Presentation on theme: "Water Budget III: Stream Flow P = Q + ET + G + ΔS."— Presentation transcript:
Water Budget III: Stream Flow P = Q + ET + G + ΔS
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?
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
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
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?
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. Wetland Hydrological Services
200,000 m 3 of Stormwater Runoff; Channel Peak Flow capacity of 1m 3 /s All in one day Peak Flow =2.3 m 3 /s Spread over 3 days Peak Flow = 0.8m 3 /s Why Does Storage Matter?
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…
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)
Rating equations (stage vs. discharge) allow continuous flow monitoring
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 HHQ tt Stage Hydrograph Stage-Discharge Curve or Rating Curve Discharge Hydrograph
Stage-Discharge Relation Typical relationship: Q = a(H +b) c The relationship between H & Q has to be calibrated locally for different stations
Stage Discharge Relationship for the Ichetucknee River At low stage, positive relationship between stage and discharge At high stage, negative relationship Why? Stage Discahrge
Type The GoodThe Bad WeirLow cost Easy installation Won’t work on low gradient streams Upstream flooding Clogs Changes WQ Wildlife barrier FlumeWorks ok in low gradient streams Better for WQ and wildlife Self cleaning High cost Difficult to install Weir vs Flume
What if there’s no rating curve? New watershed, new conditions Areas where it’s hard to develop rating curves –For example, the Everglades
Q= 1/n * A * r 2/3 * s 1/2 Q = estimated flow m 3 /s n = Manning’s roughness number (0.02 smooth to 0.15 rough or weedy, 0.5 dense vegetation) A = cross sectional area (m 2 ) r = Hydraulic Radius (wetted perimeter = WD/(W + 2D) W > 10D, R → D) s = Hydraulic Gradient ΔH/L Manning’s Equation - Flow Estimation without a rating equation
Predicting Flow in the Everglades Dense vegetation channel (n = 0.4) Shallow slope (s = 3 cm per km = 0.00003) Wide channel (100 m wide, 0.3 m deep, A = 30 m 2, r = 30 m 2 / 100.6 m = 0.3 m) What is Q? What is flow velocity (u)? Q = (1/n) * A * r 0.67 * s 0.5 V = Q / A Q = (1/0.4) * 30 m 2 * 0.3 m 0.67 * 0.00003 0.5 = 0.183 m 3 /s V = 0.183 m 3 /s / 30 m 2 = 0.006 m/s = 0.6 cm/s