The hydrologic cycle Jean-Marc Mayotte

Earth is neat 70% of Earth is covered by water – Probably arrived during the very early stages of Earth as it was being bombarded by comets and asteroids Earth is unique in that almost all of its water is allowed to remain in its liquid form – Life is supported by liquid water The movement of water around the globe is what sustains life

Where is the water (globally)? Berner and Berner, Global Environment: Water, Air and Geochemical Cycles, Second Edition, Princeton University Press, 2012, table 1.1 Total: ⁶ km³

What does that look like? All the water in the world ( billion cubic kilometers of it) including sea water, ice, lakes, rivers, ground water, clouds, etc. Right: All the air in the atmosphere (5140 trillion tonnes of it) gathered into a ball at sea-level density. Shown on the same scale as the Earth.

Oceans 97% is seawater Total: 1400 x 10⁶ km³

Ice Total:28,39 10⁶ km³ 1,97 % of global water 65% of freshwater Ice Shelves (floating): 2,47% Ice caps and glaciers: 0,32% Ice Sheets: 97,22% Antarctica accounts for 90% of all ice

Groundwater Total: 15,95 10⁶ km³ 1,06% of global water 35% of freshwater

Atmosphere Total: 0, ⁶ km³ 0,0011% of global water (but it is very important) Terrestrial: 0, ⁶ km³ Oceanic: 0, ⁶ km³

Surface freshwater Total: 0, ⁶ km³ 0,0088% of global water 0,2886% of freshwater

Fluxes of water Ground surface Precipitation P Evaporation Transpiration Run-off R E Precipitation Evapotranspiration Runoff Δ (Change in) Storage P = E + R + ΔS

Global Water cycle Atmosphere 13 (0.0009%) (9d) Ocean 1.37x10⁴ (97.61%) (37000y) Soil moisture 67 (0.005%) (280d) Groundwater (active) 4000 (0.29%) (300y) Polar ice, glaciers (2.08%) (16 000y) Freshwater lakes 125 (0.009%) (1-100y) Salt lakes 104 (0.008%) ( y) Rivers 1.2 ( %) (12-20d) /y /y /y /y /y Evaporation Evapotranspiration precipitation Evaporation precipitation River runoff Reservoir Pool size [10³km³], pool size [%], turnover time Schlesinger, 1993; Murray, 1992

How does water move between reservoirs? Evaporation Precipitation Runoff Groundwater flow Two most important mechanisms. Driven by the sun’s energy

Evaporation Water changes phases (liquid to gas) – Requires energy (from the sun) Difference in vapour pressure provides gradient (move from high to low pressure) – Depends on the surface’s temperature and the relative humidity of the air Wind can have a profound effect by constantly moving moist air and replacing with dry air thus increasing the difference in vapor pressure

Potential evaporation (PET) The ammount of evaporation that would occur if there were no limits on water supply (demand-side calculation) – Actual evaporation is the net of the atmospheric ”demand” for water and the ability of a water surface to supply the water Many methods available to estimate PET but they are empirically derived and can result in a lot of error – Thorthwaite Eqn. – Penman Eqn. – Penman-Monteith Eqn.

Global Evaporation

Annual mean evaporation in Sweden Difficult to measure. Can be estimated as a residual term in the water balance: E = P – R –  S Precipitation Evapotranspiration Runoff Δ (Change in) Storage

Transpiration The loss of water vapor from plants Clouds forming over the amazon as a result of transpiration

Precipitation Result of saturation and condensation of atmospheric water Saturation can occur either through cooling or by addition of more water Three types of precipitation: – Liquid (drizzle, rain) – Freezing (sleet) – Frozen (snow, graupel, hail) Liquid freshwater deposits are a result of precipitation

Causes of precipitation Frontal Two bodies of air meet (warm and wet meets cold and dry) Convective Strong updrafts fueled by intense evaporation force large bodies of warm, moist air high into the atmosphere where it rapidly cools and condenses Orographic Air is forced up by the landscape, cools and precipitates

Global precipitation

Annual mean precipitation in Sweden Notice: more precipitation at the higher elevations

Snowfall Note: More snow at the higher elevations as well Note: Snowfall increases from south to north

Precipitation - Evaporation Available surface water is determined by the difference in evaporation and precipitation rates S. Scandinavia S.Europe E. Africa Precipitation (mm/year) Pot. evapotranspiration (mm/year) Start After one year After two years 2m (Falkenmark)

Atmospheric circulation (General model)

Atmospheric circulation

Runoff Occurs when the ground is saturated and water pools on the surface. Water-saturated soil Unsaturated soil Pooling

Interception I = P – crown drip – trunk flow – penetrating precipitation P Penetrating precipitation Interception (I) Trunk flow Crown drip

Groundwater Porosity Total volume Pore volume Water content Porosity = 0.30 Water content = 0.05 Water-saturated soil Unsaturated soil Porosity = 0.30 Water content = 0.30

Groundwater Discharge Recharge area Discharge area Regional groundwater flow Recharge area Three near lying wells sample groundwater that originally comes from widely separated locations Location of landfill?

Groundwater in Sweden

Surface water discharge Soil surface Precipitation P Evaporation Transpiration Groundwater Storage  S (Rising/lowering of groundwater level) Surface run-off E Run-off R Groundwater discharge Infiltration Percolation Reservoirs, lakes, rivers... Usable freshwater

Practical Applications