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./cm 2 ; SI => joule/m 2 = 4.18 x 10 4 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 Warm, 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 1 O C/100m. Wet lapse rate – Rising moist air 0.5 O C/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