Hydrological Cycle and Water Hydrological Cycle and Water ResourcesResources

OverviewOverviewIntersection of Atmosphere, Hydrosphere, Intersection of Atmosphere, Hydrosphere, Lithosphere and BiosphereLithosphere and BiosphereHydrologic CycleHydrologic CycleComponentsComponentsWater BudgetWater BudgetThe supply of water available for useThe supply of water available for useGroundwaterGroundwater

aquifersaquifersexamples of aquifers as resourcesexamples of aquifers as resources

Global distribution of water resourcesGlobal distribution of water resourcesPollutionPollution

Atmosphere Biosphere

Hydrosphere Lithosphere

precipitation

precipitation

evapotranspiration

evaporation

runoff

percolation

groundwateraquifersKarst topography

vegetation

soilinfilitration

Root uptake for photo-synthesis

transpiration

evaporation

watersheds (drainage basins)river systems

oceans

Hydrologic CycleHydrologic CycleDescribes the way that water passes Describes the way that water passes between hydrobetween hydro--, , atmoatmo--, litho, litho-- , biospheres, biospheresIn generalIn general

describes the balance between water leaving describes the balance between water leaving the atmosphere (the atmosphere (precipitationprecipitation) and water re) and water re--entering the atmosphere (entering the atmosphere (evaporationevaporation, and , and transpirationtranspiration))

More specificallyMore specificallydetails the processes that occur in between the details the processes that occur in between the general processes described above (general processes described above (infiltrationinfiltration, , percolationpercolation, , runoffrunoff, , photosynthesisphotosynthesis))

InfiltrationInfiltrationWater access to subsurface regions of soil moisture Water access to subsurface regions of soil moisture storage through penetration of the soil surfacestorage through penetration of the soil surfaceOccurs at a constant rate, measured the same way as Occurs at a constant rate, measured the same way as precipitationprecipitation

e.g., inches per hour, millimeters per houre.g., inches per hour, millimeters per hour

If precipitation rate exceeds the infiltration rate, runoff If precipitation rate exceeds the infiltration rate, runoff will occur.will occur.

CapillarityCapillarityForces that cause water to move upward through the Forces that cause water to move upward through the soilsoil

Plant root uptake (transpiration is essentially capillary actionPlant root uptake (transpiration is essentially capillary actionthrough the plant, from root to leaf)through the plant, from root to leaf)Evaporation of water at the soilEvaporation of water at the soil’’s surfaces surfaceCommon in arid regionsCommon in arid regions

Runoff occurs when individual soil particles Runoff occurs when individual soil particles can no longer hold onto infiltrated watercan no longer hold onto infiltrated water

Dynamic tension between attractive forces Dynamic tension between attractive forces (holding soil and water particles together) and (holding soil and water particles together) and gravity (pulling water away from soil).gravity (pulling water away from soil).

Affected by the size of soil particles: smaller particles Affected by the size of soil particles: smaller particles have a greater summed surface area, and can hold onto have a greater summed surface area, and can hold onto water more stronglywater more strongly

Sand, silt, claySand, silt, clay

Hygroscopic waterHygroscopic water (or unavailable water)(or unavailable water)held tightly by soil, not available to plants or capillary held tightly by soil, not available to plants or capillary evaporation (attractive forces much greater than gravity)evaporation (attractive forces much greater than gravity)

Capillary waterCapillary water (or available water)(or available water)soil holds water tightly enough to prevent runoff, but not soil holds water tightly enough to prevent runoff, but not capillarity (attractive forces balanced with gravity)capillarity (attractive forces balanced with gravity)SaturationSaturation

Gravitational waterGravitational water (runoff)(runoff)soil cannot hold onto water (gravity exceeds attractive soil cannot hold onto water (gravity exceeds attractive forces)forces)

The ability of soil The ability of soil to retain moisture to retain moisture is a direct is a direct consequence of consequence of hydrogen bonding hydrogen bonding between water and between water and soil particles soil particles ((colloidscolloids))

Texture of colloids Texture of colloids affects retention of affects retention of water as wellwater as well

GravelGravelSand Sand SiltSiltClayClay

Finer particles Finer particles retain more waterretain more water

2mm2mm50 50 μμmm2 2 μμmm

Root exerts Root exerts additional additional capillary forcecapillary force

ColloidColloid

Hygroscopic Hygroscopic water water –– held held tightly by tightly by the colloidthe colloid

Capillary Capillary water water –– held held by the colloid, by the colloid, but available but available to plantsto plants

RootRoot

Gravitational water Gravitational water –– not held by soil, not held by soil, available to plants or available to plants or runoff and runoff and percolationpercolation

Finer particles create smaller pores, so there is less gravitatiFiner particles create smaller pores, so there is less gravitational onal water, hence better moisture retentionwater, hence better moisture retention

Finer particles have more surface area per volume, which allows Finer particles have more surface area per volume, which allows greater opportunity for water to attach, and causes greater amougreater opportunity for water to attach, and causes greater amounts nts of hygroscopic water as well as capillary water. As particle siof hygroscopic water as well as capillary water. As particle size ze decreases, capillary water increases initially, then decreases adecreases, capillary water increases initially, then decreases as s hygroscopic water increaseshygroscopic water increases

44

44

Surface area = 16Surface area = 16

22

22

88 88

88 88

11

11

44 44 44 44

44 44 44 44

44 44 44 44

44 44 44 44

Surface area = 32Surface area = 32 Surface area = 64Surface area = 64

4 + 4 + 4 + 4 4 + 4 + 4 + 4 = 16= 16

Field CapacityField CapacityThis refers to the maximum amount of This refers to the maximum amount of capillary/available water a specific soil type can capillary/available water a specific soil type can hold.hold.Adding more water to the soil, after field capacity Adding more water to the soil, after field capacity is reached, results in the build up of gravitational is reached, results in the build up of gravitational water, and runoff and percolation occurwater, and runoff and percolation occur

Wilting PointWilting PointThe amount of hygroscopic water that a specific The amount of hygroscopic water that a specific soil type can holdsoil type can holdWhen all available water is gone and only When all available water is gone and only hygroscopic water remains, there is no water hygroscopic water remains, there is no water available to plants (hence the name)available to plants (hence the name)

Varies with soil particle size mixtureVaries with soil particle size mixturesmaller particles hold more watersmaller particles hold more waterclays can hold water too strongly (less available)clays can hold water too strongly (less available)Loams tend to have highest field capacityLoams tend to have highest field capacity

% Soil% SoilMoistureMoisture

% Fine Particles% Fine Particles % Organic matter% Organic matter

PlantPlant--Water InteractionWater Interaction

Plant roots exert an attractive force on Plant roots exert an attractive force on available soil water (Capillary Force)available soil water (Capillary Force)Photosynthesis (absorbs solar energy)Photosynthesis (absorbs solar energy)6CO6CO22 + 6H+ 6H22 OO CC66 HH1212 OO66 + 6O+ 6O22

Respiration (releases solar energy)Respiration (releases solar energy)CC66 HH1212 OO66 + 6O+ 6O22 6CO6CO22 + 6H+ 6H22 OOStoring energy as sugar or starch also stores Storing energy as sugar or starch also stores water; plant growth and life processes water; plant growth and life processes release waterrelease waterTranspiration: respiratory water released as Transpiration: respiratory water released as water vaporwater vapor

SoilSoil--Water BudgetWater BudgetA mathematical way of expressing the A mathematical way of expressing the difference between input (precipitation) of difference between input (precipitation) of water into an area against its output water into an area against its output ((evapotranspirationevapotranspiration and runoff).and runoff).Conceptually very similar to hydrological cycleConceptually very similar to hydrological cycle

PRECIP = ACTET + SURPL + PRECIP = ACTET + SURPL + ΔΔSTRGSTRGwhere ACTET = POTET where ACTET = POTET –– DEFICDEFIC

Precipitation = Precipitation = EvapotranspirationEvapotranspiration + + gravitational water + available watergravitational water + available water

Varies over space and timeVaries over space and time

VariablesVariablesPRECIP: PrecipitationPRECIP: PrecipitationACTET: Actual ACTET: Actual evapotranspirationevapotranspiration. Occurs when . Occurs when plants do not have sufficient water to reach their plants do not have sufficient water to reach their maximum metabolic potentialmaximum metabolic potentialPOTET: Potential POTET: Potential evapotranspirationevapotranspiration. Represents . Represents the maximum metabolic potential of plants, and the maximum metabolic potential of plants, and occurs when there is sufficient wateroccurs when there is sufficient waterDEFIC: The amount of water short of achieving DEFIC: The amount of water short of achieving POTETPOTETSURPL: Surplus water, corresponds to SURPL: Surplus water, corresponds to gravitational watergravitational waterΔΔSTRG: Change in storage. Corresponds to STRG: Change in storage. Corresponds to capillary water (available water). capillary water (available water).

Negative if plants are utilizing the available waterNegative if plants are utilizing the available waterPositive if available water is being recharged to field Positive if available water is being recharged to field capacitycapacity

ScenariosScenariosPRECIP > POTETPRECIP > POTET

POTET = ACTETPOTET = ACTETDEFICIT = 0DEFICIT = 0SURPL + SURPL + ΔΔSTRG > 0STRG > 0

Fall: Fall: ΔΔSTRG > 0, SURPL = 0; Available water increases STRG > 0, SURPL = 0; Available water increases until Field Capacity is achieved (Soil Moisture Recharge)until Field Capacity is achieved (Soil Moisture Recharge)Spring: SURPL > 0, Spring: SURPL > 0, ΔΔSTRG = 0 (after Field Capacity is STRG = 0 (after Field Capacity is reached), gravitational water; runoff and percolation occurreached), gravitational water; runoff and percolation occur

PRECIP < POTETPRECIP < POTETDEFIC > 0DEFIC > 0ACTET = POTET ACTET = POTET –– DEFICDEFICSURPL = 0, no runoff or percolationSURPL = 0, no runoff or percolationΔΔSTRG < 0, available water is utilized until the STRG < 0, available water is utilized until the wilting point is metwilting point is met