Hydrologic Equation

1. Hydrologic Equation • Inflow = outflow +/- Changes in storage • Equation is simple statement of mass conservation

2. Hydrologic inputs into area • Precipitation • Surface water inflow (streamflow + overland flow) • Ground water inflow from outside area • Artificial import (pipes + canals)

3. 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

4. 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

5. Mono Lake • Inputs: precipitation; streams; ground water. • Outputs: evaporation; artificial streams.

8. 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.

9. 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.

11. 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.

13. 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.

14. 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.

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

16. Ample soil-moisture storage

17. 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.

18. 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.

19. Influences on rising air mass • Convective processes. • Movement of weather fronts. • Orographic effects.

26. 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.

28. 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.

30. 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