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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). - PowerPoint PPT Presentation
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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
• 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
• 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.
Cool, moist Cool, moistWarm, dry
Limited soil-moisture storage
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 point-rainfall 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