In order to understand why growing food uses so much water, we need to explore the process of evaporation. Evaporation is a hydrologic process that we're all quite familiar with, even if you aren't aware of it. Think about hanging clothes out to dry on the clothesline, or blow-drying your hair. Both of those involve the movement of water from its liquid form to its vapor or gaseous form that we call water vapor, or in other words, both involve the evaporation of water.
In what weather conditions do your clothes dry faster? A hot, dry, windy day, or a cool, cloudy, rainy day? Why do you use a blow drier to dry your hair? Water evaporates faster if the temperature is higher, the air is dry, and if there's wind. The same is true outside in the natural environment. Evaporation rates are generally higher in hot, dry, and windy climates.
The rate at which water evaporates from any surface, whether from a lake's surface or through the stomata on a plant's leaf, is influenced by climatic and weather conditions, which include the solar radiation, temperature, relative humidity, and wind (and other meteorological factors). Evaporation rates are higher at higher temperatures because as temperature increases, the amount of energy necessary for evaporation decreases. In sunny, warm weather the loss of water by evaporation is greater than in cloudy and cool weather. Humidity, or water vapor content of the air, also has an effect on evaporation. The lower the relative humidity, the drier the air, and the higher the evaporation rate. The more humid the air, the closer the air is to saturation, and less evaporation can occur. Also, warm air can “hold” a higher concentration of water vapor, so you can think of there being more room for more water vapor to be stored in warmer air than in colder air. Wind moving over a water or land surface can also carry away water vapor, essentially drying the air, which leads to increased evaporation rates. So, sunny, hot, dry, windy conditions produce higher evaporation rates. We will see that the same factors - temperature, humidity, and wind - will affect how much water plants use, which contributes to how much water we use to produce our food!
Evaporation requires a lot of energy and that energy is provided by solar radiation. The maps below (Figure 4.1.1) illustrate the spatial patterns of solar radiation and of annual evaporation rates in the United States. Notice how the amount of solar radiation available for evaporation varies across the US. Solar radiation also varies with the season and weather conditions. Note that annual evaporation rates are given in inches per year. For example, Denver, Colorado in the lake evaporation map is right on the line between the 30-40 inches and 40-50 inches per year of lake evaporation, so let's say 40 inches per year. On average, if you had a swimming pool in Denver, and you never added water and it didn't rain into your pool, the water level in your pool would drop by 40 inches in a year. Explore the maps and answer the questions below.
Text description of the Figure 4.1.1a image.
The image is a color-coded map of the United States, including insets for Hawaii, Alaska, and Puerto Rico, showing daily solar radiation in langleys. The main map uses a gradient of colors from light yellow to dark red to represent increasing solar radiation levels. The legend on the right explains the ranges: 230–250, 250–300, 300–350, 350–400, 400–450, 450–500, and 500–550 langleys. The lightest shades, indicating the lowest solar radiation (230–250), appear in the northern states such as Washington, Montana, and the upper Midwest. Darker shades, representing higher solar radiation (500–550), dominate the southwestern U.S., particularly Arizona, New Mexico, and southern Nevada. The Southeast, including Florida and Texas, shows moderately high values (400–500). Hawaii and Puerto Rico are shaded in dark red, indicating very high solar radiation, while Alaska shows mostly light colors, reflecting low solar radiation levels. The map emphasizes regional variation in solar energy potential, with the Southwest and tropical territories receiving the most sunlight compared to northern and coastal regions.
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Text description of the Figure 4.1.1b image.
The image is a color-coded map of the United States showing annual lake evaporation in inches. The map uses a gradient of colors to represent different evaporation ranges, as explained in the legend on the right: 10–20 inches (dark blue), 20–30 (medium blue), 30–40 (light blue), 40–50 (yellow), 50–60 (light orange), 60–70 (orange), 70–80 (red-orange), and 80+ inches (dark red). The northern and northeastern states, as well as much of the Midwest and Pacific Northwest, are shaded in blue tones, indicating low evaporation rates between 10 and 40 inches annually. Central regions, including parts of the Great Plains, show yellow and light orange areas, representing moderate evaporation of 40–60 inches. The southwestern U.S., particularly Nevada, Arizona, and southern California, is shaded in red and dark red, signifying the highest evaporation rates exceeding 80 inches per year. This pattern reflects the influence of climate and geography, with arid regions experiencing significantly higher evaporation compared to cooler, wetter areas in the north and east.