Hydrology Lab #1 Units.

•      Units: Just as hydrology has taken concepts from many fields and languages, the units used in hydrology come from many sources.

•       It is common practice in the USA to use both metric and U.S. units.



•      A liter is 1 cubic decimeter hence there are 1,000 liters in a cubic meter. A liter at STP weighs 1 kilogram or 2.205 pounds. There are 3.79 liters in a gallon.

•       A gallon contains the amount of ale the king could drink at a single sitting  and weighs 8.33 pounds.

The cubic foot

•      Contains 7.480 gallons is also a unit of measure of volume of  water and weighs 62.31 pounds.

•      Cubic feet per second (CFS) is a common unit of flow.

How thirsty are you?

•      The average person needs something like 1-2 quarts of water  a day,  although consumption could be much higher in hot arid areas.

•      Or during fraternity parties (of beer anyway)…

•      Gallons are therefore good units for individual water consumption.

How thirsty is your toilet?

•      The typical consumption of water in residential settings is closer to 100 gallons per day per person.

•      Thus gallons is not a very convenient unit for measuring municipal water use.

How thirsty is your cotton crop?

•      Agriculture consumes far more water than municipal uses, so gallons are rarely used for measuring agricultural water use.

Acre Feet:

•       Rather the acre foot is the common measure for large quantities of water and is approximately 325,851 gallons.

•      This is the volume of water that will cover one acre of land to the depth of one foot. It is equal to 43,560 cubic feet of water.

Calculation I. Is water cheaper than dirt:

•      Many farmers pay as little as $5 per acre foot for water delivered from federal water projects to their farms. How many pounds of water does a farmer get for a dollar

Cost per pound.

•      1 acre equals 43,560 square feet, 1 acre foot equals 43,560 cubic feet of water,  a cubic foot of water weighs 62.31 pounds so an acre foot weighs 2,714223.6 pounds.

•      That works out to 542,844 pounds of water delivered for a dollar.

Dirt Cheap?

•      Compare that to dirt which for top soil would run about $5 per cubic yard (which would weigh about 2,600 pounds) plus another $3 per cubic yard for delivery. So this works out to about 325 pounds of dirt for $1 delivered.

•      So dirt costs over 1,600 times as much as water per pound delivered.

Calculation II. What do you pay for water?

•      In Huntsville today the rate for the first 0-3,000 gallons is $10.65. Then the rate is $1.80 per thousand gallons.

•      Ignoring the initial fixed fees this works out to 555.56 gallons per dollar or alternatively $.002 per gallon.

If farmers paid what you do:

•      Therefore an acre foot would cost: 325,851 gal -3000 gal or 322,851 gal at $.002 per gal or $645.70 plus $10.65 for the first 3,000 gallons or  $656.35 per acre foot.

•      This is more than 100 times what many farmers pay for water.

Example III. Residential consumption.

•      A typical residential suburban family (i.e. has a lawn) uses about 1/3 of an acre foot of water per year,  about 75% for landscaping.

•      A typical alfalfa farmer uses 5-7 acre feet of water per acre per year and will farm perhaps 640 acres.

•      That would be equivalent to the water used by a typical town of about 10,000 people.


•      180 Gallons per day X 365 days = 65,700 gallons per year.

•      6 Acre feet X 640 acres = 3,840 acre feet per year X 329,250 gallons per acre foot =12,3360,000 gallons/ 65,700 gallons per family

•      19,229 families..


Example IV:

•      The Mirage/Treasure Island Hotel in Las Vegas employs 7,000 people and generates about $500 million in revenues each year, it consumes 300 acre feet of fresh water per year (it recycles its water for landscape irrigation).

•      The average small farmer in Nevada that is growing an irrigated crop such as hay or alfalfa uses 2-10 times more water to support  himself, his family and some migrant labor.

Units of Flow:

•      After measures of volume, measures of flow are most important in hydrology.


•      The important measures of flow are Cubic foot per second (CFS) (which is used for streams and rivers).

•      A small stream might be 1-10 CFS a large river 100,000 or more CFS.


•      Gallons per minute (GPM) is the typical unit for production from wells. A typical well will product 20-100 GPM while very large wells will produce 2000 or more GPM.


•      Millions of gallons per day (MGD) is a typical unit for water treatment plants and sewage treatment plants. A small plant might be 1-10 MGD while a large plant might be 100-200 MGD.


•      Cubic meters per second is not as common in the U.S. but is common everywhere else.

Example V Flow in Harmon Creek

•       Harmon Creek has an average flow something like 10 CFS.

•      How many gallons is this in  a second? A day or a year?:


•      7.48 gal/cf x 10 cfs =

•      74.8 gallons per second.

•      Per day 60 sec/min x 60 min/hr x 24hr/day = 6,462,720 per day.

•      x 365 days per year = 2,358,892,800 gallons per year.

How much does this water weigh?

•      At 8.33 pounds per gallon this is:

•      1,9,649,577,024 pounds.

Creek for sale?

•      If I could own the flow in this creek and sell it in Huntsville at the Cities’ rate what would it be worth?

•      2,358,892,800 gallons / 325,851 acre-feet/gal x $656.35/acre-foot =

•      =$4,751,433.29.

Other units

•      Units of water velocity are feet/sec or meters/sec and

•      Units of stage height are feet or meters.

•      There are also units of concentration, typically parts per million PPM or parts per billion PPB.

Specific gravity

•      It is the density of a substance relative to the density of water:

•      Water has a specific gravity of 1, solid rock and the particle matter in soil typically has a specific gravity of 2.65, Gasoline has a specific gravity of .75, PCB have a specific gravity of 1.6.

Example VI: Toxic transformers.

•      A transformer with hazardous waste PCBs inside it:

•      Concentration of 480,000 ppm of PCB s and the other material is mineral oil with a specific gravity of approximately .8. Volume of transformer is 900 gallon and it is 100 % full.

•      EPA wants to know what is the weight in Kg of hazardous waste shipment?

Toxic calculation.

•      ((1.6 x .48) + (.8 x .52)) 900 gal x 8.33Ibs/gal/2.205 lbs/kg.           

•      (.77 + .42)  1.19 x 900 gal. 1,071 x 8.33 = 8,921.43/2.205 = 4,046  kilograms.


•      You should be reasonably able to convert between these units, since some exam problems will involve calculations where you need to make simple unit conversions (with supplied conversion factors).


•      Distance:

•      1 meter = 3.281 feet.

•      1 mile = 5280 feet

•      1 kilometer = 1,000 meters = 1,000,000 mm.


•      1 cubic meter = 264.2 gallons.

•      1 cubic foot = 7.48 gallons, 28.32 liters.

•      1 liter = .2642 gallons.

•      1 acre foot = 325,851 gallons.


•      1 cubic meter per second = 35.32 cubic feet per second

•      1 cubic foot per second for 1 day  = 1.98 acre feet.

Conversion factors metric to U.S. customary

•      1 hectare = 2.471 acres, 10,000 square meters.

•      1 acre = 43,560 square feet.


•      1 kg = 2.205  pounds.

•      1 cubic decimeter = 1 kg.

•      1 gallon = 8.33 pounds

•      1 cubic foot = 62.31 pounds


•      1 meter/sec = 2.237 mi/hr.

•      1 foot/sec = 1.097 km/hr.

Other factors.

•      1 inch of run-off per hour from one acre = 1 cubic foot per second of flow.