Water Cycle

Water Facts

•65-70% of human body is water

•80-95% of fresh weight of non-woody plants is water

•Most organisms are 60-65% water by weight

•Typical adult diet requires about 1 kg of food and 2.5 kg of water

•Under ideal conditions, humans could not survive more than 7-10 days without water

Water Facts (cont.)

•71% of earth’s surface is covered with water

•97% of earth’s water is salty

•3% is freshwater

–70% of Earth’s freshwater is in ice sheets and glaciers

–29 % is in groundwater

–0.55% in lakes and reservoirs

–0.004% in river waters

–0.05% in atmosphere

–0.3% in soil

Water distribution

Assume that all water on Earth is represented by 100 liters (26 gallons):

–less than one gallon (3 L) would represent all freshwater.

–less than one teaspoonful would be readily available freshwater (1 ml).

–Large portions of readily available freshwater are polluted.

Water usage

• Groundwater makes up about 40% of the water we use (nationwide).

• NC about 50% of water usage is from groundwater sources

• Surface water use requires expensive filtration (not as potable as groundwater and requires expensive treatment) - often times groundwater is the more cost-effective source of water and less easily polluted.

The hydrologic cycle is powered by _______.

energy from the sun and gravity.

–Solar energy evaporates water and forms water vapor.

–Winds and air movement transport the water vapor.

–Changing temperatures cause condensation and precipitation.

Water cycle components

•Precipitation

•Interception

•Evapotranspiration

•Infiltration

•Groundwater

•Runoff

Precipitation occurs when _______.

3 conditions are met

1) Atmosphere becomes saturated

2) Small particles are present in atmosphere (dust, ocean salt, etc) upon which condensation can take place

3) Water or ice particles grow large enough to reach earth against updrafts

Precipitation factors

• Atmospheric saturation results when air mass is cooled until saturation vapor pressure is reached or moisture is added to air mass

• Most of the time, precipitation occurs when an air mass is lifted, becomes cooled, and reaches saturation vapor pressure

Frontal systems

two air masses of different temperature and moisture come together, air is lifted at the frontal surface

ex: warm and cold front

Warm Front

warm air rides up and over a cold air mass- widespread, gentle rainfall that rains steadily

Cold Front

cold air mass replacing and lifting a warm air mass- high intensity, more localized, short duration rainfall; generates strong intense storms

Orographic

when general circulation forces an air mass up and over a mountain range- air mass reaches saturation vapor pressure, resulting in more precipitation on windward side- rain shadow on other side

Convective

excessive heating of earth’s surface and layer of moist air adjacent to surface- Air mass rises, condensation occurs, latent heat of vaporization adds more energy, more lifting- rapid uplift results in frozen water droplets, hail, and severe thunderstorms  

• common near equator

US precipitation

• Rainfall distribution is uneven

  East of Mississippi 40-45”/yr

  Pacific Northwest 80”/yr

  Midwest much lower 10-20”/yr

3 common hurricane paths in the Atlantic

Drought

a period of abnormally dry weather sufficiently prolonged for the lack of water to cause serious hydrologic imbalance in the affected area

• 3 types: meteorological, agricultural, and hydrologic

• You can have one type of drought but not the other

• Ex: You get a lot of snow for a couple of months, but then no rain → meteorological (not hydrologic because the snow melted and became groundwater)

Meteorological

based on precipitation

Agricultural

based on soil moisture

• is the topsoil wet enough to grow crops?

Hydrologic

based on streamflow, groundwater

NC Drought maps

Moderate- Beige

Severe- orange

Extreme- Red

Exceptional- Brown

Tools to measure/estimate precipitation

•Weather Stations - tell you everything on climate

•Rain gauges

–Manual

–Automated - don’t need to manually read → for research purposes

•Satellite and radar

•State Climate Office - historical and long-term data

•NOAA Weather Stations

•Volunteer reporters - report how much rain they received → helps calibrate radars and satellites

Interception

prevention of precipitation from reaching soil surface

Types of interception

Ic = canopy interception loss

Pg = gross precipitation → total amount of rainfall measured above the canopy, usually by a standard rain gauge placed in an open area; full precipitation input to an ecosystem before any interception or partitioning occurs

Th = throughfall → portion of incident precipitation, such as rainfall or snowfall, that reaches the ground surface beneath a vegetated canopy by passing directly through gaps in the foliage or by dripping and splashing from intercepted water on leaves, branches, and stems

Sf = stemflow → captured by the above-ground structure of vegetation, such as tree stems and branches, and subsequently channeled down along the stem to the base of the plant. Transports water, solutes, and sometimes nutrients directly to the soil around the root zone.

Pn = net precipitation → actual rainfall that reaches the soil surface

Ii = litter interception → hits debris on forest floor

Controls on Interception and Throughfall

•Storm size and frequency- small and infrequent storms may have higher interception rates. Larger, more intense storms less interception.

•Hardwood vs conifer- variable, but mature conifers typically have higher interception rates than mature hardwoods

•Growing vs dormant season- growing season more leaf area

•Snow vs rain- typically more interception in rainstorms relative to snowstorms

•Stand density- more trees, more potential for interception

•Energy availability- influences evaporation

•Location under canopy- sheltered, drip line, etc.,

Evaporation

–Net loss of water from a surface resulting from a change in state of water from liquid to vapor

Transpiration

–Net loss of water from plant leaves by evaporation through leaf stomata.

Evapotranspiration

Evaporation + transpiration

The Evapotranspiration Process

•Requires:

1.A flow of energy to the evaporating or transpiring surface

2.A flow of liquid water to these surfaces

3.A flow of vapor away from these surfaces

Effects of vegetative cover on evapotranspiration

Forested soil → most evapotranspiration

bare soil → least evapotranspiration

Potential evapotranspiration estimates

•Thornthwaite method (requires air temperature data)

•Hamon method (requires air temperature data)

•Penman method (requires radiation, vapor pressure, wind speed data)

–Most physically based but requires expensive data.

Components of the Water Balance

P = ET + Q + DS + Dl

where: P = precipitation  ET = evapotranspiration 

Q = runoff (River discharge)

DS = change in storage

Dl = change in seepage

Simple water budget

•(no storage/seepage considered)

•Suitable when conducted over a period when changes in storage would be small

•Typically conducted from October-October (1 water year)

•ET for a watershed can be estimated from P-Q = ET

Infiltration

–The process by which water enters the soil surface

Infiltration rate

–Rate of water passing into the soil (inch/hr, ft/day, cm/hr, etc)

• what it is actively doing right now

Infiltration capacity

– Maximum rate of water infiltration for given soil conditions

Infiltration rate depends on _______.

soil properties and the soil surface conditions

• Soil surface conditions:

  –Plant material or litter Protects soil from raindrop impact (compacts soil); Stores water, retards flow

• •Overall, conditions that reduce vegetative cover and compact the soil surface cause infiltration to diminish

    (Causes runoff to increase)

Controls on infiltration

•Soil water content- soils already saturated will be more likely to produce runoff & less infiltration

•Soil porosity- soils with more connected void spaces can transmit water more readily than soils with less voids

•Soil biological activity- worm holes, animal burrows can increase porosity and infiltration

•Soil litter layer- protects soil surface from crusting/compaction and increases infiltration

•Soil frost- can reduce infiltration

•Clay type- some clays expand when wet, close pores and reduce infiltration

•Quality of infiltrating water- “clean” water will infiltrate quicker than water with sediment, organics

Land use affects water cycle dynamics →infiltration impact on surface runoff

• surface runoff rate = how much h2o is moving across

• Because rainfall rate is greater than infiltration capacity → surface runoff

Forested soils have the _______.

• Greatest infiltration rates

• Organic litter at surface

• Root cavities

• Burrowing animals

• Earthworms

• Overland flow and flooding is least likely

Measuring Infiltration Rates 

• Double Ring Infiltrometer = Two metal cylinders (usually 12-inch inner and 24-inch outer rings) are driven into the ground. Both rings are filled with water. Water level is kept steady using a constant head setup or measured with a dip float. The water drop speed in the inner ring shows the soil infiltration rate.

• Constant Head Permeameter = used for deeper depths → tubes fill hole with water → as water seeps out of bore hole into the soil → water drop is observed in ruler tube

Even in forested areas clay can ______.

reduce infiltration rates

• sandy soils and high vegetation cover → greater infiltration rates

Recharge to groundwater systems

• Varies seasonally 

  • More common during periods when plants are dormant and rainfall is high

  • during the winter, water is more likely to reach the water table → restores groundwater supply

  • groundwater levels are lowest in the summer → more evaporation & transpiration

• Varies spatially

  • Usually occurs in uplands

  • Also depends on vegetation water uptake

Water table

• Varies seasonally  and annually

• Is drawn down by pumping of wells

• Where it intersects the land surface wetlands, streams, and springs occur

• Often may have multiple water levels below the land surface due to aquifers and aquitards and different pressure 

Confining bed

largely impermeable layers that hinder water’s movement (ex. clay)

• water doesn’t move quickly → low hydrologic conductivity

Aquifers

permeable rock or sediments that transmit water easily (ex. sand or gravel)

• high hydrologic conductivity

Runoff generation

When rainfall rate > infiltration capacity

Surface runoff or ponding occurs (overland flow)

The water cycle is much different in _______.

urban settings as compared to natural settings.

• In urban settings, there is much more impervious surfaces like parking lots, rooftops, and roads. 

• more hard surfaces → more runoff → less infiltration

Urban water cycle

• natural = takes a while for creek/stream water to rise → runs down vegetation and infiltration → moves through the groundwater system → gradual

• parking lot →funnels waters at the same time to creek →large spikes & then low → flash floods