Chapter 2A Main Ideas

1. The Hydrologic Cycle and Freshwater Uses

The hydrologic cycle describes the movement (flow) of water from one location (store) to the next. All water stores on the planet are connected through a complicated network of flows.

Most of the water on the planet is saltwater, with only a small portion as freshwater, and most of that is frozen. Groundwater is the largest store of unfrozen freshwater on the planet.

The main use for freshwater worldwide is for agriculture (irrigation). In the U.S., the largest freshwater usage is for thermoelectric production, but unlike agricultural usage this is mostly non-consumptive use. The largest consumptive use of freshwater in the U.S. is irrigation.

2. Controls on Evaporation and Transpiration

The amount of evaporation and transpiration changes based on humidity levels, temperature, and wind conditions. This means plants in different climatic conditions will require different amounts of water to survive and thrive.

3. Crop Water Needs

Plants need enough water to replace any water lost to evapotranspiration. Comparing crop water needs with precipitation and effective precipitation for an area, the amount of irrigation required can be determined.

4. Groundwater

Groundwater is the water found in the pore spaces of rocks and sediments. Water moves from the surface through the unsaturated zone into the ground through infiltration where it collects in the saturated zone below ground. The water table is the boundary between the unsaturated and saturated zones below ground. The porosity and permeability of the ground material determines whether it will make a good aquifer or aquiclude. Unconfined aquifers are the top aquifer under the ground while confined aquifers are located beneath an aquiclude, or confining layer. The water table elevation adjusts up and down depending on how much water is being recharged and discharged in the aquifer.

5. Impacts of Groundwater Withdrawals for Irrigation

Excessive groundwater withdrawals can lead to groundwater depletion and result in subsidence of the land surface. In coastal areas, saltwater intrusion can also result from excessive groundwater withdrawals. These effects are compounded by drought conditions brought on by climate change.