Water & Climate
1
Topic 8
Water & Climate
è Water in the Earth
- Water enters the atmosphere through evaporation and transpiration (release of water vapor from plants)
- Water leaves the atmosphere through condensation and precipitation
- This constant circulation of water from hydrosphere to atmosphere and back again is called the Water Cycle or Hydrologic Cycle, and maintains dynamic equilibrium on the Earth.
- Water that falls on the ground as precipitation can:
1. evaporate
2. run downhill or collect on the surface, called runoff
3. sink into the ground, called infiltration
Infiltration
- Infiltration can occur wherever the surface is permeable, that is, wherever there are spaces (pores) that water can enter.
- If all the pores are filled, the soil is saturated and excess water becomes runoff.
- Infiltration also depends on the slope of the land -- if the land is steep, water will run off due to gravity before infiltration can occur
- The rate of infiltration (how fast water sinks in) is determined by the porosity and permeability of the soil.
- Porosity refers to the amount of space (pores) into which water can enter
- Permeability refers to the ease with which water can enter and flow through these spaces
Porosity
All soils, and even rocks, have pores
Porosity is the percentage of open space in a sample compared with its total volume.
Characteristics that determine porosity:
1. Particle shape
Rounded particles (like sand) have greater porosity than flat particles (like clay)
2. Packing
Loosely packed, freshly deposited soil has greater porosity
3. Particle sorting
Sorted soil (all particles are the same size) has greater porosity than unsorted (mixed size) soil
When considering round particles (tightly packed), the size of the particles doesn’t affect the porosity: large particles have a few large spaces; small particles have many small spaces…..they average out the same, usually about 35% porosity.
Permeability
- A material through which water can pass is said to be permeable. Permeability is a measure of the rate at which water can pass through a material (how easily it flows).
- Permeability depends on pore size and on whether or not the pores are interconnected.
- Usually, soil that has a high porosity is very permeable.
- Infiltration stops when water reaches a layer of soil or rock that is impermeable. [This layer is rarely more than 3 km deep due to pressure of overlying material.]
Runoff
Runoff will occur when:
- the soil is saturated (or)
- infiltration can’t keep up with rainfall (or)
- the slope is too steep for water to sink in
è Zones of Subsurface Water
- When water passes through soil, it leaves a thin film behind.
- Water clings by adhesion to soil particles and the walls of pore spaces. Adhesion is the attraction between molecules of different materials.
- Water also exhibits capillary action, due to adhesion and cohesion. Cohesion is the attraction between molecules of the same material.
- Capillary action occurs whenever liquid is in contact with very narrow spaces. The more narrow the space, the higher the liquid will rise.
- Therefore, the extent of capillary action increases with a decrease in the size of soil particles and pore spaces (inverse relationship).
- The lowest portion of a permeable surface layer (directly above the impermeable layer) is saturated with water. Water in this zone of saturation is called groundwater, and the top of this zone is called the water table.
- Above the water table is the capillary fringe in which the pores are kept filled with water by capillary action from the groundwater.
- The upper portion of the surface layer is the zone of aeration, where the pore spaces are coated with water but also contain air. Plant roots receive their water from this area.
- The water table is usually not horizontal because water infiltrating the ground also flows slowly downhill below the surface. Therefore, the water table usually shows a pattern of ups and downs similar to the land above it.
- The zone of aeration usually dries out during periods without rain due to evaporation and absorption by plant roots.
Aquifers & Wells
- Layers of permeable rock or loose sediment whose pore spaces are filled with water are called aquifers.
- Groundwater in aquifers can be tapped by digging a well to the water table. Then water can be pumped to the surface. If the water table drops, the well can become dry.
- A well in which water under pressure rises to the surface (no pump necessary) is an artesian well. This forms when an aquifer is sandwiched between two impermeable layers confining the water as it seeps down slope, building pressure.
Springs & Geysers
- Where the water table meets the surface on a hillside is a spring.
- If groundwater is heated by rocks in contact with molten material under Earth’s surface, it’s a hot spring.
- A geyser is a hot spring that erupts periodically due to pressure
Caves
- Limestone is a common bedrock that dissolves easily in ground water, leaving a cave or cavern behind.
- Sinkholes form when part of a cave roof collapses, forming a depression on the surface.
- Where ground water drips from the roof, deposits of calcite called stalactites hang down. Deposits on the cavern floor are called stalagmites.
è Water Pollution
- Only 3% of the Earth’s water is fresh water.
- Water is almost a “universal solvent,” meaning that most substances (natural or manufactured) will dissolve in water, so it is easy to pollute.
Sources of pollution:
- Water is sometimes polluted by natural events (EX/ flowing through a sulfur deposit or getting dumped on by a volcanic eruption), but most pollution is the direct result of human activity. There is a direct relationship between pollution and human population.
- Sewage (treatment plants are costly)
- eutrophication: the addition of chemicals from fertilizers and detergents provides nutrients for algae, resulting in less oxygen for other organisms
- groundwater can be polluted by sewage or by salt water where wells are dug too close to the ocean
- industrial wastes -- chemical dumps damage water supply
- radioactive wastes can pollute water with dangerous toxic substances but nuclear power plants also cause thermal pollution -- water is used to cool reactors and released back into rivers at very high temperatures, killing organisms
- coal burning -- causes sulfur dioxide and nitrogen oxides to combine with water in the atmosphere forming sulfuric acid and nitric acid which produce acid rain
Water Budgets
[The back of your lab book contains detailed instructions for computing a water budget.]
A local water budget is a mathematical model of the water cycle of a region. It shows how precipitation, evapotranspiration, and soil storage of water vary throughout the year.
Due to seasonal variations, it is constructed on a monthly basis.
P = precipitation
Ep = potential evapotranspiration (depends on temperature)
Ea = actual evapotranspiration
St = storage (soil moisture storage can never exceed 100)
S = surplus
D = deficit
Streams and the Water Budget
- During heavy rains, most surface runoff forms streams
- A stream is a channel in the Earth in which runoff collects and moves downhill due to gravity
- If the stream bed is above the water table, water will steadily infiltrate and drain away, so the stream will dry up between periods of rain…..this is an intermittent stream.
- However, most stream beds are below the water table, fed by runoff during periods of rain and groundwater during dry periods.
- Water that enters a stream from the groundwater of the surrounding land is called the base flow of the stream
- The discharge of a stream is the volume of water that passes a certain point during a given amount of time
- Stream discharge varies directly with precipitation and seasonal changes in the water budget…….least during periods of usage and deficit, most during recharge and surplus
- A lake or a stream is, in most cases, simply a place where the water table comes up above the land’s surface
- The water table depth varies slightly with periods of heavy and little precipitation
è Climate
- Climate: the average weather conditions in a region over a period of many years
- To describe climate, you need two characteristics: temperature and moisture
- For temperature, you need average monthly temperatures and average yearly temperatures, as well as the yearly temperature range
- For moisture, you need to compare the amount of precipitation with the region’s natural demand for water……in other words, look at the ratio of P to Ep.
If P/Ep 0.4 ¢ arid climate
If P/Ep = 0.4 – 0.8 ¢ semiarid climate
If P/Ep = 0.8 – 1.2 ¢ subhumid climate
If P/Ep 1.2 ¢ humid climate
EX/ Reno, Nevada Precipitation = 193, Potential Evapotranspiration = 628
P ¸ Ep = 193 ¸ 628 = 0.3 ¢ arid climate
Factors Affecting Climate Patterns
1. Latitude -- temperature decreases as latitude increases
2. Elevation -- temperature decreases as elevation increases
3. Nearness to large bodies of water -- coastal (marine) climates have milder temperatures, a smaller temperature range, and more precipitation than inland (continental) climates (although wind direction can play a major role here also)
4. Ocean currents -- water coming from colder regions at higher latitudes will have a cooling effect on the land the currents encounter (lower temperatures year round) [see ESRT]
5. Mountains -- as barriers to wind, mountains produce the “orographic effect.” Moist air hitting the windward side is forced upward where it cools to its dew point and forms clouds and rain, making the windward side of the mountain cool and moist, but the leeward side warm and dry. [Many deserts are located on the leeward side of mountain ranges.]
6. Planetary Wind Belts -- low pressure zones create regions that are warm and humid, while high pressure zones create regions that are cool and dry [see ESRT]