Lecture 19: November 18

From Last Lecture (we didn't get to):

Multiple Stressors to an ecological system- diffuse and hard to identify what of many possible causes are responsible for degradation

  • challenge is this: at an endpoint change in biological assemblage, there can be multiple causes. Models can help you diagnose.
  • Metrics aren't good for that. i.e. some measure of biological integrity— it's responding to changes of all sorts (sediment, turbidity, etc.)

Earth's Water

  • 3% fresh, most in glaciers (global warming will take care of that)

Hydrologic cycle

  • you know the deal— water evaporates, plants transpire, precipitation happens, water runs off, infiltrates (water from surface into soil), percolates (after infiltrates), is transferred through the hydrologic cycle across contintent surfaces through lakes and streams, from glaciers and snowpack, ground water and soil water, and vegetation.
  • water in landscape includes surface water, water in soil (soil moisture), saturated zone (ground water— think artesian well where ground water comes up through surface.)

What determines waters infiltration into soil?

  • Precipitation- magnitude and intensity and duration
  • Soil characteristics- lot of rain on sand goes right down, clay absorbs less water at slower rate.
  • Soil saturation— how well water infiltrates. Higher saturation leads to more runoff instead.
  • Land cover
  • Slope
  • Evapotranspiration

Basic Soil Water and Runoff Terms

Can go to this website to play around with infiltration rate relative to saturation. If saturation high, rainstorm will result in surface runoff. Most intact forests have sufficient infiltration capacity— no runoff. But if have more impermeable surface, water will flow over surface.

Imperveous surface created with urbanization

  • Built environment: roofs and roads and parking lots.
  • Changes in hydrologic flow as impervious surface cover increases.
  • Evapotranspiration goes down when paved landscapes remove role of evapotranspiration (by removing vegetation), thereby preventing inflitration.


  • More leaf tissue lots of transpiration.
  • On average in US evapotranspiration accounts for 2/3 precipitation (runof is therefore 1/3).
  • Want to know how important plants are to evapotranspiration? Remove plants. All water removed in that system will be from runoff (not evapotranspiration.)

Bill went over charts to discuss how hydro cycle can be thought of on a month by month basis. Charts show precipitation, runoff and ET. Summer months solar radiation evap water and plants take up a lot of water, so stream flow difference between precipitation and ET. Low in summer because of more evaportranspiration, more water to sky, less available to run downhill. Can also see why diff patterns of stream through in diff parts of co. based on seasonal precipitation and seasonal loss of water to the atmosphere.

Catchment Water Cycle

  • Mass balance water— water in and out.
  • In as precip, out as ET, q is runoff, ground water (GW) is storage that may increase or decrease.
  • P=ET_Q +-GW
  • Q= (P-ET)=- GW
  • ~Q=P- ET

Consumptive use

  • The reason agriculture uses so much water? Water not returned to source after use, i.e. take water from surface and irrigate crops, a lot evaporates, if it doesn't goes into plant roots and evapotranspires. Take from river, river will dry up if not replenished.
  • Non-consumptive (water returned to surface runoff— usually contaminated. i.e. goes through waste water treatment, ag use, domestic use, and becomes contaminated. Pollution mostly municipal, industrial, and agricultural.

How much water do you need to drink/ year?

  • 1000 litres per person (1000 water bottles/yr.)— equals 1 cubic meter
  • BUT- all of us need more water to grow food. Actually need 1000 m cubed per person. Less than this = water scarcity.
  • Virtual water- import food not water (because bringing in food would require less water.)

Water Budget

  • 40,000 cubic kilometers— all fresh water for ALL uses per year (total)
  • Evapotranspiration taken over by humans (terrestrial NPP, 30% under human manipulation in crops or forest plantations, we're taking all ET water available with that.)
  • Runoff- there's a lot we can't get to. Amazon is single bigest source of runoff. All rivers flow into sea, most of it runs off. spatial availability— hard to transport. Temporal availability- Dams store some but not all. about 73% is floodwater.
  • Bottom line- 12,500 km^3 is accessible runoff.

For a good time, and to play with a model of various effects on dams, check out: [http://dameffects.org]

Human alteration of flows

  • Between the late 19th and late 20th century the variability and frequency of flood events increased markedly.
  • Highs higher, lows lower. Combo of imperveous surface and water projects (wetland draining and infill, dams, dikes, levees, etc.)
  • US Dam database taken offline after 9/11
  • Snake River- dam perennial discussion to remove dams to allow salmon to migrate
  • 3xs annual river flow stored in Colorado River (most overallocated river in US)
  • Most dams are small, and most storage volume is from large dams. Owned privately (large), many small dams abandoned. Wisconsin has most progressive dam removal program b/c of safety issue. Public utilities own a small fraction.
  • Most dams are multiple use (recreation, farm ponds, hydroelectric…)
  • Getting rid of big dams difficult.

Why manage river flow to maintain flow regime?

  • A lot of dams you have to live with (big ones.) Small ones, crumbling, easier to get rid of.
  • Want to achieve channel capacity, allow fish to migrate, create habitat complexity (natural systems are more resistant to invasions.)
  • Only in the last few decades have we begun to implement more positive policies that seek to reestablish natural flow systems.

Sustainable Water Use: A Vision

  • Basic human needs for water should be top international priority
  • Water-related diseases should be largely conquered
  • Agricultural water should be used and allocated efficiently
  • Basic ecosystem needs should be identified and met
  • Serious water-related conflicts should be resolved through formal negotiations
  • Consequences of climate change for water resources should be accounted for

(Gleick, 1999) For a look at Nelson Mandela's "Some, for all, forever" vision, here's a paper I found: [http://www.princeton.edu/~mauzeral/wws402f_s03/JP.Priscilla.Delgado.pdf]

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