From GeoClasses

Water Resources

hydrologic cycle

Water cycle.

Transfer of water through Earth's reservoirs (oceans, atmosphere, ice, surface fresh water, groundwater) hydrologic cycle

• processes of movement - evaporation, precipitation, transpiration, surface runoff, groundwater flow

satellite image of the Aral Sea in 1973

Aral Sea in 1987

Aral Sea in 2000

• table 11.1 (Keller, 2005) - size and residence times of each reservoir

• balance within cycle includes liquid, solid, gas

Surface water

Darling River, Australia

• runnoff creates erosion of sediments

North Carolina coast before and after Hurricanes Dennis and Floyd, 1999

Groundwater

Groundwater flow and flow times

Surface water that infiltrates through soils down into rock

Vadose zone

Vadose zone

Above the water table (boundary below which ground is saturated with water) - seldom saturated except just after large rainstorms.

Subsurface monitoring can provide early warning of pollutants heading into groundwater (aquifers)

Zone of saturation

Below water table - where actual flow occurs

Areas within this zone can be aquifers tapped for freshwater supplies.

Aquifer

Aquifers and confining units

An aquifer is any type of earth material capable of supplying groundwater at a useful rate from a well. It requires;

• high enough flow (large connected pore spaces)
  • eg. gravel, sand, soil, fractured rock

If a confining layer is on top (confined aquifer) then water pressure may be high and create an artesian well (water rises without needing to be pumped)

diagram showing how an artesian well is formed

an artesian fountain at Fountain Point, Michigan, that has been continuously discharging since 1867

• Confining layer
  • A confining layer is a layer of earth material that restricts the flow of water.

Recharge

Any process that adds water to an aquifer. It tends to only occur in certain places.

Case Study: Ogallala Aquifer, Great Plains, U.S.

• recharge limited by deposits of caliche, limited to ephemeral playa lakes
• discharge extensive

crops in Kansas irrigated with groundwater from the Ogallala aquifer

Groundwater discharge

Groundwater discharge into a saline lake.

Any process that removes groundwater (man-made pumping, natural discharge at spring at surface)

• If artificially pumped then a cone of depression forms (removing water faster than replacing) and pumping progressively deeper water. This may induce water from the polluted zone, salty (saline) zone or the well may dry up and have to be drilled deeper.

Groundwater flow

Occurs according to Darcy's law

• Flow rate ∝ hydraulic gradient × hydraulic conductivity
• gradient - usually the slope of the water table - water flows down - steepness determines speed
• conductivity - ability of water to move through material ** large pore space (porosity) and many connections (permeability) = faster flow

Groundwater supply

Central Valley, California: the larger reservoir is drained by the Tuolumne River, the smaller by the Merced River; North is toward the lower right; eastern foothills are at the lower left and humans' land use can be seen in top half of photo

Can be renewable resources but if pumping exceeds recharge then becomes non-renewable, such as if recharge areas become urbanized.

Note: any amount of groundwater pumping will change the natural system. The artificial discharge replaces natural discharge (to streams) that would have occurred.

Surface/groundwater interactions

Blue Nile Falls in Ethiopia: the ultimate in surface runoff

Surface water can either runoff (rivers/lakes) to ocean or infiltrate (groundwater)

Groundwater can discharge naturally into rivers, lakes, oceans

adding dye tracer to a stream to measure travel time of water in the stream

filling sample bottle from a well for measurement of contaminants in groundwater

graph showing variability of discharge in an effluent-dependent river (Santa Cruz River)

rivers

• effluent streams - fed by rain and groundwater = perennial
• influent streams - only fed by rain = often ephemeral
  • can cause pollution to infiltrate into groundwater

karst

Karst cave formations.

Forms only in limestone which is easily dissolved (chemical weathering) and makes caves and sinkholes that groundwater can flow through.

Base of sinkhole is usually at water table level -don't throw your garbage in!

• Exploring Karst: Blanchard Springs Caverns

Water use

Offstream use

thermoelectric power plant

Water is removed or diverted from source

• irrigation, thermoelectric power, industrial processes, public supply.

Consumptive use

Offstream use that is not returned to its source immediately

• evaporated, incorporated into plants of factory products, consumed by animals and humans

Instream use

Hoover Dam (hydroelectric power plant) seen from the air

Used but not withdrawn from source

• navigation, hydroelectric power, fish and wildlife, recreation

Trends

Fig. 11.20 (Keller, 2005) shows trends of use in US over time

conservation - biggest uses are irrigation and thermoelectric power

1. more efficient irrigation or alternative farming
2. alternative energy (not coal or nuclear)

Water shortages

Global problems

• lakes running dry (Aral Sea)
• rivers do not reach the sea (Colorado River, Yellow River in China, Nile River only 10% to Mediterranean Sea)

Colorado River: explanation from NASA Visible Earth

Yellow River, China: sediment discharge

Nile River before and after rainy season flooding in 2000 and 2001: NASA's expanation

• demand for irrigation water 3× in last 50 years
• human population 2× in 50 years

graph of human population growth since 10,000BC

estimated global water scarcity for 2025, made by the Interational Water Management Institute

• water shortage = food shortage
• groundwater being depleted globally for irrigation of grain.