The Hydrological System

Course Outline

Environmental Studies home

Environmental Engineering Technology home

The Hydrological System

Picture by Elementary Science Teacher Resource Book,
Utah State Office of Education

Water in Circulation

The hydrologic cycle

the hydrologic or water cycle is nearly closed, but

some water is released from crustal or other rocks by volcanic action
some H₂O is split by cosmic rays
some H₂O is split by plants in photosynthesis

most water is unavailable for use as freshwater

primarily in the ocean (97.3%),
or in polar or glacial ice (2.1%)

the ocean is nonetheless a dynamic part of the cycle, as one of the sources of water vapor produced by evaporation

the vapor condenses, falls as precipitation
water on the ground:

evaporates
infiltrates (sinks into the ground

infiltrated water can be taken up by plants and transpired (hence, with the above "evapotranspiration")

runs off

streams
lakes
rivers
the oceans
the proportion of run off increases in urban areas (parking lots have low rates of infiltration)

Where is the water?	Fraction of total
Oceans	97.3%
Polar or mountaintop ice	2.1%
Groundwater	0.6%
Freshwater Lakes	0.009%
The atmosphere (vapor)	0.001%
Rivers	0.0001%

precipitation = runoff + evapotranspiration

30 inches/year = 9 inches + 21 inches
(the ocean has to make up that 9 inches or the rivers would run dry)

water cycles through the air in about 9 to 10 days; the rivers would take about 40K years to refill the oceans

Precipitation

most rain in the US (65%) falls on the eastern half of the country
most water (85%) is used by the western states, primarily for irrigation (folks in the east eat the food grown with that water too)
the average precipitation in the US is about 30 inches but it varies from essentially nothing in the southwest to 200 inches or more in the east and northwest

Runoff

Effects of soil

soil characteristics greatly affect the degree of runoff versus infiltration

clay content reduces infiltration, especially after the ground is saturated with water (which swells the clay)

sandy soil or friable loams, both of which have low clay content, have higher rates of infiltration

more densely compacted soils have lower rates of infiltration

note: rainfall can actually compact soil
rainfalling directly on soil also tends to plug the opening in the ground with fines from near the surface (like espresso grind coffee in a melitta filter)

waterlogged soil has a lower rate of infiltration than dry soil

frozen soil has almost no infiltration (it may actually be coated with ice

thus the timing and dynamics of a storm event can have a great influence on the proportion of runoff

generally speaking, longer or repeated rains result in more runoff

human development of an area usually has a dramatic effect on increasing runoff

paving and roofs have essentially no infiltration
one of the engineering calculations that must be done when planning a development is the excess flood flow--this must be taken into account
diverted stormwater may have substantially poorer quality than native runoff (metals and oils etc from the streets and yards)

commercial and industrial establishments that discharge stormwater at a point are now subject to the restrictions of categorical wastewater discharge permit

Effects of vegetation

vegetation can greatly increase the rate of infiltration

hold water in place, buffer the release of water to the groundsurface
shield the soil surface from some of the effects of compaction by rainfall

clear cutting a forest can increase runoff and erosion disastrously, as can overgrazing and improper cropping

proper management reduces the effects
in fact, well conducted forestry (including some clear cutting) can increase the amount of snowmelt which is available for human use without dangerously increasing erosion
plowing along hillside contours (conservation tillage) tends to slowdown runoff and thus decrease erosion and increase infiltration

bare soil is bad

Erosion and rivers

water flow in streams is often turbulent (ie, not laminar or in smooth lines parallel to the stream bed)

different flow velocities will lift particles of different sizes

generally, smaller particles will be lifted by lower velocities
but, clay and silt (<0.06 mm) can be harder to erode

can be tightly packed (consolidated)
may not stick up into the flow enough

suspended matter abrades rocks etc
as the river cuts a stream channel, the
some material is dissolved by surface water passing over it

the process of erosion gives rise to stream beds

a drainage pattern develops like the veins on a leaf (assuming the surface is uniformly erodable-irregularities change the pattern (trellis or rectangular)
the entire area from which a river and its tributaries draw runoff is called a drainage basin or watershed

the divisions between drainage basins are ridges called topographic divides
these can be drawn on topo maps by always drawing at right angles to the contours

a fast young stream cuts rapidly and sharply into the rock (V-shaped cross section)

these narrow valleyed rivers have waterfalls and rapids caused by sudden drops in elevation along the stream bed

may be caused by erosion (Niagara) or glaciation (Yosemite)

slope wash and mass movement (collapse) of the valley walls tend to flatten the walls of the valley

a more mature river

cuts more laterally, increasing the number and size of side canyons
deposits some eroded material in the base of the valley
alternatively, the valley itself can be broadly eroded to form a plain

an old river has a broad floodplain at the base of the valley, the channel of the river can meander across it

the periodic flooding of the river enriches the floodplain with nutrients carried by the swollen river
the bed of the river can actually sit above the surrounding floodplain, with the stream channel separated by natural levees
the low-lying flood plains are also popular for housing, which can be a problem in the rainy season

building dikes can make the problem worse downstream

the base flow of a river is fed by springs

in a rain storm flow increases, the more widespread a storm is (over the drainage basin) the more flow increases
the plot of flow of a river is called the hydrograph

Flood protection

planning based on the 100 year (or other) flood

flood control or management measures:

reducing peak flows
dams
land use controls
early warning systems
insurance (often from Uncle Sam)

Lakes

Stratification

the slow moving water of lakes is not necessarily mixed by turbulence, thus there is an opportunity for layers to form
cool water sinks, warm rises

in summer (if the lake is deep enough), the lake separates into the warm epilimnion and the cool hypolimnion
the two layers are separated by a thermocline

you have felt this with your toes when swimming
sometimes you can pick it up with a fishfinder

the hypolimnion is isolated from the surface, ie oxygen thus the bottom of the lake can become anoxic due to aerobic decomposition

this condition is deadly to fish
the anaerobic environment favors reduction reactions

sulfide can form
solubility of metals and nutrients is affected

in winter, the stratification is slightly different with ice and cold water on top and water at 4° C on the bottom

if photosynthesis stops due to snow and ice cover, there can be anoxia under the ice

in spring and fall, lakes turnover

important in the seasonal changes in the lake ecosystem
have affects on the distribution and movement of nutrients and metals

Groundwater

infiltrating water sinks through the permeable substrate until it reaches impermeable material (bedrock or clay)

underground, essentially everywhere, there is a layer of permeable soil or rock saturated with water
this groundwater is more than 98% of the freshwater on earth
such a layer is called an aquifer

water table or unconfined
artesian or confined

flow of water in an aquifer is determined by the permeability (ft/day) and porosity (percent) of the substrate material and the hydraulic head or gradient that the water is under
water is drawn for human use from aquifers by wells

if water is drawn faster than it can be replenished the aquifer can be depleted (so-called mining of water)

subsidence
saltwater intrusion (can be counteracted with wastewater injection)

groundwater not withdrawn by humans eventually emerges at springs (or underwater discharge zones in streams) where it rejoins runoff to the oceans

Limestone & Sinkholes

groundwater can "erode" soluble rocks like limestone (calcium carbonate)

an underground chamber so eroded is called a cavern
the ground above such erosion can collapse and form a sinkhole
the underground channels in a limestone area can contribute to rapid underground spread of pollutants

groundwater movement is difficult to model in Karst areas (as it is in ordinary fractured rock)

Glaciers

when more snow falls in the winter than melts in the summer, the level of snow rises
when the wet of snow and ice gets great enough, they start to flow (very slowly) as a glacier

the river of ice has its own erosional characteristics

huge boulders and massive quantities of lighter material are carried along
when they melt or recede they leave behind fields full of boulders and sandy morraines (like Long Island or Cape Cod)

the polar glaciers seem to extend toward the equator in some sort of cycle (peaking at the ice ages)
the volume of water frozen in glaciers effects sea level

dramatic increase in rate of rise in the last 70 years or so

Water Supply

the most readily available water is the scarcest (rivers and lakes)

Water rights

riparian ("river-ian"): all those who own land fronting on a river are entitled to the water; your riparian rights protect you from those upstream

based on English common law; this theory is used for water rights in most of the eastern US (where water is plentiful and cheap)
no one may deprive those downstream of their use of the water
conversely, you can't flood out those downstream (don't increase or decrease useful flow)

prior appropriation

this theory is used for water rights in most of the western US (where water is scarce and expensive)
first come, first serve (and shoot the guy who comes after you); your right to use water is based on historical precedent

Transbasin diversion of water

a raindrop falling in one watershed ends up in another
New York City is not in the Delaware River watershed, yet it draws its water supply from there

there is an agreement between the city and the Delaware Basin Commission (this is a sort of prior appropriation)
other large cities (such as LA) divert water from faraway

Irrigation

if rainfall is less than 20 inches per year, farming is generally not feasible without irrigation

irrigation consumes a lot of water

about 200 to 300 gallons of water have to be taken up by a tomato plant to produce one pound of tomatoes
more water than that must be poured onto the plants for them to take up that much water
a good portion of that poured onto the ground evaporates
irrigated soil becomes saline form salts in the irrigation water; soil can have nutrients leached out of the root zone; leached nutrients and salts can contaminate the groundwater

irrigation of feed grain and watering of livestock consumes a large amount of the water used in the US

about 2500 gallons of water are required to produce one pound of hamburger

Water uses (data for 1995 from the USGS)

as you can see from the table, direct water use is approximately 100-120 gallons per person per day
the largest use of water is for irrigation and livestock watering

this usage is almost two thirds consumptive (consumed water is not returned to the environment in reusable form, instead it is evaporated or contaminated)

US Daily Water Use by Sector for 1995
Category of Use	per capita daily use	fraction consumed
Domestic	86 gallons	26%
Commercial	36 gallons	14%
Industrial	78 gallons	16%
Thermoelectric	500 gallons	2.5%
Agricultural	528 gallons	61%

Return to:

Top

Course Outline

Environmental Studies

Environmental Engineering Technology

ANTHONY G BENOIT
ROOM 201B
(860) 885-2386

abenoit@trcc.commnet.edu