Earth's natural cycles Water / Hydrological Cycle

Dimitra Konsta IAASARS/ NOA

Athens, 5 April 2015

Plan

1. Water Cycle Water Balance – Water distribution 2. Precipitation 3. Evapotranspiration 4. Infiltration 5. Runoff 6. Hydrological changes 7. Hydro Exercise

1. Water Cycle

Linkages between the 3 major water reservoirs (not to scale!)

l

http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/hyd/bdgt.rxml

Global water balance (volumetric) Units are in volume per year relative to precipitation on land (119,000 km3/yr) which is 100 units Precipitation 100

Atmospheric moisture flow 39

Precipitation 385

Evaporation 424

Evaporation 61 Surface Outflow 38

l l

Land (148.7 km2) (29% of earth area)

l

Subsurface Outflow 1

l

Ocean (361.3 km2) (71% of earth area)

Presence of water on Earth Form of water

Total quantity

Quantity of fresh water

km3

%

km3

%

Oceans

1 338 000 000

96.54

-

-

Ice caps and glaciers

24 364 100

1.758

24 364 100

69.55

Groundw ater

23 416 500

1.690

10 546 500

30.11

Lakes

187 870

0.014

102 470

0.293

Atmosph ere

12 900

0.009

12 900

0.037

Rivers

2 120

0.0002

2 120

0.006

Biospher e

1 120

0.0001

1 120

0.003

Total

1 385 984 610

100

35 029 210

100

Circulation of water on Earth Surface reference

Surfac e 109 km2

Transport

Mean annual volum e, 103 km3

Mean annual height, mm

Mean supply, km3/s

Percentage of precipitation

Total surface of Earth

510.0

Precipitation = Evapotranspirati on

577

1131

18.28

100.0

Oceans

361.1

Precipitation

458

1268

14.51

100.0

Evapotranspirati on

505

1399

16.00

110.3

Precipitation

119

799

3.77

100.0

Evapotranspirati on

72

484

2.28

60.5

Total runoff

47

316

1.49

39.5

Surface component of runoff

44.7

300

1.42

37.6

Growndwater component of runoff

2.3

16

0.07

1.9

Land

148.9

Global water resources

105,000 km3 or 0.0076% of total water

Blue and Green Water Green Water Water that is stored in the soil and is taken up by plants and lost by evapotranspiration Blue Water Water that is found in rivers and lakes as well as groundwater that is used for agriculture, industrial and domestic purposes.

All components formed by H2O are part of the Hydrology cycle

The hydrologic cycle is the system which describes the distribution and movement of water between the earth and its atmosphere. The model involves the continual circulation of water between the oceans, the atmosphere, vegetation and land.

Describing the cycle Solar energy powers the cycle. Heat energy from the sun causes evaporation from water surfaces (rivers, lakes and oceans) and transpiration from plants Evaportranspiration : water loss to the atmsophere from plants and water surfaces. The warm, moist air (containing water vapour) rises and, as it cools, condensation takes place to form clouds. Advection: wind energy may move clouds over surfaces where Precipitation occurs, either as rain or snow depending on altitude The rainwater flows, either over the ground (runoff/ surface flow) into rivers and back to the ocean, or Infiltrates downwards through the soil and rocks where it is returned to the oceans through groundwater flow.

The Water Cycle Balance The hydrologic cycle is a good example of a closed system: the total amount of water is the same, with virtually no water added to or lost from the cycle. Water just moves from one storage type to another. Water evaporating from the oceans is balanced by water being returned through precipitation and surface run off. A water balance can be established for any area of earth's surface by calculating the total precipitation input and the total of various outputs.

Water input and outputs is in balance globally P = R + ET BUT: Water input and output is not always in balance locally l P ≠ R + ET

Water Balance Equation P = Q + E + dS/dt l l l l

P : precipitation [mm/a] Q : Discharge [mm/a] E : Evaporation [mm/a] dS/dt : Storage changes per time step [mm/a]

Characteristic hydrological spatial scales Watershed/ catchment/ drainage basin A logical unit of focus for studying the movement of water within the hydrological cycle . Determined by : - topography - shape - size - soil type - land use

Water Districts of Greece Division of Greece into 14 River Basin Districts with as far as possible similar hydrological-hydrogeological conditions. 5 of them are cross-border with Albania, FYROM and Bulgaria to the north and Turkey to the east. They consist of 45 catchments. Obligation to prepare Management Plans of the 14 Water Districts

2. Precipitation

Precipitation Definition: fall of moisture from atmosphere in any form. It as any product of the condensation of water vapours that fall under gravity. Factors determining precipitation: - Climate - Geography - Ocean surfaces - Source of moisture

Forms of precipitation: - Liquid Precipitation : Drizzle, Rainfall - Frozen Precipitation: Snow, Hail, Sleet, Snowflakes

Mechanism of Precipitation 1. Creation of thermodynamic state of water vapor saturation. Almost always is a result of the swelling and cooling of of the air enriched in humidity during his rising movements. The upwelling movements are favored in unstable atmospheric conditions., i.e. conditions of sharp reduction of the temperature with altitude.

Mechanism of Precipitation 2. Condensation into fine droplets of indicative average diameter 10 to 30 μm or small crystals (depending on the ambient temperature). In order to be accomplished it is required the process of nucleation. nucleation Nucleation process: The effect of specific particles (nuclei) within the condensation of water vapor in clouds. Likefaction requires the creation of interface between liquid and gas phase, and thus the existence of hygroscopic nuclei. The process has high energy requirements and is reached more difficult if the core consists of water molecules (homogenous nucleation). On the contrary the process is favored if the core has a different origin such as dust, combustion products, salt crystals (heterogenous nucleation)

Mechanism of Precipitation 3. Great increase of the mass of the drops (or of ice crystals) in precipitation sizes. The increase may be 106 times and to be performed until the gravitational forces of the individual droplet overcome suspension created by turbulent diffusion (processes of adhesion of droplets – growth of ice crystals). crystals 4. Continuous supply of new water vapor, so that the processes of the three previous steps are preserved for some time.

Causes of Precipitation

Convective Precipitation It results from heating the earth's surface that cause air to rise rapidly. As the air rises it cools and moisture condenses into clouds and precipitation.

Cyclonic Precipitation When two air masses due to contrasting temperatures and densities clash with each other, condensation and precipitation occur at the surface contact.

Orographic Precipitation When heavily moisture laden air stops due to topographic conditions (mountains) and precipitation occurs on the windward side of the mountain while leeward side receives very little.

Measurement of Precipitation Amount of precipitation Intensity of precipitation Duration of precipitation Spatial extent of precipitation

Measurement methods Measurement of precipitation (Rain and Snow) can be done by various devices. These measuring devices and techniques are: Rain Gauges (non-recording, recording) Snow Gauges Radars Satellites Scratching of snow packs Water equivalent in snow packs

Average precipitation over an area In order to compute the average rainfall over a basin or catchment area, the rainfall is measured at a number of raingauge stations suitably located in the area. The no. of raingauge stations depends upon the area and distribution of rainfall. If a basin or catchment area contains more than one raingauge station, the computation of average rainfall may be done by the following methods: 1. Arithmetic average method 2. Thiessen polygon method 3. Isohytel method 4. Inverse distance weighting

Arithmetic Mean Method ●

Simplest method for determining areal average

P1 = 10 mm

P1

P2 = 20 mm P3 = 30 mm

1 P N

N

P i 1

P2

i

10  20  30 P  20 mm 3 •

Gages must be uniformly distributed

•

Gage measurements should not vary greatly about the mean

P3

Thiessen polygon method

•

P1

Any point in the watershed receives the same amount of rainfall as that at the nearest gage

•

Rainfall recorded at a gage can be applied to any point at a distance halfway to the next station in any direction

•

Steps in Thiessen polygon method 1. Draw lines joining adjacent gages 2. Draw perpendicular bisectors to the lines created in step 1 3. Extend the lines created in step 2 in both directions to form representative areas for gages 4. Compute representative area for each gage 5. Compute the areal average using the following formula

1 N P   Ai Pi P  12 10  15  20  20  30  20.7 mm A i 1 47

A1 P2 A2 P3 A3

P1 = 10 mm, A1 = 12 Km2 P2 = 20 mm, A2 = 15 Km2 P3 = 30 mm, A3 = 20 km2

Isohyetal method

●

Steps – Construct isohyets (rainfall contours) – Compute area between each pair of adjacent isohyets (Ai) – Compute average precipitation for each pair of adjacent isohyets (pi) – Compute areal average using the following formula

1M N PP   P Ai pA i i i A i 1 i 1 P

5  5  18 15  12  25  12  35  21.6 mm 47

10 20

P1

A1=5 , p1 = 5

A2=18 , p2 =

P2

15 A3=12 , p3 = 25

30

P3

A4=12 , p3 = 35

Inverse distance weighting

• • •

Prediction at a point is more influenced by nearby measurements than that by distant measurements The prediction at an ungaged point is inversely proportional to the distance to the measurement points Steps – Compute distance (di) from ungaged point to all measurement points.

d12 

 x1  x2  2   y1  y2  2

– Compute the precipitation at the ungaged point using the following formula  Pi   2    i 1  d i  N

Pˆ 

 1   2  i 1  d i  N

10 20 30  2 2 2 Pˆ  25 15 10  25.24 mm 1 1 1   25 2 152 10 2

P1=1 0 P2= 20

d1=25 d2=15

p

P3=3 0 d3=10

Radar and satellite measurements A weather radar is a type of radar used to locate precipitation, calculate its motion, estimate its type (rain, snow, hail, etc.). and forecast its future position and intensity. Weather radars are mostly Doppler radars, capable of detecting the motion of rain droplets in addition to intensity of the precipitation. Both types of data can be analyzed to determine the structure of storms and their potential to cause severe weather.

Radar detecting the cloud by collecting reflected microwaves

Satellite observe earth in microwave or infrared channels from space and estimate precipitation using retrieval techniques or carry radar (CloudSat)

3. Evapotranspiration

Evapotranspiration EVAPORATION Definition: Process by which water is changed from the liquid or solid state into the gaseous state through the transfer of heat energy (ASCE, 1949) It occurs when some water molecules attain sufficient kinetic energy to break through the water surface and escape into the atmosphere (~600 cal needed to evaporate 1 gram of water) Depends on the supply of heat energy and the vapor pressure gradient (which in turn depends on water and air temperatures, wind, atmospheric pressure, solar radiation, etc). TRANSPIRATION Transpiration is the evaporation occurring through plant leaves (stomatal openings) Transpiration is affected by plant physiology and environmental factors, such as - Type of vegetation - Stage and growth of plants - Soil conditions (type and moisture) - Climate and weather

Evapotranspiration (ET) Combined “loss” of water vapor from within the leaves of plants (“transpiration”) and evaporation of liquid water from water surfaces, base soil and vegetative surfaces. Globally, about 62% of the precipitation that falls on the continents is evapotranspired (~ 72.000 km3/y); 92% of which from land surfaces evapotranspiration and 3% from ocean water evaporation (source: Dingman, “Physical Hydrology”) In practice, the terms E and ET are often used to mean the same thing – the evaporation from the land surface. Therefor, you must use the context to determine what the term evaporation means in a specific case (i.e., is it just from an open water surface or the entire land surface? ).

POTENTIAL EVAPOTRANSPIRATION (PET) is the ET that would occur from a well vegetated surface when moisture supply is not limiting (often calculated as the PE). Actual evaportanspiration (AET ; ET) drops below its potential level as the soil dries.

Methods for estimating Evaporation Water budget methods Energy budget methods Mass transfert techniques (e.g., Meyer, Thornthwaile-Holzman) Combination of energy budget and mass transfert methods

Water budget method ΔS/Δt = (P + Q + Qr +Qs ) - (Q0 + Qd +E)

Characteristics : Simple Difficult to estimate Qd Unreliable, accuracy will increase as Δt increases

Precipitation - P Evaporation- E

Inflow- Q

Surface runoff - Qr

Subsurface runoff - Qs Outflow- Q0

Subsurface seepage losses- Qd

Energy budget method Es= (Ea + Rt ) - (Rr + Ee +Hn + R1) E (mm/day)=10 Ee/Hv Hv=596-0.52T – latent heat of vaporization T (oC) – temperature of the water surface Total solar radiation - Rt Net energy advected (net energy content of incoming and outcoming water -

Ee

Characteristics Most accurate method (evaporation is a function of the energy state of the water system) Difficult to evaluate all terms Energy balance equation has to be simplified Empirical formulas are used (although radiation measurements are preferable)

Reflected solar radiation - Rr Energy used for evaporation (latent heat)- Ee

Sensible heat loss from the water body to the atmosphere - Hn

Net long-wave radiation exchange between the atmospere and the water body- R1

Energy stored - Es R1 includes long-wave (LW) radiation from the atmosphere, reflected LW radiation, LW radiation emitted by water

Mass transfer methods Definitions es=6.11 exp [17.3T/ (T+237.3)] es [mm Hg] = es [mb] / 1.36 Rh= e/es e - actual vapor pressure (difference in the atmospheric pressure with and without the vapor) es - saturated vapor pressure (partial pressure of water vapor in saturated air) T (oC) – air temperature Rh - relative humidity Evaporation is a diffusive process (moves from where its concentration is larger to where its concentration is smaller at a rate that is proportional to the gradient of concentration): E = bo (eso-ea) eso – vapor pressure of the evaporating surface; saturation vapor pressure at the water surface temperature Ts ea – vapor pressure of overlying air at the same height bo – empirical coefficient that has to be calibrated

Mass transfer methods E = bo (eso-ea) Studies showed that bo = function (air turbulence) = fn(v) E = ba fn(v)(eso-ea) Meyer's formula: E = 14 (1+0.1v30) (es-ea) v30 – monthly mean wind speed [km/h] at 9.14m height; es; ea [in mm] ; E [in/month] bo=f(v,es,ea, Ta, Tw) Thornthwaite-Holzman equation bo=f(v,T,k), k- Von Karman constant (0.41) 2

E=

833 k ( e1 −e 2 )( v 2 −v 1 ) ln

z2

( ) z1

2

( T + 459 . 4 )

Combination approach – Penman-Monteith equation Combination of mass-transfer and energy-balance equations

E '=

Δ Rn γ' + F (u) D Δ +γ ' λ Δ +γ '

[Kg/(m2d)]

where γ'=(1+0.33u)γ [hPa/ºC] – psychrometric constant, u in [m/s], τypical value of γ=0.67 hPa/ºC F(u)=90/(T+275) *u [kg/hPa m2d)] wind function , T in [ºC] and u in [m/s] Δ [ hPa/ºC] -slope of the saturation vapor pressure curve

Δ=

4098 e s

17.27 T T +237.3

e s =6.11 e (T + 237.3)2 Rn = total net irradiance at the earth's surface [kJ/(m2d)] λ = lantent heat of vaporization [kJ/(kg)] λ = 2501 – 2.361*Τ D= vapor pressure deficit [hPa] D = es – e = es – U*es , U=e/esX100 – relative humidity

Combination approach – Penman-Monteith equation Rn = total net irradiance at the earth's surface [kJ/(m2d)] Rn = Sn - Ln Sn – amount of energy absorbed, Ln – net outward flow of longwave radiation Sn = So (1-a) (as+bsn/D) So – total possible radiation for the period of estimation; it is function of latitude and season a – albedo, surface reflection coef. (0.05 – 0.12) as,bs – empirical coef. (as=0.2; bs=0.5) n/D – fraction of possible sunshine (from climatic atlas)

n 4 Ln= σ ( Τ + 273) ( 0.47−0.077 √ e )(0.2+ 0.8 , ) D e – actual vapor pressure T – air temperature n/D – fraction of possible sunshine

σ=1.1777x 10-7

Measuring Evaporation Evaporation Pan Evaporation from an open water surface (E) is usually estimated from the pan evaporation (Ep) as: E = k Ep where k is the pan coefficient (regional coef, usually around ~ 0.7 and always