Discharge and Evaporation
Discharge
Some of the Precipitation evaporates or is transpired. Some of the
infiltration, the interflow, stays above the Groundwater Table and
resurfaces in the stream valley as return flow. The overland and return
flow become runoff. They reach the stream, and eventually become
outflow = discharge
Surface Runoff redirected to a stream by an artificial
drainage system; most will reach the watershed outlet
Pesticides and herbicides included
Discharge aka Outflow, etc. Flow Q [m3/s] leaving the Control Volume.
A Hydrograph plot of flow rate vs. time
Q [ft3/s]
Hydrograph
Hyetograph
A graph of discharge vs. time is called a Hydrograph, measured at gaging
stations. During a rainstorm some initial Precipitation fills permeable
depressions, which infiltrate after a delay.
Some initial precip. fills detention basins which slowly return most water
downstream. As these fill, the streams receive increasing amounts of
direct runoff flow. Once the precip stops all the runoff is from storage.
Different watershed shapes and permeabilities have different Hydrographs.
Elongated watershed has a broader hydrograph because some water has a
long way to go.
Developed watershed (Macadam, Concrete, grass) or Desert (desert
pavement, desert varnish) arrives sooner because little infiltration
• To obtain the
hydrograph data for
your project, you
will rely on online
data taken from the
flow gage at the
outlet of your
watershed.
For discharge
flow rates, you
will rely on
automated
USGS gages
http://nh.water.usgs.gov/gauge_station/3_howusgs.htm
Stream Gages
Measure Discharge
Old: Float has a variable resistor attached to turning float pulley.
New: Bubbler measures pressure (as current drawn by the bubbler pump)
required to supply constant gas flow.
At each gauging station, the cross-section is well
known and frequently checked, so the flow rate,
and wetted area, with each depth of water is
known precisely.
Stage
• Flood stage is the water level of a stream as
read by a gauge for a particular location,
measured from the level at which a flowing
body of water threatens lives, property, or
commerce. The term "at flood stage" is
commonly used to describe the point at which
this occurs.
• "Stream stage" (also referred to as "gage
height" or simply "stage") is the level of the
water surface above an established zero level
at a given location.
Evaporation
• Evaporation is often the most difficult
parameter to estimate
• In many areas it is also the most important
parameter of Water Balance. Excessive
evaporation leaves salts behind, making the
land incapable of growing crops.
Heat of Vaporization
• There are two basic mechanisms by which
moisture gets into the atmosphere;
evaporation and transpiration.
• Evaporation is just a phase change in water
(from liquid to vapor) induced by the addition
of enough energy
• Water has a high heat of vaporization—it
takes about 580 calories to vaporize 1 gram of
liquid water at standard surface conditions.
Liquid to Gas
• Relatively loose bonds between water
molecules (remember, because they’re polar,
they have a loose hydrogen bond) are broken
with the addition of enough energy, allowing
individual molecules of water to fly free.
• The two things that drive this process are how
much energy is available (sunlight), and how
much water is already in the atmosphere.
Saturation
• Once the atmosphere is saturated (T = Tsat = Td) with
water vapor, the air cannot “carry” any more. If you
lower the temperature slightly, condensation starts
and we see clouds, including Fog.
Transpiration
• Transpiration is also the conversion of liquid
water to water vapor, but it’s done by plants.
• Plants have developed a very efficient system
for pulling water up from the ground based on
capillary action. This moves nutrients to
tissues. They must lose water at leaves for the
process to continue.
• Water vapor is lost from leaf surfaces through
small openings called stomatae during carbon
dioxide diffusion for photosynthesis.
Conditions favorable for Evaporation
• Evaporation is greatest on hot, windy, dry
days; and is greatly reduced when air is cool,
http://www.earlham.edu/~biol/desert/irrigation.htm
calm, and humid.
Estimating Evaporation
Class A Evaporation Pan
Filled to 8 inches and observed daily.
Plus a rain gage, thermometer for water
temp, and a psychrometer for air
temperature and wet bulb temperature
(calculations reveal dewpoint Td)
Keep animals
from drinking
Pan Evaporation
• We estimate how much
water evaporates from
an area with a pan of
water.
• Measure how much
water leaves the pan in
a day.
• Out in the open, so you
need a rain gauge.
Class A Pan, 4’x10”
Stilling Well
Pan evaporation is used to estimate the evaporation from lakes.
Evaporation from a natural body of water is usually at a lower rate
because the body of water does not have metal sides that get hot with the
sun. Most hydrologists suggest multiplying the pan evaporation by 0.75
to correct for lakes.
Evapotranspiration E + T
• The pan actually measures evaporation, not evapotranspiration.
• You have to multiply the pan evaporation by a
different pan coefficient to estimate the E+T over
plants http://www.eijkelkamp.com/Portals/2/Eijkelkamp/Fil
es/Manuals/M4-1689e%20Evaporation%20pan.pdf
pasture
coastal plain
golf course
pasture
Notice some vegetation has high transpiration, pan coefficient. > 1
Estimates of Pan Coefficient
The K pan is high if: the pan is placed in a fallow (plowed but unseeded) area, the humidity is
high (i.e. humid), the wind speed is low
The K pan is low if: the pan is placed in a cropped area, the humidity is low (i.e. dry), the wind
speed is high
If the pan factor is not known the average value .7 - .75 could be used. If more accuracy is
required, the pan factors can be taken from the table above( for class A pan only).
An Example
•
•
•
•
•
•
•
•
Type of pan: Class A evaporation pan
Water depth in pan on day 1 = 150 mm
Rainfall (during 24 hours) = 1 mm
Water depth in pan on day 2 = 144 mm (after
24 hours)
Formula: E = K pan × E pan
K pan = 0.75,
E pan = start + rain - finish
Calculation: E pan = (150 + 1 – 144) = 7 mm/day
E = 0.75 × 7 =5.2 mm/day
Estimating evaporation with mass transfer
• In this model, water is transferred from the
water surface to the air because of the
difference in vapor pressure between the
water surface and air. Once there, the wind
“sweeps away” the newly arrived moist air
and brings in new, dry air, and the cycle
restarts.
The model part 1
Suppose water is covered by dry air. At the surface water
evaporates and an equal amount of water vapor
condenses to liquid water. Air is saturated near the surface.
The addition of water vapor to the air near the surface
makes it less dense: that parcel is more buoyant
The model part 2
The parcel lifts. The vapor pressure difference between the
surface, e ~ esat , and dry air 2 meters above the surface
where e is low, is a pressure gradient, the moist air also
diffuses (spreads out) toward the low pressure altitude
The model part 3
When the expanding parcel arrives at the 2 meter elevation, it
has the lower vapor pressure e. We know how to calculate
this from RH and esat
The model part 4
Then the wind sweeps away the moisture
The model part 1
And the process restarts
The Harbeck and Meyers Equation
•
Harbeck and Meyers developed this idea. They set the
reference heights for Tair and RH and wind and e to be 2m
above the surface, and used one tuning constant, b, derived
for individual areas. The result is:
Tuning constants b are all around 0.012 cm.sec/(m.mb.day) for lakes
Estimating Evaporation (continued)
Remember that you can use tables, such as
Appendix C, to get the saturation partial
pressure esat for water at a particular
temperature. You can get the Relative
Humidity at the 2 meter level above the
surface from a sling psychrometer, and
then calculate
e2m = (es x RH)/100
The Penman-Kohler Nomograph
• A second approach is the Penman Equation. It
combines mass transfer and energy budget
methods, but deliberately avoids energy
budget terms that are difficult to measure.
Because of its complexity, most of the time we
Kohler’s graphic Nomograph of the Penman
equation to calculate evaporation.
Use this to estimate
shallow lake
evaporation.
For vegetation, use
the tables included in
this lecture and your
book to estimate E+T
Upper left T air = 70F and daily radiation 650 ly /day langleys /day, 1 langley = 1
calorie/cm2
Upper right, the dew point Td = 50oF and Tair = 70oF
Lower right, Td = 50F and avg wind speed = 40 miles/day measured 6 inches above pan
rim
Lower left, intersection of vertical from upper left and horizontal from lower right
Estimates daily pan evaporation in hundreds of inches = 0.22 inches.
Examples
• As usual I’ll do a couple of examples, and then
you’ll do similar problems for homework
• The last slide is a full page Penman-Kohler
Nomograph if you need it for your project.