Download presentation
Presentation is loading. Please wait.
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
Similar presentations
© 2025 SlidePlayer.com. Inc.
All rights reserved.