LCC – Water MT

Limiting Factors on the Global Water Supply

1) Problem of Unsuitabiilty or Inaccessibility

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-less than 1! of total global water supply is fresh and usable.. rest is salt, frozen, or underground

 

2)Problem of Unequal Distribution

A)Spatially: water not where people are

B)Temporallly: hydrograph variable… demand is more constant

-deficits would have to be made up by storage

 

3)Increasing Demand

A)Due to Increasing Population

B)Due to Increasing Industrialization/Affluence

-early 1900s average was 2650L/Person/day

-in 1974 was 8175

-household in 1986 335L/p/d in Canada

-Nicaragua around 30L/p/d

-about 65% water use is in bathroom

-largest peak in summer… lawns

 

4)Problems of Increasing Competition Among Users

 

5)Problem of Increasing Degradation of Water Resources

-acid rain, industrial effluents, groundwater contam

Hydrologic cycle

input = output +/- storage

 

inputs = precip

outputs= surface runoff, groundwater flow, evapo-transpiration

storage= lake and reservoir storage, soil and rock storage, plant storage, animal storage

definition of hydrology
The science which attempts to understand, describe and predict the processes governing the replenishment and depletion of the water resources on earth.
watershed

That portion of land which slopes or drains to a particular river system or lake at a defined point.

-also called a drawinage basi

Drainage Divide

Boundary of the drainage basin.

Can be delineated with a topographic divide

-the phreatic divide is the subsurface drainage divide (rarely have information on this)

 

-within drainage basin lie sub-drainage divides, which divide land draining to various tributaries of the main river of the basin

 

 

**contours bend upstream when crossing a stream

 

**a concentrated hydrograph is one of high discharge (high peak flow and flood hazard) relative to a flatter hydrograph (extended over time)…AREA BENEATH CURVE = VOLUME**

Drainage Area

Can either be defined as gross drainage area (entire area enclosed within drainage divide) or effective drainage area (that portion of drainage basin contributing runof to the system).

-certain areas may not contribute due to flatter slopes, deeper/drier soils, lower elevations and higher temperatures.

 

**As drainage area increases, the volume of runoff and therefore the flood hazard will also increase (all other things being equal), but the rate of increase in the flood hazard will become progressively less as the basin area increases.

 

Small Basin = <100km2

Medium = hundreds

Large Basin = >1000km2

 

-use a planimeter to measure

Slope

Average basin slope = measure of the average slopes of the sides of the basins draining to the main channel

 

ABS = CI x L/DA x100%

CI=selected contour interval

L=total ground length of contours at the selected interval enclosed within the basin

DA=drainage area

**all must be same units

 

-ABS values 25% or higher mean high flood hazard…. less than 10% indicate low hazard

 

average channel slope = the average slope of main channel

 

ACS = rise/run X100%

rise = difference in elevation b/w upper point and lower in the channel

run = total length of  stream

 

ACS > 5 = high

less than 1 = low

Basin Shape

-circular basin leads to concentrated hydrograph

 

-the compactness coefficient provides a shape factor to quantify the effect of shape.

-it is a ratio b/w the length of the perimeter of the drainage basin and the circumference of a circle having an area equal to it.

-closer to 1 means higher flood hazard (cannot fall below 1)

 

CC=WP/2pi(SQRT(A/pi))

 

If CC is 1.15 or less = high runoff

 

-units must cancel!! (CC has no units)

Elevation

There is a direct relationships b/w elevation, temperature and precipitation.

-results in increasing the volume of runoff whic occurs, and to also concentrate the runoff seasonally by enhancing spring runoff.

 

-air molecules have ability to hold moisture

DALR = 1C/100m

rainshadow= leeward side of mountain

 

??Greater than 2000 is high, less than 1000 is low (2000m is ~ treeline)

 

**make hypsometric curve an XY graph, not line.

 

A hypsometric curve can be used to compare elevations within a basin. Has elevation on Y axis and % basin above given elevation on X axis.

-from these curves, the median elevation can be interpolated from the 50% point of the X axis, which forms the index for comparison.

 

-elevation is plotted at the lower interval (ie. if lowest is between 900-1200, 100% of basin is above 900.

Drainage Density

-The higher the density of drainage channels, the greater the flood hazard

 

DD=L/DA

 

less than 5km/km2 = low

greater than 10 is high density and high flood

Vegetation Cover

Volume of runoff increases from deforestation

Three influences:

1)interception (by leaves)

2)infiltration (into the soil from roots which break up the soil and provide channels for the water to enter, and by acting as a barrier to the movement of overland flow, allowing more to enter soil.

3)plant storage and evapotranspiration

 

-burned area is warmer stream temperature…. more vegetation = lower temperatures

Soil Conditions

Four properties affect:

Texture, degree of sorting, packing arrangement, and grain shape

 

1)Texture

-coarser means more infiltration, less runoff

 

2)Degree of Sorting

-better sorted means larger relative pore spaces will be between them and the higher the inflistration rates. Better sorted means less runoff

 

A measure of sorting is the Uniformity Coefficient

Cu=D60/D10

-A uniformity coef less than 4 would be indicative of a well-sorted soil, while a value greater than 8 would be indicative of a poorly-sorted soil.

 

Uniform particles within a textural class packed or arranged in a regular pattern at right angles would have larger pore spaces, than those that are offset, allowing more infiltration.

Similarly, elongated grains would pack more tightly together, have smaller pore spaces b/w them, allowing less infiltration of precip, and yielding more runoff

Basin Orientation

Effect has to do with shade and direct sunlight, and therefore evaporation losses, infiltration capacity of soils, and less mid-winter melting of snow.

 

East and West differences not as pronouned. Prevailing wind affects though

 

Use 16 point system.

Anything between N and W is High flood hazard

Anything between S and E is low.

B/W N and E and W and S is moderate

Artificial Drainage

The RC rarely exceeds 0.25 in the most extreme natural conditions.

By contrast, RC in urban areas can be as high as 0.3-0.5 in residential areas, 0.5-0.7 in light industrial areas, and 0.7-0.9 in downtown core areas.

 

Effect is a function of both proportion of the basin altered and the location of the alterations.

 

If urban area located towards outlet of basin, additional runoff will occur early, and have minimal impact.

If same area located in middle, urban runoff will tend to be added to the central part of the hydrograph when most of natural runoff is converging.

 

-as area increases, so too does the peak flow and the volume of the urban runoff, making a greater impact on the runoff response

Runoff Processes

Runoff can be defined as water which originates as precipitation, and through some physical process reaches a stream channel or lake.

 

**There are four processes associated with runoff which together create total runoff (TRO)

1)Baseflow (BF)

2)Channel Interception (CI)

3)Subsurface Runoff (SSRO) (or interflow)

4)Surface Runoff (SRO)

 

CI, SSR, and SR result directly from a specific precipitation event, and are therefore termed components of Direct Runoff (DRO), while baseflow represents a runoff component amalgamated from many precipitation events..

 

TRO = BF + DRO

DRO = CI + SSRO + SRO

Types of stream

perenial stream = all year high water table

ephemeral = seasonal (spring runoff.. high up in shed)

intermittent =during certain times

Baseflow

Baseflow is water discharged to a stream channel from the zone of saturation lying below the water table, which represents the upper boundary of the zone of saturation.

 

Recession (rate of change in the decline of baseflow) is nearly constant from year to year, as it is controlled by the physical characteristics of the soil and rock layers which are constant.

 

Baseflow will contribute to TRO as long as the water table lies above the stream bed in the valley cross-section so as to have a hydraulic head relative to the stream bed.

 

effluent stream: when bf is contributing to runoff

 

influent stream: if the flow of the stream lies above the water table, then some of it will be lost to the zone of aeration (the zone lying about he zone of saturation, and characterized by soil moisture being less than saturation).

Direct Runoff Components

CHANNEL INTERCEPTION (CI)

-proportion of precip that falls directly into flowing stream channel. (not dry)

-it is a minimal contribution


INTERFLOW (SSRO)

-that portion of runoff which infiltrates into the ground surface and travels laterally though the zone of aeration.

-when water enters medium, it spreads laterally in all directions.

-when that medium is inclined, the point pattern of spread will elongate in the downslope direction (intro stream channel)

 

SURFACE RUNOFF (SRO)

-results from precip exceeding the infiltration capacity of ground surface

-LARGEST COMPONENT OF DIRECT RUNOFF!!!

Hydrographs

a hydrograph is a plot of stream discharge vs time measured at a specific point of the river.

 

-represents all four components together

-only possible to separate BF component

Baseflow Recession

Factors controlling BF recession defined by Darcy’s Law, and include soil/rock permeability and hydraulic gradient.

* rate of recession in BF will be a relatively constant decay function

;

EX

If the BF recession factor was 0.9, the BF at the end of a given time period would be 90% of the BF at the end of the previous time period…. and so on so forth…

Therefore, if BF were 10m3/s at a reference time 0, then the baseflow at time 1 would be 10m3/s times 0.9 = 9m3/s. At the next time period it would be 8.1… etc

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*****plotting BF values on semi-log graph paper represents the recession curve as as traight line, the slope of which is the recession factor. Lines should be approx parrallel from year to year.

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***keep units constant in formula!!!!!!!!!!!!!

Baseflow Separation

METHOD A

-usually selected, since it recognizes a lag effect between the occurrence of precipitation and the resulting recharge of the zone of saturation.

-make a straight-line approximation of the BF recession curve, extend it from the point where DRO starts to a point directly beneath the peak of the hydrograph. This point is then connected to twhere DRO ends.

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METHOD B

-involves connecting the two points with a straight line

-simple, though assumes BF recharge is immediately coincident /w start of DRO

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METHOD C

-make a straight-line approximate of the BF recesion curve after DRO ends, and extending baxkward to point directly beneath the peak of the hydrograph.

-assumes both immediate and rapid recharge of BR. This is only likely to occur in highly permeable substrate, such as very coarse textured areas.

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1941 paper by Brater explaining method C… it is real world.

Hydrograph Analysis of Direct Runoff

Analyzed to determine:

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1)Volume DRO

2)depth DRO

3)RC

4)ALR

5)Peak DRO Flow Value

6)Time of Peak Flow after Start DRO

7)Duration DRO

8)Derivation of Unit Hydrographs

Volume DRO

Volume is the area enclosed by DRO hydrograph

-can be done with a planimeter, but usually done by calculating a series of rectangular approximations of the area beneath the curve.

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-formulate rectangles of constant flow, using constant x-axis (time) increments

-under and overestimate theoretically balance

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The volume of flow in m3 represented by a rectangle is equal to the product of the flow rate in m3/s times the time increment in seconds

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Volume DRO = SumDRO times increment (deltaT(s))

-more rectangles = more accurate!!

Depth DRO

Is the depth that the volume of runoff would assume if it were spread evenly across the entire drainage area to the point where the runoff was measured.

;

Depth DRO = (Vol DROm3 times 1000mm/m)/DA(m2)

Runoff Coefficient

The ratio b/w depth DRO and average depth of precipitation falling on the basin

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RC = Depth DRO(mm)/Depth Rain (mm)

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It determines the proportion of precip that went to runoff.

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Higher RC will occur in steep mountainous basins than in flat prairie basins.

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Typical values of RC for natural basins are less than 0.3, while in downtown urban areas /w large proportion of impervious surface they can range as high as 0.7 to 0.9

Average Loss Rate (ALR)

The portion of precip that goes to vegetation intercetion, storage, interflow, of BF recharge. These are considered losses to DRO

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ALR = (Depth Rain(mm) – Depth DRO(mm))/Duration Rain(h)

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the ALR is variable over time

-therefore calculation provides an average, but does not indicate the variability of loss rates over time.

Peak DRO
The max difference b/w total runoff and corresponding BF.
Time to Peak Flow after the start of Direct Runoff

This parameter can be determined either graphically or from tabulation.

Difference b/w time of peak and beginning DRO

Duration DRO
The time difference b/w the start and end of DRO.
Field Trip- Stream Gauging

Q = AV

A=wd

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Since a natural river is irregular in chape and velocity distribution, it is divided into a series of rectangular panels, approximating the equivalent area of the channel.

Field Trip- Slope Survey

Run is measured as slope distance, and the rise is determined as the difference in readings from an automatic level on a rod placed at each end of study reach.

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Do rise over hypothenuse (river length) to get slope, as rise/run is not possible.

Field Trip- Cross Sectional Profiles

To obtain the elevation of each tagline point, the height of instrument is first established by taking a backsight onto a benchmark (a point of established true or relative elevation) and adding it to the benchmark location.

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Each successive point of elevation is determined by taking an intermediate foresight (a shot onto a point of unknown elevation between benchmarks), which is subtracted from the HI.

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To verify the accuracy of the profile when complete, at least one turning point is made before a foresight (a shot onto a benchmark to determine it’s elevation in order to verify accuracy) is taken onto the benchmark and its elevation determined. The difference between the A/F (as found) elevation and the established A/S (as sighted) elevation is the error of closure. Should be within 1 cm.

Field Trip- Characterization of Flow Regime

subcritical flow= slow, deep, tranquil flow. Not hydraulically efficient as it requires great channel capacity. Minimizes erosion and sediment transport. Good for fish and wading.

 

critical flow= transitional flow which is highly unstable and unpredictable in nature. Characterized by standing waves, and should be avoided.

 

supercritical flow – swift, turbulent shallow flow. It is efficient, erosive, and represents a safety hazard to human traverse and a barrier to fish.

 

When Froude Number (Nf) is less than one, the flow is subcritical, when it equals 1, the flow is critical, and when it is greater than 1, the flow is supercritical.