Winter 2003 GEOG 4350 Water Resources and Management

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Winter 2003 - GEOG 4350Water Resources and
Management

• Instructor Michael D. Lee Ph.D.
• Geography and Environmental Studies

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The Hydrologic Cycle

• The movement of water between the lands, the
  oceans and the atmosphere we call the
  hydrologic/hydrological cycle.
• On the land, the hydrologic cycle takes place
  within the geographical unit called the
  watershed.
• The watershed is basically the area of land from
  which water drains to a particular water body,
  defined by high points in the landscape.
• The geological, geomorphological, vegetative, and
  human character of the watershed together with
  the weather/climate determines the pathways that
  water takes following its input as precipitation.

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The Hydrologic Cycle
As summarized by Cech (2003), four main
components precipitation, runoff, storage and
evaporation (its a little bit more complex than
that!).
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Driving Forces

• Solar energy from the sun drives the hydrologic
  cycle.
• It causes the evaporation of water and
  transpiration by photosynthesizing plants,
  creating water vapor.
• It also causes temperature differentials that
  give rise to pressure differentials that result
  in air masses moving both vertically and
  horizontally.
• Vertical movement of air gives rise to cooling
  and condensation, leading to precipitation, and
  horizontal movement distributes that
  precipitation from oceans and other water bodies
  onto lands.
• Topography also forces air to rise, leading to
  cooling and to orographic precipitation in
  specific areas.

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Precipitation

• Precipitation takes a variety of forms and occurs
  when the forces of upward air movement are
  exceeded by the weight of water droplets
  contained by the air and the effects of gravity.
• Precipitation is measured usually by rain gauges
  (which can also be modified to collect snow) or
  by measuring snow-pack accumulation.
• If we know the depth of precipitation in a
  particular time, we can convert that to a volume
  if we know the area over which it fell.

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Runoff

• Runoff is the amount of water that flows across a
  land surface as a result of a precipitation
  event.
• Climate (antecedent conditions), topography,
  surface characteristics, precipitation intensity
  (depth per unit time), and precipitation duration
  (determining total depth) largely determine
  runoff.
• All things being equal, the wetter the prior
  period, the more intensive the precipitation, the
  longer the storm duration, the greater the
  runoff.
• Whether the soil has a high infiltration capacity
  or is impermeable, and whether the land is
  sloping or flat will play modifying roles.

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Storage

• Water that runs off the land surface or
  infiltrates and percolates down to groundwater is
  stored, sometimes for very short periods (e.g. in
  streams) and sometimes for very long periods
  (e.g. in deep aquifers).
• Water is usually moving through these stores from
  store to store on its way to its base level, most
  commonly the oceans.
• Lakes and reservoirs are our most obvious
  terrestrial stores of water, but more water is
  stored underground than above.
• Reservoirs are water bodies with the specific
  purpose of retaining water for human benefits.

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Evaporation

• Liquids convert to vapor either by evaporation
  from wet soil, wet surfaces or water bodies, or
  by the vaporization of water from plants through
  stomata during photosynthesis.
• Evaporation is measured using Class-A pans,
  evapotranspiration using lysimeters, and both can
  be estimated using empirical formulae.
• Potential evapotranspiration, the amount of water
  that could theoretically be vaporized from a
  given area, is an important concept for
  irrigation and storage losses.

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Class A Evaporation pan
Source American Meteorological Society
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Source UC Davis, Greg Pasternak
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Climate and Hydrology

• The stores and flows of water behave differently
  in different climate zones under different
  weather conditions.
• Hydrologists often specialize in one climate type
  e.g. arid, temperate or tropical conditions.
• Of most concern to water managers are the
  extremes of climatic and weather variation
  floods and droughts.
• For floods, large intense storms (the 150 or
  1100 year maximums) and prolonged wet periods
  (e.g. El Niño years) are critical.
• For droughts, hydrologists are concerned with
  abnormally dry periods how long this has to be
  depends on the climate and the management
  situation.
• Droughts are climatic, but can be mitigated or
  aggravated by management or the lack of it.

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Surface Hydrology
Groundwater Hydrology

• Surface water hydrology is the study of moving
  water across and into the surface of the earth
  i.e. includes infiltration.
• Groundwater hydrology is the study of the
  movement of water through the interconnected
  pores in the rock layers of the earth.
• Between these exists soil hydrology,
  alternatively claimed by both types of
  hydrologists.

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Managing Watersheds

• The hydrologic cycle takes place over and under
  watersheds.
• Water is input to a watershed by precipitation
  (also by diversion and conveyance) and moves
  through it and out of it by different pathways
  some surface, some subsurface.
• Streams and rivers converge down through the
  watershed towards a common base level.
• Water that falls in the headwater reaches will
  eventually move downward above or below ground
  until it reaches this base level.
• Any characteristics the water has or attains
  within the headwaters or middle reaches of the
  watershed will be transmitted to the lower
  reaches of the watershed.
• To manage the quantity and quality of a water
  resource, we must thus attempt to control the
  conditions within the whole watershed that might
  affect it.

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Surface Water Hydrology

• Clearly of great importance here is how
  watersheds react to precipitation events to
  produce stormwater discharge.
• Water inputted will take different pathways of
  different speeds depending on
• the state of the watershed (wet, dry, frozen,
  etc.),
• the nature of the input (gentle, intense, rain,
  snow), and
• the physical conditions of the surface
  (vegetation, surface type soil, rock, asphalt,
  etc., and underlying geology)
• Hydrologists/water managers are very interested
  in what happens in storms and how watersheds and
  streams react (for an excellent resource click
  here)

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Pathways

• A raindrop can take a variety of pathways on
  falling to earth and have different fates.
• Evaporate back to the atmosphere
• Be interceptied by vegetation (then evaporate,
  drip to the ground, or run down as stemflow)
• Infiltrate the surface
• Pond on the surface (then evaporate or
  infiltrate)
• Runoff across a hillslope as overland flow toward
  a stream or other water body
• Fall into a stream or water body directly.
• Note that infiltrating water can return to the
  surface as interflow or emerging groundwater.

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Some Key Points

• Please read Cech Chap. 3 thoroughly and remember
• The more intense the precipitation event, the
  quicker runoff will occur.
• Combined with this, the greater the depth of a
  precipitation event, the more likely surface
  runoff is to occur and in proportionally greater
  quantities.
• Where multiple precipitation events occur in a
  given period, the greater the probability of
  runoff and of greater quantities during
  subsequent events.
• The wetter the soil, the closer the water table
  to the surface, and the more impermeable (lower
  infiltration capacity) the surface materials, the
  more likely the watershed is to produce surface
  runoff.

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Hydrographs

• Stream flow (cfs or cumecs) can be measured at a
  given location by use of gauging apparatus and a
  relatively simple formula (Q A.V) see Cech p74.
• The volume of water moving through a stream will
  increase in proportion to its depth (exploits a
  bigger A) and its velocity (as it moves faster,
  more water passes through a given area in a given
  unit time).
• If too great a volume of water arrives at a given
  stream segment from upstream and the stream is
  not able to convey it quickly enough (due to a
  shallow slope and/or high friction), water levels
  will rise and burst the stream banks a flood.
• The expected peak stream flow from a watershed
  can relatively accurately be predicted by a
  rationale formula (QKiA) see Cech p73.

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Some useful graphics (see Cech p75, 79 for
similar)
Source Schafersman UTPB - http//www.utpb.edu/sci
math/schafersman/flooding/page-3.htm
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Groundwater hydrology

• Geographers frequently become hydrologists -
  surface hydrology is taught in geography depts
  outside the US.
• Generally groundwater hydrology, usually termed
  hydrogeology, is the realm of geologists.
• Hydrogeology is the study of the characteristics,
  movement and occurrence of water found below the
  earth.
• Water percolates down through rocks by the force
  of gravity.
• When an impermeable barrier is reached, water
  collects and saturates the rock, forming a zone
  delimited by the water table.

• Water will then move move laterally down a
  gradient determined by the slope of the water
  table in unconfined aquifers, or the distribution
  of hydraulic pressure in confined (artesian)
  aquifers.

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Aquifers

• Aquifers are layers of rock saturated with water
  from which useful volumes of water can be
  abstracted from springs, wells or boreholes.
• Where groundwater is confined, artesian pressure
  may build up due to the hydraulic head generated
  by higher water levels far away in the aquifer
  that may force water to the surface where the
  overlying impermeable layer is breached.
• Where groundwater is unconfined, water will need
  to be pumped out of wells against gravity.
• The rate at which water can be pumped out or
  flows out a well will be a function of the
  hydraulic conductivity, the rate at which water
  can flow through the interconnected pore
  spaces/cracks in the rock.

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Groundwater
Water Table
Unconfined Section
Confined Section
Pumping necessary
Artesian pressure
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Hydraulic Conductivity

• How much water is available and can move through
  a given cross-sectional area of rock in a given
  time will be a function of the hydraulic gradient
  and head (pressure pushing the water), porosity
  (size of voids) and void connectivity, viscosity
  of the water, and the frictional resistance
  offered by the material.
• Porosity, void connectivity, and frictional
  resistance all interact to determine the
  permeability of a rock layer.
• Hydraulic conductivity is a function of
  permeability and viscosity and has the same units
  as a velocity (e.g. meters per day) and is the
  rate of flow per unit area through a rock layer.
• The discharge of water through an aquifer is thus
  the product of the hydraulic conductivity the
  hydraulic gradient the cross-sectional area of
  flow (note that Cech p103 is a little confusing
  on this).

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Assessing Groundwater

• Hydrogeologists use the knowledge of rock
  formation and stratigraphy to determine the
  underground structure of rock layers and the
  likely whereabouts of exploitable groundwater
  reserves.
• This can be enhanced by satellite images, ground
  penetrating radar, seismic surveys and so forth.
• Studying the patterns of water table levels in
  neighboring wells and conducting pumping tests to
  see what happens to water tables is also helpful.
• Radioisotope tracers can be introduced to
  recharge areas or injected into test wells and
  recovered to show the movement patterns of
  groundwater.

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Some Important Points

• Different types of rocks store and/or yield more
  water than others coarser, more porous
  sedimentary rocks (sands, limestones) tend to
  store and yield more, although weathered and
  fractured igneous rocks (granites, etc.) can also
  transmit water well.
• Artesian aquifers are favored for water supplies
  because they can be tapped without pumping costs.
• Water in aquifers can come from a long way away
  and thus management of recharge areas can be
  difficult.
• Groundwater can accumulate and recharge slowly
  and can frequently be used up faster than it is
  replaced (e.g. Ogalala aquifer), leading to
  drawdown of water tables and drying up of wells
  and surface water bodies like wetlands.
• Groundwater frequently has a much longer
  residence time than surface water and thus
  contamination can take a long time to flush from
  an aquifer, moving through slowly and attaching
  itself to the aquifer pore materials.

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The Basic Water Balance

• The law of continuity applies to the hydrologic
  cycle in all watersheds. In a given period of
  time
• InputOutput ? ?Storage
• PQET??SMS??GWS??DS?GWO
• P total precipitation input
• Q total streamflow discharge at outlet
• ET total evapotranspiration loss
• ?SMS change in soil moisture storage
• ?GWS change in groundwater storage
• ?DS change in depression and snowpack (surface)
  storage
• GWO groundwater outflow at depth

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Applying the Balance Concept

• Water resource managers use watershed balance
  concepts and develop complex management systems
  that can combine hydrological engineering models
  with economic models to track and predict
  supplies and demands.
• While mathematically complex, such models are
  based on relatively straightforward rules of
  operation.
• Models can be used to manage resources over a
  given water year (Oct 1-Sept 31) or to predict
  supplies and shortage potentials over future
  years.
• For example, models exist for the State of
  California and its various river and reservoir
  systems.

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Lake/Reservoir Balance Models

• Similar continuity principles can be applied to
  lakes and reservoirs and are used in their
  management.
• Note the slight modification here (from Cech
  p77)
• Reservoir storage QiPGi-S-E-Qo
• The volume stored in a given lake or reservoir
  varies non-linearly with depth depending on the
  geometry of the water body how its volume
  varies from its minimum to maximum water
  elevation.
• The geometry is determined by bathymetric
  analysis that produces a contour map of the
  storage area.
• How much evaporation is lost is a function of the
  surface area which will also change non-linearly
  with volume and depth according to geometry.