Hydrologic Equation

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Hydrologic Equation

• Inflow outflow /- Changes in storage
• Equation is simple statement of mass conservation

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Hydrologic inputs into area

• Precipitation
• Surface water inflow (streamflow overland flow)
• Ground water inflow from outside area
• Artificial import (pipes canals)

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Hydrologic outputs into area

• Evapotranspiration from land areas
• Evaporation from surface water
• Runoff of surface water
• Groundwater outflow
• Artificial export of water through pipes and
  canals

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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

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Mono Lake

• Inputs
• precipitation streams ground water.
• Outputs
• evaporation artificial streams.

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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.

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Condensation

• Condensation occurs when air mass can no longer
  hold all of its humidity.
• Temperature drops gt saturation humidity drops.
• If absolute humidity remains constant gt relative
  humidity rises.
• Relative humidity reaches 100 gt condensation gt
  Dew point temperature.

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Factors affecting Evaporation

• 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 gt
  joule/m2 4.18 x 104 Langleys.

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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 gt Osmotic pressure.

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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.

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Limited soil-moisture storage
Cool, moist
Cool, moist
Warm, dry
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Ample soil-moisture storage
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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.

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Precipitation cont.

• Adiabatic expansion
• P decreases gt V increases gtT decreases
• Dry lapse rate Rising dry air 1OC/100m.
• Wet lapse rate Rising moist air 0.5OC/100m.

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Influences on rising air mass

• Convective processes.
• Movement of weather fronts.
• Orographic effects.

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Effective uniform depth (EUD) of precipitation

• Arithmetic mean method the rain gauge network
  is of uniform density.
• EUD an arithmetic average of the
  point-rainfall data.

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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.

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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

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