Hydrologic Equation
• Inflow = outflow +/- Changes in storage
• Equation is simple statement of mass
conservation
Hydrologic inputs into area
• Precipitation
• Surface water inflow (streamflow +
overland flow)
• Ground water inflow from outside area
• Artificial import (pipes + canals)
Hydrologic outputs into area
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•
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Evapotranspiration from land areas
Evaporation from surface water
Runoff of surface water
Groundwater outflow
Artificial export of water through pipes and
canals
Changes in storage
• Changes in volume of:
-- surface water in streams, rivers, lakes, and ponds.
-- soil moisture in vadose zone
-- ice and snow at surface
-- temperature depression storage
-- water on plant surfaces
-- ground water below water table
Mono Lake
• Inputs:
precipitation; streams; ground water.
• Outputs:
evaporation; artificial streams.
Humidity
• Absolute Humidity
number of grams of water per cubic meter
[ML-3]
• Saturation Humidity
maximum amount of water air can hold
[ML-3]
• Relative Humidity = % ratio of absolute
humidity to saturation humidity.
Condensation
• Condensation occurs when air mass can no
longer hold all of its humidity.
• Temperature drops => saturation humidity
drops.
• If absolute humidity remains constant =>
relative humidity rises.
• Relative humidity reaches 100% =>
condensation => Dew point temperature.
Factors affecting Evaporation
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•
•
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Water temperature.
Air temperature above water layer.
Absolute humidity of air above water surface.
Wind – keeps absolute humidity low.
- may increase the molecular diffusion.
• Solar radiation – Langley = 1 cal./cm2; SI =>
joule/m2 = 4.18 x 104 Langleys.
Transpiration
• Plants pump water from ground to atmosphere;
accounts for most vapor losses in land-dominated
drainage basin.
• A function of
plant density
plant size
limited by soil water.
• Wilting point – surface tension of soil water
interface > Osmotic pressure.
Evapotranspiration
• Evapotranspiration = total water loss due to
1) free water evaporation, 2) plant
transpiration, 3) soil moisture evaporation.
• Potential evapotranspiration – the water
loss, which occur if at no time there is a
deficiency of water in the soil for the use of
vegetation.
• Actual evapotranspiration.
Limited soil-moisture storage
Cool, moist
Warm, dry
Cool, moist
Ample soil-moisture storage
Formation of Precipitation
• Humid air mass cooled to dew point
temperature.
• Condensation or freezing nuclei (clay
minerals, salt, combustion products …).
• Droplets coelesce to form raindrops.
• Raindrops must be large enough such that
they do not evaporate.
Precipitation – cont.
• Adiabatic expansion
P decreases => V increases =>T decreases
• Dry lapse rate – Rising dry air 1OC/100m.
• Wet lapse rate – Rising moist air
0.5OC/100m.
Influences on rising air mass
• Convective processes.
• Movement of weather fronts.
• Orographic effects.
Effective uniform depth (EUD)
of precipitation
• Arithmetic mean method – the rain gauge
network is of uniform density.
EUD = an arithmetic average of the pointrainfall data.
Effective uniform depth (EUD)
of precipitation
• Arithmetic mean method – the rain gauge
network is of uniform density.
• Isohyetal line method:
- draw isohyets.
- EUD = the weighted average of each
isohyetal area.
Effective uniform depth (EUD)
of precipitation
• Arithmetic mean method – the rain gauge
network is of uniform density.
• Isohyetal line method.
• Thiessen method.
- construct polygons
- weighted by polygon areas