Surface Water BalanceSurface Water Balance

Review of last lecture: Surface energy balanceReview of last lecture: Surface energy balance

dT/dt

SWdn =Scos

SWup =SWdn

LWdn =Tair4

LWup=Ts4

LH=CdLV(qsurface- qair)

SH=CdCpV(Tsurface- Tair)

Fc = - dT/dz

Incoming shortwave + Incoming longwave = Reflected shortwave + Emitted longwave + Latent heat flux + Sensible heat flux + Subsurface conduction

• What is sensible heat flux? What is latent heat flux?

• Bowen ratio B= SH/LH = Cp(Tsurface - Tair) / L(qsurface - qair) provides a simple way for estimating SH and LH when the net radiative flux Fr is available LH=Fr/(B+1), SH=Fr B/(B+1)

• Factors affecting soil thermal conductivity

• Other heat sources: precipitation, biochemical, anthropogenic

The global water cycleThe global water cycle

Video: Weather WetVideo: Weather Wet

Components of global water cycleComponents of global water cycle

• Atmosphere (water vapor, clouds, precipitation)• Land (soil moisture, rivers, snow, ice sheet and glaciers)• Sea ice• Ocean• Biosphere (including human beings)

Atmosphere: water vaporAtmosphere: water vapor

Atmosphere: PrecipitationAtmosphere: Precipitation

GPCP Annual Mean Precipitation for 1979-2005 (mm/day)

Inter-tropical convergence zone (ITCZ)

Strong rainfall (heating)

Weak rainfall

Land snow/Ice cover provide a reservoir Land snow/Ice cover provide a reservoir

Flow of >1000 rivers on the seven continentsFlow of >1000 rivers on the seven continents

Amazon river Yangtze riverMississippi river

Land: Soil moistureLand: Soil moisture

Flow of ocean currentsFlow of ocean currents

A significant fraction of the human body is A significant fraction of the human body is water (~75%)water (~75%)

About every 16 days nearly 100% of the water in a human body is exchanged.

The remaining: fat, protein, carbonhydrate, other solids

So the water we drink may come from …So the water we drink may come from …

Therefore we need to protect the environment because any pollution we put into the environment may someday come back into our bodies

Surface water balanceSurface water balance

dS/dt

Precipitation (P)

Evaportranspiration (E)

Runoff (Rs)

Irrigation (I)

Infiltration (Rg)

The changing rate of soil moisture S

dS/dt = P - E - Rs - Rg + I

Evaporation from bare soil (Eb)

EvaportranspirationEvaportranspiration• Is equivalent to latent heat flux

• Has four components: E = Eb + Ei + Es + TR

Evaporation from inception storage

(Ei)

Transpiration (TR)

Snow sublimation (Es)

Evaportranpiration: Penman-Monteith equationEvaportranpiration: Penman-Monteith equation

where λ is latent heat of evaporation, Rn is the net radiation, G is the soil heat flux, (es - ea) represents the vapour pressure deficit of the air, a is the mean air density at constant pressure, cp is the specific heat of the air, represents the slope of the saturation vapour pressure temperature relationship, is the psychrometric constant (=66 Pa/K), and rs and ra are the (bulk) surface and aerodynamic resistances.

Soil moistureSoil moisture• Typically expressed as ‘volumetric soil water content’

S = Vwater / Vsoil

• Increases with depth• Complicated to measure

Root zone

Intermediate zone

Ground water

Soil moisure regimesSoil moisure regimes

US Soil moisture mapUS Soil moisture map

Palmer drought severity index (PDSI)Palmer drought severity index (PDSI)

• was developed by Wayne Palmer in the 1960s and uses temperature and rainfall information in a model to determine dryness of soil moisture.

• is most effective in determining long term drought (a matter of several months) and is not as good with short-term forecasts (a matter of weeks).

• It uses a 0 as normal, and drought is shown in terms of minus numbers; for example, minus 2 is moderate drought, minus 3 is severe drought, and minus 4 is extreme drought.

Change of PDSI in the last 100 yearsChange of PDSI in the last 100 years

PSDI for US in January 2015PSDI for US in January 2015

A different index: U.S. Drought MonitorA different index: U.S. Drought Monitor

Video: Crippling Drought in the Video: Crippling Drought in the Golden State: California Soul Golden State: California Soul

• https://www.youtube.com/watch?v=lmUwjk4S3gw

Glacier mass balanceGlacier mass balance

Structure of a glacierStructure of a glacier

• A glacier forms when the accumulation of snow and ice exceeds its overall loss of mass by ablation (sublimation and melting).

• A glacier can be divided into two zones; the zone of accumulation and the zone of ablation. They are separated by the “equilibrium line”.

Accumulation zoneAccumulation zone

• Surface accumulation processes include snow and ice from direct precipitation, avalanches and windblown snow.

• There may be minor inputs from hoar frost (radiation frost).

• The snow and ice is then transferred downslope as the glacier flows.

Ablation zoneAblation zone

• Surface ablation processes include surface melt, surface meltwater runoff, sublimation, avalanching and windblown snow.

• Glaciers on steep slopes may also dry calve, dropping large chunks of ice onto unwary tourists below.

• Other processes of ablation include subaqueous melting, and melting within the ice and at the ice bed

Glacier mass balance for the globeGlacier mass balance for the globe

Glacier mass balance for different regionsGlacier mass balance for different regions

Summary:Summary:Components of global water cycleComponents of global water cycle

• Atmosphere (water vapor, clouds, precipitation)• Land (soil moisture, rivers, snow, ice sheet and glaciers)• Sea ice• Ocean• Biosphere (including human beings)

Surface water balanceSurface water balance

dS/dt

Precipitation (P)

Evaportranspiration (E=Eb+Ei+Es+TR)

Penman-Monteith eq

Runoff (Rs)

Irrigation (I)

Infiltration (Rg Darcy’s law)

The changing rate of soil moisture S

dS/dt = P - E - Rs - Rg + I

(PDSI, desertification)

Works citedWorks cited

• http://ffden-2.phys.uaf.edu/212_spring2011.web.dir/Jocelyn_Simpson/Slide3.htm

• http://www.antarcticglaciers.org/modern-glaciers/introduction-glacier-mass-balance/