Stream hydrographs. Stream hydrographs. Baseflow. Graphs of river stage or discharge at a single location as a function of time

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1 Stream hydrographs Graphs of river stage or discharge at a single location as a function of time Hydrologic og budget Discharge: units? How is it measured? Show fluctuating water levels in response to rainfall events, seasonal fluctuations in stream flow or variations in rainfall patterns depending on the length of the observations Stream hydrographs Baseflow Groundwater Component of Streamflow Total Baseflow 1

2 Baseflow recessions Baseflow recessions Baseflow Recessions: No excess precipitation Discharge is entirely from GW at 0 e 0 : flow at start of recession a: recession constant for a drainage basin t: since start of recession 0 Period of recession t Function of: Overall topography Drainage pattern Soil and geology of the watershed Fetter: Fig Recession constant Problem: calculate a for this basin 3500 m3/d ln ln 0 a t a = d m3/d 100 days at 0 e t Rainfall-Runoff relationships How much runoff will occur from a given storm? Rational equation = CIA = peak runoff rate [L 3 /T] I = average rainfall density [L/T] = drainage area [L 2 ] C = runoff coefficient [-] depending o land use (table 2.3 on Fetter) 2

3 Determining GW recharge from baseflow Mayboom method: The total potential groundwater discharge (V tp ) to the stream is given by: 0 t 1 V tp t = baseflow at the beginning of the recession [L 3 /T] = time it takes to go from 0 to [L/T] Determining GW recharge from baseflow Mayboom method: The amount of potential baseflow (V t ) remaining after some time t from the beginning of the recession is given by: V Vt 10 t tp ( t / 1) Calculate V tp Measurement of Streamflow 0 V tp 0 t V tp t 1 0 = 350 ft 3 /s t 1 = 7.5 months 3 ft d min mo s mo d s min Domenico & Schwartz ft 3 1. Stream Gauging = V A Or divide stream channel into m segments and measure area and velocity individually: m i i1 v d w 2. Weirs = 2.5 H 5/2 i i V: flow velocity wi A: total cross section area H 90 o V-notch 3

4 Stage-discharge rating curve Manning Equation Empirical relationship between stream stage and discharge V 1.49R n 2 3 S 1 2 a a V: average velocity (ft/s) R: hydraulic radius (ft), or, cross sectional area/wetted perimeter S: slope of water surface n: Manning roughness coefficient b a b R 2 a b Global Water Budget Calculating Water Budget Statement of mass conservation inflow-outflow=s (change in storage) types of inflow/outflow for a groundwater basin inflows outflows 1. from recharge through unsaturated zone, R 1. to atmosphere through evapotranspiration, ET 2. from other parts of groundwater, gi 2. to other parts of groundwater, go 3. from bodies of surface water, si 3. to bodies of surface water, so (Winter et al., 1999) 4. from injection wells and/or other types of artificial recharge, wi 4. to withdrawal wells and/or other types of artificial discharge, wo 4

5 Example of Groundwater Budget Equation R + gi + si + wi - ET - go - so - wo =S if inflow=outflow, S=0 (steady-state) if inflow>outflow, S>0 (rising water table) if inflow<outflow, S<0 (resource depletion) Preparation of Water Budgets define a system which part of the hydrologic cycle? identify all inflow/outflow components depend on the system keep mass balance where the water goes? Example: Long Island, NY Overall and Groundwater Predevelopment Budgets Entire Island Aquifer Alley et al. (1999) 5

6 In-Class Problem Physical Dimensions Hypothetical river basin: Precipitation (P): 35 in/yr Evapotranspiration (ET): 20 in/yr Streamflow ( river ): 10 in/yr Subsurface outflow( gw ) : 7 in/yr P ET A physical dimension is associated with a physical quantity. Three basic physical dimensions are Mass [M], Length [L], Time [T] Dimensions for many other composite quantities can be expressed in terms of the basic three, for example, Write a water budget equation. if not balanced, why? P ET river gw S (in/yr) gw river Water storage [L 3 ] Flow [L 3 T -1 ] Flow per area [L 3 T -1 L -2 ] or [LT -1 ] Velocity [LT -1 ] Concentration [ML -3 ] It is imperative that for any equation, the resulting dimensions on both sides of the equation must be equivalent. Check = K i A [L 3 T -1 ] = [LT -1 ] [-] [L 2 ] Units of Measurement Each physical dimension may be associated with multiple units of measurement, for example, [M]: gram, kg, lb, ton, [L]: meter, km, inch, ft, [T]: second, hour, day, year, In carrying out a calculation, units may need to be converted from one to another, as in 2 Storage 11 mile 15 m ft 5280 ft 12 in 12 in 11 mile 15 in 2 1 mile 1ft 1ft in Significant Digits Number of significant digits used in measurement is important. A measure of cm has 4 significant digits. It means the measurement is somewhere between and , 17.0, and 17.00, are not the same. 17 has 2 significant digits; it could be somewhere between 16.5 and 17.5; 17.0 has 3 sds; it could be somewhere between and 17.05; has 4 sds; it could be somewhere between and When 2 or more numbers are multiplied (or divided), their product (or quotient) should have the same significant digits of the multiplier (or divider) with the least digits x = When numbers are added (or subtracted), the sum (or difference) should not have any significant digits to the right of the last significant digit of the addends (or subtrahends) = 21 6