Transcript Document

Hydro-Geography:
• Natural Water Budget
• Impacted Water Budgets
• Management of Water Resources
1
Water Budgets; the Geography of Water:
• Water is as important in terms of global warming as temperature: increasing
risk of flooding and drought in a changing climate (Milly et al, 2002)
• Water has also long been a source of fascination
• in art and even poetry: e.g. John Masefield:
“I must go down to the sea again, to the lonely sea and the sky...”
• conjecture about water has demonstrated a deep-felt curiosity:
e.g. Ecclesiastes 1(7):
"And all the rivers run into the sea yet the sea is not full;
unto the place from whence the rivers come, thither they return again.“
• certainly biological, personal and scientific interest
• water is essential to life
• habitat of the origin of all life, for most of earth history the sole habitat
supporting life
• organisms carry water with them (as 65% of body tissue); critical for all
nutrition to be absorbed; we can only last a few days without it
• at a resource level, society needs a steady supply of water domestically
(“potable water”)
• all food requires it to grow.
• salinity excludes water from many uses
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Water Budgets; the Geography of Water, continued
• focus on its natural distribution
• from description, explanation and prediction to management
• as well as management of ourselves to avoid the consequences of too little
and excess
• Water is not evenly distributed; it covers more than 71% of the earth's surface.
Both excess and shortage are issues of temporal and spatial concern:
• crises associated with water
shortage include drought and
contamination since unusable water
through natural or human actions is
no longer a resource and an
alternative supply must found
immediately
• crisis of excess includes drowning,
flooding, with its associated
property damage, injury, and death
3
Water Budgets; the Geography of Water, continued
Waterside sites were the hearths of civilization:
• the first central places of the Agricultural Revolution (on flood plains of the
Tigris/ Euphrates, Nile, Yangtze, Huang Ho, Indus, etc)
• and the Industrial Revolution (mill streams first in Britain, then spreading to
Europe, North and South America etc)
• these early sites of economic growth have created many of the patterns of
the modern economy:
• locally (water for domestic and commercial use, irrigation, effluent
dilution, tourism appeal)
• but also to meet the industrial and transport demands of the global
economy
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Water Budgets; the Geography of Water, continued
• society places a high social and dollar value on maintaining water supplies
• however, people take it for granted. Evidence of ignoring abundance and
risk of shortages abounds: taps and fountains, toilets, lawn sprinklers, car
washes, abandoned/neglected water bodies, etc.
• waste management is also a water crisis:
• issues about what to do with garbage are largely precipitated by
concern for the surface and subsurface contamination potential from
landfills
• extreme care is made in site selection due to the high cost and
uncertainty of ensuring attenuation of leachate
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Water Budgets; the Geography of Water, continued
• society also demands care in the construction of built environment
• to exclude rain and groundwater from most structures
• to protect the natural values of waterways
• to avoid liability and hazard
• to protect the sensitivity to nature although many exceptions to this
• environmental infrastructure (agencies, policies and procedures) is involved
in protecting and maintaining water systems
• this treats water as a resource, although a remarkable one:
• it exists in nature as a solid, liquid and gas
• is renewable and recyclable,
• capable of multiple uses
• Transportable
• but it also demonstrates the commitment society can demonstrate to
sustain essential components of nature
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Water Budgets; the Geography of Water, continued
Management of water necessitates the making of decisions that may be
virtually irrevocable; extremely high infrastructure investment:
•
•
•
•
•
•
•
sanitary sewers
treatment facilities
dams, reservoirs
canals, aquaducts, tunnels
bridges
pumping stations
storm sewers (stormwater
management can add significantly
• to the land area and expense of
constructed facilities)
• also the ethical issues of diminishing
the quality and/or quantity of water,
and of ever curtailing water supply
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2. Water Budgets; Global Disposition
• hydrology:
hydros: water
logos: logical proof, rational assertions
• science concerned with the quantity of water
• in a particular ”store” and
• transferred between one store and another (flux)
• it is common to treat such a system as budget analogous to financial systems
in which the balance of transfers to and from stores is the primary concern
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Water Budgets; Global Disposition, continued
Stores:
• volume: storage where water resides for a lengthy period
• e.g. oceans, lakes
• globally 1.386 x 106km3 (Shiklomanov,1999)
Fluxes:
• Volume per unit time: process of removal from one store or to another
• e.g. evaporation, condensation, precipitation
• globally 5.77 x 105 km3yr-1 (Shiklomanov,1999)
Residence time:
• how long it takes to replace the volume of the store
• not all molecules actually cycle through, but an equivalent volume is replaced
• a dramatic indicator of how dynamic the water balance is
• particularly for the atmosphere and surface runoff
• also indicates how long water is secured (on average) in frozen environments
and below ground
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Water Budgets; Global Disposition, continued
Stores:
1.386 x
106km3
Location
salt water
fresh water
solid as ice, snow, glaciers
Storage volume
97.5 %
2.5%
68.7% of fresh
Mean Residence Time (yr)
2500
subsurface (groundwater)
29.9% of fresh
1400
surface: lakes, wetlands, soils,
0.26% of fresh
rivers, etc
atmosphere
Fluxes:
5.77 x 105
km3yr-1
evaporation from oceans
evaporation from land
precipitation on oceans
precipitation on land
runoff
interflow
(Shiklomanov,1999)
trace
Polar ice: 9700;
Mountain glaciers: 1600
Permafrost ice: 10 000
lakes: 1700
bogs: 5
soils: 1
rivers: 16 days
8 days
5.03 x 105 km3yr-1
7.42 x 104 km3yr-1
4.58 x 105 km3yr-1
1.19 x 105 km3yr-1
4.26 x 104 km3yr-1
2.2 x 103 km3yr-1
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Water Budgets; Global Disposition, continued
• global limits of water seem unfathomable
• little scientific work and management are conducted at this scale
• But it needs to be appreciated that water is and almost completely confined
to the earth in both its stores and fluxes
• quantities are finite, essentially fixed, having been released over the 4 x 109
years of earth history
• very little “juvenile water” continues to form and little is lost deep into the
earth
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Water Budgets; Local Watersheds
• the scales at which water is most frequently addressed is local:
• watershed (US)
• drainage basin or catchment (Europe)
• in Canada: both terms
• defined as the land drained to the mouth/outlet of a river
• span a range of scales:
• from the extremely local (e.g. a rooftop)
• to continental (e.g. Hudson Bay and all its tributaries)
• convenience of geographic watersheds is due to their role of enabling the
isolation of precipitation and runoff, and because smaller subwatersheds nest
conformably within larger watersheds
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Water Budgets; Local Watersheds, continued
Geographic concerns relate to:
• where water is
• in what store
• how fluxes affect it
• quantities are important
• water surpluses and deficits of
different magnitudes have different
importance
• water demand exceeds supply in
increasing parts of the world to the
point that once-major rivers now
frequently run dry before reaching
their mouths (Brown, et al, 1999)
• there are clearly limits to the natural
supply of water to and from a
drainage basin.
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Water Budgets; Local Watersheds, continued
• if the volume of water in a river passing a point is reached by the rate of water
withdrawal, then the river will not flow
• as withdrawal approaches “normal” (average) flows there will be deficits at
times when below average flows are experienced.
• in a perfectly symmetrical frequency distribution, the river would be dry half of
the time (the below-average half)
• limits to water use cannot be allowed to approach even “average” if no
alternative sources are available
• what has happened in water-deficient areas (actually areas where demand
exceeds natural limits) is that water is imported from other areas
• demand is therefore allowed to rise, and as growth continues more and more
distant sources are diverted toward the “need”
• one such scheme is the “Grand Canal”
(http://home.thezone.net/~deltaprt/aquarius/greatlakes.htm)
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Water Budgets; Local Watersheds, continued
• conventional wisdom is that when it rains or if snow melts runoff results,
however, rivers continue to flow under natural conditions, well after the
precipitation stops
Storm Hydrograph shows pattern:
• stores in the basin hold back water from immediately running off
• a lag time, between when water is
input into a basin and when it
crests
• and until it has completely passed
out the basin via the mouth of the
river
• the storm hydrograph depicts the
immediate stream runoff peak
attributed to a specific precipitation
event, and that which slowly seeps
from storage within the watershed.
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Water Budgets; Local Watersheds, continued
Hydrologic Budget Components
• P - precipitation
• I - interception
• DS - depression storage
or surface detention
• SM - soil moisture
• IF - interflow
• G - groundwater
• ET - evapotranspiration
• R – runoff
I
A local water balance involves the
distribution of water in each of these stores
(albeit temporary) across the watershed
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Water Budgets; Local Watersheds, continued
• the only input to the system is Precipitation
• as rainfall, mist, condensation (dew), snow, hail, sleet, freezing rain (may persist
until melting later)
• these inputs are geographically and temporally variable and in fact are
discontinuous
• measured in units of depth (mm
for rain, cm for snow) in rain/snow
gauges
• concern for the quality of the
data (e.g. Larson and Peck, 1974),
has led to refinements in the
technologies of measurement,
however the geography of
measurements, particularly the
density of observations remains an
issue
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Water Budgets; Local Watersheds – Precipitation, continued
• temporal variation is very familiar to most people, from 0 to torrential.
• precipitation intensity declines rapidly
with the duration of the event:
• heavy rains are short-lived, less
frequent
• lighter rains may persist, more
frequent
• heavy rains or sudden snowmelt may
overload basin storage capacities,
but this may be infrequently enough
for people to overlook the risk of
flooding
From: http://www.cdnarchitect.com/asf/en closure_design_strategies/fundame
ntal_considerations/fundamental_c onsiderations.htm
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Water Budgets; Local Watersheds – Precipitation, continued
• spatial variation is also familiar – passing showers
• precipitation generally shows spatial variation within a single event, but this
may be inconsistent enough over time that local variations are unnoticeable
when aggregated to monthly or annual quantities
• spatial patterns are commonly analysed and displayed via “isohyets” or
Thiessen polygons
• Canadian precipitation data are not
available for free except for the current
day’s conditions
Mapping to demonstrate spatial
variation in precipitation
from: www.aceweather.ca
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Water Budgets; Local Watersheds, continued
Hydrologic Budget Components
• P - precipitation
• I - interception
• DS - depression storage
or surface detention
• SM - soil moisture
• IF - interflow
• G - groundwater
• ET - evapotranspiration
• R – runoff
I
• liquid or solid precipitation that is lost from the watershed by being caught while
falling by plants and later evaporating
• collects on trees, shrubs and ground covers (leaves, stems, etc) , and
sometimes even on human structures (buildings) too, but this seems to be
inconsistent with most usage of the term
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Water Budgets; Local Watersheds - Interception, continued
Interception
• can be significant; it is instinctive to seek the sheltering effect of tree cover from
rainfall (despite warnings about lightning)
• also shelter for birds and other animals; there is respite only until the capacity to
store is reached, and water drips off the leaves to the ground below
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Water Budgets; Local Watersheds - Interception, continued
22
Water Budgets; Local Watersheds - Interception, continued
• measurements seldom record the volume of intercepted precipitation
• expressed either as millimetres of precipitation equivalent or as a percentage of
incident precipitation
e.g.:
• Millet et al. (1998): 4 or 5% of the rainfall
• Hebda, et al., (2000): 30% to 100%
• Rowe (1983): maximum canopy storage for deciduous forests range from 1.2 1.5mm/m
• Thurow et al (1987) for grasslands: 1.8 mm and 1.0 mm /yr
Basin-wide quantification of interception is a problem:
• uncertainties arise from differences in vegetative growth form such as
different species, types, densities, and leaf and stem areas, which reflect
seasonality and vary rapidly for fast-growing plants like crops (Van Dijk, and
Bruijnzeel, 2001)
• ground covers, wetland areas with emergent species, stemflow and
intercepted snow and ice are also problematic
• heavy rain or snow would likely have their capacities exceeded more often
than areas where drizzle or mists dominate
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Water Budgets; Local Watersheds - Interception, continued
• one measure of the amounts retained is to
shake a sapling after a rainstorm!
• quantities are usually measured as the
difference between rain gauges placed
above and below intercepting plants.
• basin-wide variations coincide with
seasonality for deciduous tree covers
• interception variations influence the
subsequent evaporative losses
• some intercepted water will eventually pass
through vegetative layers (leaf drip, stemflow
etc) and will need to be known since it is not
lost from the water budget of the drainage
basin
• determination of these remains a topic of
ongoing research.
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Water Budgets; Local Watersheds, continued
Hydrologic Budget Components
• P - precipitation
• I - interception
• DS - depression storage
or surface detention
• SM - soil moisture
• IF - interflow
• G - groundwater
• ET - evapotranspiration
• R – runoff
I
water that passes through vegetation (if any) to pool on the ground surface; standing
water accumulation filling low spots (puddles)
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Water Budgets; Local Watersheds – Depression Storage, continued
• some water may infiltrate initially
• if the ground surface becomes saturated, pooling begins
• if it deepens sufficiently,
water will spill over and
eventually contribute to:
• overland flow toward
the stream channel
• seeps into the ground
• or evaporates
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Water Budgets; Local Watersheds – Depression Storage, continued
• depression storage is usually expressed in millimetres of precipitation equivalent
• very difficult to measure
• assessing the collective storage capacity across a watershed is only done with
considerable uncertainty
• furthermore, under frozen conditions, depression storage is readily confounded
with interception: precipitation and melting events initiate the filling of surface
depressions simultaneous with activation of fluxes such as infiltration, overland
flow and evaporation
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Water Budgets; Local Watersheds – Depression Storage, continued
from: Kouwen, 1987
(http://sunburn.uwaterloo.ca/Watflood/manual.p df)
Type of Surface:
Depression Storage
(Retention, in mm)
(ASCE, 1969)
SPL8
Impervious Urban Areas
1.25
1. 25
Pervious Urban Areas
3.0
2.0
Smooth Cultivated land
1.3 - 3.0
2.0
Good pasture
Forest litter
5.0
8.0
3.0
10.0
from:http://www.ci.gillette.wy.us/pub w/en/drainage.html#Table6
Depression/Detention Values
Recommended
(mm)
(mm)
Paved Areas Roofs
Flat Roofs
Sloped Roofs
1.2 - 3.7
2.5 - 7.4
1.2 - 2.5
2.5
2.5
1.2
Lawn grass
2.5 - 12.4
7.4
Wooded areas and open fields
5.0 - 14.9
Assess each
situation
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Water Budgets; Local Watersheds, continued
Hydrologic Budget Components
• P - precipitation
• I - interception
• DS - depression storage
or surface detention
• SM - soil moisture
• IF - interflow
• G - groundwater
• ET - evapotranspiration
• R – runoff
I
Infiltrating water that penetrates the ground surface and is held against the pull of
gravity within the rooting zone of plants
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Water Budgets; Local Watersheds – Soil Moisture, continued
• the only source of water for plants, though not all of the water which reaches the
soil is available:
• hygroscopic water is actually chemically bound to minerals in the soil
• held too tightly to be removed by plant roots
• only in episodes of very intense dehydration is it depleted or otherwise
involved in the soil water budget
• gravity water infiltrates/percolates deeper into the ground, becoming
interflow or groundwater
• drawn down rapidly (especially in coarse soils – sands and gravels –
where porosity and permeability allow rapid drainage)
• plants are unable to draw upon it
• capillary water is held against the pull of gravity but is not static, it is
augmented by infiltration of rainwater and snow melt, and depleted
• by gradual penetration to groundwater
• by uptake via plant roots
• and by direct surface evaporation
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Water Budgets; Local Watersheds – Soil Moisture, continued
• Capillary water (in humid environments/seasons) normally infiltrates, carrying
solutions into groundwater
• upward migration of soil moisture (capillary rise) may also occur (under high rates
of evaporation from the ground surface, evaporating and depositing salts from
solution)
Field Capacity
• water-holding ability of soil
• mass of moisture expressed as a “percentage” of dry soil mass
• for hydrologic purposes it is more convenient to recognize Field Capacity as the
maximum capillary water a soil can hold
• expressed as millimetres of precipitation equivalent for the rooting zone (about
1.0 to 1.2 m below the ground surface
• the intrinsic composition and texture of soil has a great influence on the
geographic pattern of stored and transmited water
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Water Budgets; Local Watersheds – Soil Moisture, continued
Field Capacity (mm water / m soil)
Material
Class
H2O as % of dry
weight
BC Agriculture
*
Alberta Agriculture
**
Best Estimate
mineral soil or sediment
Clay/clay loam
~ 40
200
203.2/ 177.8
~200
Silt loam
~ 25
208
Loam
175
Fine sandy loam
142
Sandy loam
organic material
~7
125
Loamy sand
100
Sand
83
Gravel
<5
peat
140-170
~160
152.4
101.6
BC Agriculture: http://www.agf.gov.bc.ca/resmgmt/publist/600series/619000-1.pdf
Alberta Agriculture: http://www.agric.gov.ab.ca/crops/wheat/moisture.html
~110
~ 50
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Water Budgets; Local Watersheds – Soil Moisture, continued
• soils have organic (once living) and mineral (rock
particles) constituents
• mineral constituents classified as clay (sub-microscopic)
silt (dust-sized), sand (up to 2mm diameter) and gravel
(over 2mm)
• capillary water varies considerably with particle size
• finer soils have higher field capacities but the greatest
water storage capacity is related to the organic (nonmineral) content of the soil
• surfaces covered by vegetation and “plant litter”
commonly maintain natural rates of infiltration, by
impeding surface compaction due to rain splash and
desiccation from direct sunlight
• burrowing organisms open up the soil and produce
composted organic matter that facilitates water retention
and aggregation of soil particles
33
Water Budgets; Local Watersheds – Soil Moisture, continued
• surfaces covered by vegetation and “plant litter” commonly maintain natural rates
of infiltration, by impeding surface compaction due to rain splash and desiccation
from direct sunlight
• burrowing organisms provide tilth that opens up the soil and produce composted
organic matter that facilitates water retention and aggregation of soil particles
Soil moisture storage is critical to the water balance:
• generation of runoff depends on there being from an excess of precipitation/snow
melt over infiltration; saturation of the available storage in the uppermost part of
the soil will impede further infiltration so that surface depressions become filled
and overland flow is initiated
• if the soil can absorb the water supplied to it surface, runoff will not be initiated
• Irrigation needs arise from the depletion of capillary water to the point where
plants are threatened with drought. Recharging soil water within the limits of field
capacity will ensure an adequate supply for plants to grow
34
Water Budgets; Local Watersheds – Soil Moisture, continued
• measurement of soil moisture was traditionally conducted by collecting a field
sample, weighing it, drying it, determining the weight loss and converting the
mass (as a volume per sampled area) to millimetres
• now many sensors rapidly detect moisture based on inferences regarding the
physical, electrical and chemical properties of water (http://www.sowacs.com/) as
some mounted on aircraft and spacecraft
Nevertheless, watershed-wide soil
moisture data is not readily available.
Modelled and generalized patterns are
to be found:
From:
http://www.agric.gov.ab.ca/climate/springsm.pdf
http://www.gov.mb.ca/natres/watres/nohrsc_soil_m
oisture_nov_2001.htm
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Water Budgets; Local Watersheds, continued
Hydrologic Budget Components
• P - precipitation
• I - interception
• DS - depression storage
or surface detention
• SM - soil moisture
• IF - interflow
• G - groundwater
• ET - evapotranspiration
• R – runoff
I
Below the rooting zone is an area of occasional saturation where interflow –
lateral and downslope movement of percolating water – is found
36
Water Budgets; Local Watersheds – Interflow, continued
• in an unsaturated, or vadose zone
• water only occupies the pore spaces as it is
pulled by gravity either to the zone of
permanent saturation (groundwater) or to
point where it surfaces as a “seep” or
ephemeral spring
• in porous and permeable materials
interflow may represent a significant
component of the water balance, but it is
very difficult to measure at all, let alone to
monitor on an ongoing basis
37
Water Budgets; Local Watersheds – Interflow, continued
• quantities are most often inferred by examining the delay of stormwater reaching
channels
• vadose zone caves may form in soluble bedrock (limestone)
• Karst geomorphology requires the constant percolation of water to remove the
carbonates released by weathering of the rock
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Water Budgets; Local Watersheds, continued
Hydrologic Budget Components
• P - precipitation
• I - interception
• DS - depression storage
or surface detention
• SM - soil moisture
• IF - interflow
• G - groundwater
• ET - evapotranspiration
• R – runoff
I
Water that has percolated to the zone of permanent saturation of pore spaces
referred to as groundwater, and its upper limit, the (ground) water table
39
Water Budgets; Local Watersheds – Groundwater, continued
• also referred to as the phreatic zone
• from deep infiltration
• Very gradual discharge to:
• springs
• streams (as base flow)
• or standing water
• Ponds
• lakes
• even the ocean
• groundwater is ubiquitous, but the abundance and potential for extraction do vary
considerably
• flow rates within this store are generally gradual (down to millimetres per century)
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Water Budgets; Local Watersheds – Groundwater, continued
• flow rates depend on the nature of the geological materials:
• porosity refers to the volume of void spaces in the rocks or sediments
• permeability is the connectivity of these for percolating groundwater
• both are properties of the host materials
• earth materials that are both porous and permeable are called aquifers
(sand, gravel, sandstone, limestone, fractured rock)
• materials which impede water flow are called aquicludes or aquitards (clay,
shale, dense bedrock).
• hydraulic conductivity is the rate of
passage of water through the pore
spaces
• local aquicludes can create small
isolated perched aquifers and springs
41
Water Budgets; Local Watersheds – Groundwater, continued
• in the phreatic zone, hydrostatic pressure influences water movement
• (whereas in the vadose zone, it is gravity that governs almost all flows)
• under hydrostatic pressure water flows from areas of high (impelling force)
pressure to low pressure, just as air does in the atmosphere
• the weight of confining water (above) is what applies the pressure; if an aquifer is
sandwiched between aquicludes on a slope, then hydrostatic pressure may be
sufficient to allow the well to discharge up onto the surface (an Artesian well)
• the water table coincides with the elevation
of the receiving surface waters, since a
lower water table would draw from the
surface and a higher water table would
discharge as a spring
42
Water Budgets; Local Watersheds – Groundwater, continued
• hydrostatic pressure forces water to rise in
wells, to the elevation of the groundwater
table at that point
• monitored from the surface by inserting
piezometric tubes to depths of aquifers for
measuring water pressure
• mapping of these elevations produces a set
of points that can be interpolated to form a
potentiometric (piezometric) surface
• the slope of this surface defines the
hydrostatic pressure gradient and the
horizontal direction of groundwater flow
• anomalies on the surface arise from the
presence of impermeable materials
(aquicludes) in the stratigraphic column.
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Water Budgets; Local Watersheds – Groundwater, continued
• knowledge of subsurface stratigraphy is extremely limited
• anisotropy can have a huge impact on percolation and discharge rates
• measurement of groundwater is often expressed as the depth to the water table,
or as the rate at which water can be pumped (yield) and the depth of
precipitation equivalent added to or removed from groundwater storage
• as a store these additions and removals must balance if the storage is to remain
in equilibrium
• because it is difficult to monitor the many inputs and outputs from aquifers,
changes in the elevation of the water table are monitored as an indicator of
disequilibrium
44
Water Budgets; Local Watersheds – Groundwater, continued
• if one well (or more) continues to extract faster than the groundwater can
recharge itself, then the water table becomes drawn down
• this deprives adjacent wells,
springs and streams
• leading to deepening of wells
which creates more drawdown,
resulting in surface de-watering
subsidence
• near oceans salt water incursion
into the aquifer can occur
• What sets the limit on removal
rates is recharge rates, otherwise
the use is consumptive and
unsustainable, but may go
unnoticed for some time
45
Water Budgets; Local Watersheds – Groundwater, continued
• similar to salt water incursion, contamination of groundwater was a latent issue
until the crisis situation at Walkerton, Ontario; May 2000, rain washed e. coli
bacteria into the town’s water supply: in a town of 5000 about half wee sick and
7 died from continuing to drink the water;
• groundwater is complicated by:
• the diffuse extent of infiltration
• uncertainty of and our ignorance of percolation pathways
• water’s inaccessibility for clean-up once contamination is discovered
• diverse potential sources of contaminants include:
• livestock
• cemeteries
• fertilizers and pesticides from rural, suburban and urban application
• pit and quarry operations, mine tailings and ponds
• abandoned and active industrial sites (spills and disposal)
• land fill leachate and old dumpsites
• winter road salt
• military activities, etc.
46
Water Budgets; Local Watersheds – Groundwater, continued
•
•
•
•
natural sources of toxins (asbestos, arsenic et al.)
“spring water” was once a standard for purity
groundwater can be no purer than the media it has passed through
Science continues to monitor groundwater:
• “Groundwater is important to health, economy and ecosystems in Canada. It
provides drinking water to about one third of all Canadians and up to 80% of
the rural population. It has been routinely surveyed since early last century,
yet groundwater has not been mapped in a systematic way across the
country. The NRCan Groundwater Mapping program, a current federal
groundwater initiative, aims to establish a conceptual framework of
national, regional and watershed-scale groundwater flow systems.”
http://ess.nrcan.gc.ca/gm-ces/index_e.php
•
Outputs from a watershed are restricted to very deep, very slow groundwater
“leakage”, evapotranspiration and surface runoff.
47
Water Budgets; Local Watersheds – Groundwater, continued
• evaporation and transpiration pathways recognized, but difficulties in
measurement of these separately as well as in combination
• necessitates development of procedures for predicting evapotranspiration from
more-readily measured atmospheric energy exchange:
• temperature
• radiation
• ground-surface properties:
• open water (unlimited H2O)
• soil: texture
• vegetation: crop type, forest, grass
• land use
open water evaporation is normally based on measurements from a standard pan of
water
48
Water Budgets; Local Watersheds, continued
Hydrologic Budget Components
• P - precipitation
• I - interception
• DS - depression storage
or surface detention
• SM - soil moisture
• IF - interflow
• G - groundwater
• ET - evapotranspiration
• R – runoff
I
Evapotranspiration is a composite concept encompassing the change of state of water
from liquid to vapour. It includes evaporation from :
• storages exposed to the atmosphere (interception, depression storage, soil moisture)
• the vascular systems of plants (stomata releasing soil moisture drawn upwardly by
transpiration).
49
Water Budgets; Local Watersheds – Evapotranspiration, continued
Canadian Normals Evaporation Data:
“Monthly averages of calculated lake evaporation (mm) derived from evaporation
pan measurements are given. Lake evaporation represents water loss from ponds and
small reservoirs but not large lakes. Lake evaporation tends in general to be about a
third less than measured pan evaporation.”
(http://climate.weatheroffice.gc.ca/climate_normals/climate_info_1961_1990_e.html)
Lysimeters are used for more comprehensive
evapotranspiration measurement:
Lysimeter
P
ET Stilling
Well
Soil
Displacement
Fluid
By monitoring weigh loss, evapotranspiration
values are obtained, but the expense of the
device prevents its widespread use.
Evapotranspiration is very sensitive to both
atmospheric and surface conditions; its inclusion
in water balance determinations can introduce
uncertainty.
50
Water Budgets; Local Watersheds – Evapotranspiration, continued
The work of Thornthwaite, Penman and Monteith have popularized the notion of
calculating potential evapotranspiration to aid in determination of water balances.
• the amount of water that would be evaporated and transpired if an unlimited
supply of water were available
• based on observations of temperature, wind speed, daylength, incoming and
outgoing radiation, vapour pressure, and atmospheric pressure
• online_thornthwaite: http://saltonsea.sdsu.edu/onlinethornthwaite.php
• modelling of these, including the use of remote-sensing techniques can lead to
spatially-generalized values for PE and ET
51
Water Budgets; Local Watersheds, continued
Hydrologic Budget Components
• P - precipitation
• I - interception
• DS - depression storage
or surface detention
• SM - soil moisture
• IF - interflow
• G - groundwater
• ET - evapotranspiration
• R – runoff
I
Surface runoff is overland flow of surplus moisture; watershed runoff is the channelized
flow that leaves the basin at its outlet.
52
Water Budgets; Local Watersheds – Runoff, continued
• could be expressed in millimetres of precipitation equivalents
• stream flow is normally reported as discharge in units of volume per unit time (e.g.
m3sec-1)
• position of channel networks on a map is referred to as hydrography and their locations
indicate the imprint of running water on the landscape (fluvial geomorphology)
• hydrologically ,hydrography is important because of how long it takes for water to
reach the channel (time of concentration) from overland flow, throughflow or
groundwater seepage
• “flashy” basins are those that deliver very quickly, such as those found on impervious
bedrock
• buffered basins have channels that respond slowly to precipitation or snowmelt events.
53
Water Budgets; Local Watersheds – Runoff, continued
• long recognized that storm hydrographs for streams
change shape as land use changes
• in general land development and urbanization create
a flashy basin from a buffered one:
• the rising limb steepens
• peak flow increases
• the recession limb steepens
• base flow decreases
• removal of basin storages in most cities and
suburbs has brought this about
• recently, stormwater management plans have become requirements of subdivision
approvals
• unless retrofitted, however, existing urban and suburban areas continue to experience
flashy stream discharge, and its associated flooding, erosion and expensive damagereduction procedures.
54
Water Budgets; Local Watersheds – Runoff, continued
• responses of streams therefore differ, depending on the mix of land uses within
their catchments it is common to generalize by broad land use categories (open
space, residential, commercial, industrial), but more detail is provided by actually
mapping pervious and impervious areas
• with current remote-sensing capabilities, extremely high resolution images now
enable very fine details, and are beginning to be incorporated into runoffprediction procedures
• useful data however remains to be stream
discharge records collected at stream gauging
stations
• non-automated stations merely have a
labelled post, allowing an observer to record
the height the river has risen
55
Water Budgets; Local Watersheds – Runoff, continued
Careful measurement of the channel cross-section (A,
in m2) and stream velocities (V, in m min-1) allows
conversion of stream stage (elevation) to discharge
(volume per unit time, in m3 min-1): Q= AV
This involves the use of a stage:discharge rating
curve. By calibrating this curve at a gauging site,
observations of water level can be converted to
actual stream discharge.
Discharge data:
Water Survey of Canada
http://www.ec.gc.ca/rhcwsc/default.asp?lang=En&n=4EED50F1-1
Ontario: Surface Water Monitoring Centre
http://www.mnr.gov.on.ca/en/Business/Water/2C
olumnSubPage/STEL02_163613.html
56
Water Budgets; Local Watersheds – Runoff, continued
A geographic approach (mapping) is necessary in understanding runoff and indeed the
whole water balance since the system of water capture, storage and discharge is
spatially connected over a watershed
A water budget involves spatially-aggregated fluxes (transfers from one location to
another) and spatially-variable stores (interception, surface detention or soils across the
catchment, or storage specific to an individual reservoir, or even to a rooftop)
To balance, data on the input fluxes, available storage and output fluxes must all be
“accounted for”.
The most common concern being addressed by water balances has been flood
forecasting, however the water budget itself has been gaining attention as a concern
arising from climatic change and variability, and as part of land-development
procedures.
57
Naturalized
Impacted
58
References
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Hebda, R.J., K. Gustavson, K. Golinski and A.M. Calder, 2000. Burns Bog Ecosystem Review
Synthesis Report for Burns Bog, Fraser River Delta, South-western British Columbia, Canada.
Environmental Assessment Office,Victoria,
BC.http://www.eao.gov.bc.ca/special/burnsbog/reports/trm/result.htm ).
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ml#2.%20Water%20storage
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http://www.geo.vu.nl/~trendy/C5.pdf)
60