1.1: Water Cycle
Learning Objectives
After reading this section, you should be able to:
- Identify processes in the water cycle that influence the water supply
- Analyze situations in terms of precipitation, condensation, evaporation, and transpiration
- Evaluate yearly data for evapotranspiration
You've probably heard on the news the statewide water supply in California described in terms of snowpack and rainfall. These are critical measures of how much water is available for human use. Water managers in California follow both snowpack measurements and rainfall measurements closely. In the diagram above from the United States Geological Survey (USGS), you can see that precipitation is shown as rainfall and snow.
Precipitation also includes something you don’t see too often in California: hail and sleet. All forms of water that fall from the sky, including rain, snow, hail and sleet, are forms of precipitation. Rainfall can provide much needed water in the ground by the process of infiltration, the process by which water seeps into the ground and eventually recharges our groundwater, water stored in the ground. Using the diagram above, you can also trace the flow of groundwater in some cases to both rivers, lakes, and even to the ocean.
Above ground, rainfall can also be stored on earth’s surface in lakes. Precipitation in California also provides snow for snowpack, which can be our largest area of storage of water in the winter. In fact, it is common for water managers to view the snowpack in the Sierra as a reservoir; it is simply a seasonal reservoir that melts in the spring. Snowmelt runoff from the snowpack can fill streams and lakes. Runoff from rainfall is also captured in streams and lakes and can serve to recharge aquifers through percolation and infiltration.
The diagram also shows the key process of evaporation, or the process through which liquid water turns into a gas. Evaporation occurs over bodies of water like the ocean or lakes, but also over the land. If you own a home with a pool, you have probably noticed that if you leave the pool without a cover in the summer, you have to add water more often in the summer than in the winter. This is because the rate of evaporation is higher in the summer than the winter, often dramatically so.
Transpiration is the process through which plants lose water. This may seem like an inconsequential process, but it is the entire process that drives irrigation. In the diagram below you can follow the transpiration process. In Step 1, plants bring in moisture from the soil with their roots. Then, in Step 2, water travels up through the plant. In Step 3, water leaves through their pores or stomates (plural of stomata) and enters the atmosphere again.
The combination of water loss from liquid water in the ground evaporating and water in plants losing water is called evapotranspiration (ET). Evapotranspiration is frequently used by scientists as a measurement of the plant water needs. If a location has a high ET, then the plant needs are greater than a location with a low ET.
Most locations in the state of California are close to a weather station that is part of the California Irrigation Management Information System (CIMIS) network. A CIMIS station measures a variety of variables in order to calculate the ET including: temperature, solar radiation, humidity, wind speed, and wind direction. With a complicated formula, the CIMIS station will calculate the ET for a specific geographic location. This information is frequently used to make a water budget, an estimate of how much water a location should use, including outdoor watering based on the amount of landscaping, the type of landscaping and the evapotranspiration.
The monthly inches of ET data in the table for Santa Clarita reflects the water needs in terms of inches of water that grass would need to receive per month. You can see that the highest needs are in summer, primarily July and August, with needs decreasing rapidly in September through December. Even though September is typically almost as hot as August, the days are shorter and the sun is less intense so the evapotranspiration is less in September than August. This means the plant needs for irrigation are significantly less.
CIMIS Station #204 Santa Clarita - 2016 Evapotranspiration Data
|
Months |
Jan |
Feb |
Mar |
Apr |
May |
Jun |
Jul |
Aug |
Sep |
Oct |
Nov |
Dec |
|
Monthly ET (in) |
1.97 |
4.80 |
4.51 |
5.24 |
5.77 |
7.77 |
8.99 |
8.13 |
6.30 |
4.68 |
3.40 |
2.69 |
Knowledge of evapotranspiration has a practical application - in Southern California, many people find they can turn off their irrigation systems for grass in normal years from November through February and that their plants’ water needs will be met with rainfall alone.
The opposite process of evaporation is condensation, the process through which water as a gas turns into liquid water again. Condensation is the process through which clouds form. This is also the process through which water beads on the outside of a glass of iced tea on a humid day or on a mirror in a steamy bathroom. In terms of the water supply picture, condensation is the key process that can lead to precipitation, so clouds are carefully monitored by scientists.
What makes the water cycle work? In short, gravity and the sun. Notice that there is always groundwater flow and streamflow from a higher altitude to a lower altitude. Gravity drives this flow of water in the water cycle, and is also a key force in most water distribution systems. If you’re not using gravity, you’re going to need a pump to go from a lower elevation to a higher elevation. Systems are generally designed to avoid pumps if possible and use the force of gravity. The sun drives the water cycle in a different way, by heating the water and causing it to evaporate. It also melts the snowpack in the mountains while gravity causes the water to flow into streams, rivers and lakes.
In Section 1.1, you've learned the basic processes in the water cycle: precipitation, infiltration, evaporation, transpiration, and condensation. Next, you'll see how these processes create surface and groundwater supplies and how these supplies are used for water management.
Try It!
Describe a process at work in each scenario and how the process works:
- Analyze the effect on the water cycle if California had no precipitation for a year.
- You hang a wet swimsuit out on the balcony to dry overnight. What process is at work? How is water changing states?
- Using the CIMIS web site (www.cimis.water.ca.gov/), register for an account (free). Then under the Data tab, find the nearest CIMIS station to your home create a monthly web report for 12 consecutive months, which should include ET. When should you water the most? When should you water the least?
Key Terms
Condensation—The process through which water as a gas turns into liquid water again; opposite of evaporation
Evaporation—The process through which liquid water turns into a gas; opposite of condensation
Evapotranspiration—The combination of water loss from liquid water in the ground evaporating and water in plants losing water
Groundwater—Water stored in the ground
Infiltration—The process through which water seeps into the ground
Precipitation—All forms of water that fall from the sky, including rain, snow, hail, and sleet
Transpiration—The process through which plants lose water
Water budget—An estimate of how much water a location should use, including outdoor watering based on the amount of landscaping, the type of landscaping and the evapotranspiration