Water quality
What are the fluxes (flow x concentration) of contaminants to a lake or estuary?
What are the effects of land use change on water delivery to downstream systems?

3. Stream Flow Network (http://water.usgs.gov)

4. The Santa Fe River

5. Rainfall between Oct 1. 2010 and January 21, 2011 is 2.68 inches
Extreme Low Flows
Rainfall between Oct and January 21, 2011 is 2.68 inches

6. Hydrograph- graph of flow over time

7. Flow Over Time – Santa Fe River

8. Seasonality of Flow

9. Watersheds as Filters Rain falls
Storages “buffer” the rainfall signal, letting water out slowly
More storage = more buffering
The result is that the rainfall signal looks stochastic, the flow looks more “organized”
Watershed properties AND size affect the filtering effect

10. Rainfall Filtering – Santa Fe River

11. Rainfall Filtering – Finer View

12. Consider the “Filter” Effects of:
Watersheds with steep vs. shallow slopes
Watersheds with deep vs. shallow soils
Watersheds with intense vs. extended rainfall
Watersheds with forests vs. parking lots
Watersheds with dams vs. not
Big vs. little watersheds
Watersheds with big shallow aquifers

13. Basin Filtering Creates Lags – Hatchet Creek

14. Where is Stream Flow From?
At any time, flow is a composite of water with different sources and residence times
Some water is stored in the watershed for a very long time, some very short
During low flow conditions, water is mostly old
During storms, the contribution of new water increases
How does an aquifer affect this?

15. 23% 8% 2%

16. Streamflow - A Convolution of Storms

17. Flow and Rainfall Intensity
If Rainfall Intensity > Infiltration Capacity then surface runoff occurs
Stream flows are composites of
Surface runoff
Subsurface flows

18. Variable Source Area (Stormflow Generation in Florida)

19. Variable Source Area Makes Antecedent Rainfall IMPORTANT

20. Land Use (Cover) Affects Runoff Generation
Impervious surfaces preclude infiltration
Less infiltration means more runoff
Runoff also MOVES faster
Less “filtering”
Compare land uses…

22. More stormflow, higher peak flow, sooner.

23. The importance of storage – the basis of filtering
Same total flow (area under the curve), lower peak flow.

24. Water Storage in the Forest

26. Wetland Hydrological Services
Depressions and vegetation (swamps) slow runoff.
Upper watershed wetland storage delays runoff and reduces peak flows.
Wetland flood plain has a dominant influence on downstream peak flow and solute transport.

27. Why Does Storage Matter?
200,000 m3 of Stormwater Runoff;
Channel Peak Flow capacity of 1m3/s
All in one day
Peak Flow =2.3 m3/s
Spread over 3 days
Peak Flow = 0.8m3/s

28. How Big a Flood Can We Expect?
The size of the flood is inversely proportional to it’s frequency
Big event happen rarely
Big events shape the landscape
Medium events maintain the landscape
Small events control the biology
How would we predict the size of a flood that happens roughly once in 25 years?
Think back to the rainfall lab…

29. Rainfall Recurrence Series

30. Flow (Santa Fe River Station 3)

31. Daily Flow Recurrence Series

32. The 100-yr Floodzone Map

33. How Do We Measure Streamflow?
Funny you should ask…basis of Lab #4.
Basis is to estimate:
Cross-sectional area (A; through which water flows)
Water flow velocity (V)
Q = A * V

34. Measuring Surface Flow

35. Typical stream velocity profile

36. Where to measure mean velocity?
Float Velocity * 0.8 for natural channels
Float Velocity * 0.9 for concrete channels

37. Velocity Instruments $2k Turn-cup 0.500 ft/s $4k $7k Electromagnetic
Sonic Doppler
0.005 ft/s

38. Sect
Width (m)
Depth (m)
m/s
Flow
(m3/s) 1
0.7
0.20
0.14 2
2.0
0.25
0.50 3
1.3
0.15
Total
0.84m3 /s

39. Discharge is HARD to Measure
We want:
Daily (or sub-daily) measurements
Multiple stations per river
Real time updating (detect changes in flow as they are happening)

40. Rating equations (stage vs. discharge) allow continuous flow monitoring

41. Stage-Discharge Relation
Water stage (elevation) is EASY to measure
Stage is related to dischage via a mathematical relationship
Applying that relationship to measured stage gives estimates of discharge Q H t
Stage Hydrograph
Stage-Discharge Curve
or Rating Curve
Discharge Hydrograph

42. Stage-Discharge Relation
Typical relationship: Q = a(H +b)c
The relationship between H & Q has to be calibrated locally for different stations

43. Stage Discharge Relationship for the Ichetucknee River
At low stage, positive relationship between stage and discharge
At high stage, negative relationship
Why?
Discahrge
Stage

44. Stage Measurements
Float-pulley
Staff gage
Ultrasonic
Pressure

45. Weirs

46. Flumes

47. Weir vs Flume Type The Good The Bad Weir Low cost Easy installation
Won’t work on low gradient streams
Upstream flooding
Clogs
Changes WQ
Wildlife barrier
Flume
Works ok in low gradient streams
Better for WQ and wildlife Self cleaning
High cost
Difficult to install

48. What if there’s no rating curve?
New watershed, new conditions
Areas where it’s hard to develop rating curves
For example, the Everglades

49. Manning’s Equation - Flow Estimation without a rating equation
Q= 1/n * A * r 2/3 * s1/2
Q = estimated flow m3/s
n = Manning’s roughness number
(0.02 smooth to 0.15 rough or weedy, 0.5 dense vegetation)
A = cross sectional area (m2)
r = Hydraulic Radius (wetted perimeter = WD/(W + 2D)
W > 10D, R → D)
s = Hydraulic Gradient ΔH/L

51. Predicting Flow in the Everglades
Dense vegetation channel (n = 0.4)
Shallow slope (s = 3 cm per km = )
Wide channel (100 m wide, 0.3 m deep, A = 30 m2, r = 30 m2 / m = 0.3 m)
What is Q? What is flow velocity (u)?
Q = (1/n) * A * r0.67 * s0.5
V = Q / A
Q = (1/0.4) * 30 m2 * 0.3 m0.67 * = m3/s
V = m3/s / 30 m2 = m/s = 0.6 cm/s

52. Next Time…
Groundwater