Evaporation and Climate

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.

Figure 4.1.1. a. Mean daily solar radiation in the United States and Puerto Rico and
b. Mean annual lake evaporation in the conterminous United States, 1946-55. Data not available for Alaska, Hawaii, and Puerto Rico.

Source: Data from U.S. Department of Commerce, 1968). From Hanson 1991.

Water is Essential for Food Production

Why do we need so much water for agriculture? Plants use a lot of water!

Plants need water to grow! Plants are about 80-95% water and need water for multiple reasons as they grow including for photosynthesis, for cooling, and to transport minerals and nutrients from the soil and into the plant.

"We can grow food without fossil fuels, but we cannot grow food without water."
Dr. Bruce Bugbee, Utah State University

We can't grow plants, including fruits, vegetables, and grains, without water. Plants provide food for both us and for the animals we eat. So, we also can't grow cows, chickens, or pigs without water. Water is essential to growing corn as well as cows!

Agriculture is the world's greatest consumer of our water resources. Globally about 70% of human water use is for irrigation of crops. In arid regions, irrigation can comprise more than 80% of a region's water consumption.

The movement of water from the soil into a plant's roots and through the plant is driven by an evaporative process called transpiration. Transpiration is just the evaporation of water through tiny holes in a plant's leaves called stomata. Transpiration is a very important process in the growth and development of a plant.

Water is an essential input into the photosynthesis reaction (Figure 4.1.2), which converts sunlight, carbon dioxide, and water into carbohydrates that we and other animals can eat for energy. Also, as the water vapor moves out of the plant's stomata via transpiration (Figure 4.1.2), carbon dioxide can enter the plant. The transpiration of water vapor out of the open stomata allows carbon dioxide (another essential component of photosynthesis) to move into the plant. Transpiration also cools the plant and creates an upward movement of water through the plant. The figure below (Figure 4.1.2) shows the photosynthesis reaction and the movement of water out of the plant's stomata via transpiration.

As water transpires or evaporates through the plant's stomata, water is pumped up from the soil through the roots and into the plant. That water carries with it, minerals and nutrients from the soil that are essential for plant growth. We'll talk quite a bit more about nutrients later in this module and future modules.

Figure 4.1.2 Photosynthesis and transpiration

Credit: Wikimedia Commons: Photosynthesis [2] (Creative Commons CC BY-SA 3.0 [3])

Click for a text description of the photosynthesis and transpiration image

This drawing shows the sunlight shining down on a flower. The roots of the flower are in the soil and there is water in the soil. Carbon dioxide is going into the flower. Water vapor and oxygen are being released from the flower (Transpiration). The chemical formula for photosynthesis is shown as 6 CO₂ (Carbon Dioxide) + 6 H₂O (Water) an arrow representing light leads to C₆H₁₂O₆ (Sugar) + 6 O₂ (Oxygen).

Evapotranspiration and Crop Water Use

How much water does a crop need?

The amount of water that a crop uses includes the water that is transpired by the plant and the water that is stored in the tissue of the plant from the process of photosynthesis. The water stored in the plant's tissue is a tiny fraction (<5%) of the total amount of water used by the plant. So, the water use of a crop is considered to be equal to the water transpired or evaporated by the plant.

Since a majority of the water used by the crop is the water that is transpired by the plant, we measure the water use of a plant or crop as the rate of evapotranspiration or ET, which is the process by which liquid water moves from the soil or plants to vapor form in the atmosphere. ET is comprised of two evaporative processes, as illustrated in figure 4.1.3 below: evaporation of water from soil and transpiration of water from plants' leaves. ET is an important part of the hydrologic cycle as it is the pathway by which water moves from the earth's surface into the atmosphere.

Remember, evaporation rates are affected by solar radiation, temperature, relative humidity, and the wind. ET, which includes evaporation from soils and transpiration from plants, is also evaporative, so the ET rate is also affected by solar radiation, temperature, relative humidity, and the wind. This tells us that crop water use will also be affected by solar radiation, temperature, relative humidity, and the wind! More water evaporates from plants and soils in conditions of higher air temperature, low humidity, strong solar energy, and strong wind speeds.

The transpiration portion of ET gets a little more complicated because the structure, age, and health of the plant, as well as other plant factors, can also affect the rate of transpiration. For example, desert plants are adapted to transpire at slower rates than plants adapted for more humid environments. Some desert plants keep their stomata closed during the day to reduce transpiration during the heat of a dry desert day. Plant adaptations to conserve moisture include wilting to reduce transpiration. Also, small leaves, silvery reflective leaves, and hairy leaves all reduce transpiration by reducing evaporation.

In summary, the amount of water that a crop needs is measured by the ET rate of a crop. The ET rate includes water that is transpired or evaporated through the plant. And, the ET rate varies depending on climatic conditions, plant characteristics, and soil conditions.

Figure 4.1.3. Evapotranspiration includes evaporation from soil and transpiration from leaves.

Credit: Figure drawn by Gigi Richard, adapted from Bates, R.L. & J.A.Jackson, Glossary of Geology, Second Edition, American Geology Institute, 1980

Click for a text description of the evapotranspiration image

Diagram of Evapotranspiration. At the bottom is soil and below that, available soil water. In the soil are two plants with roots extending into the soil water. There are lines coming up from the soil representing evaporation from the soil. Lines from the plants represent transpiration from leaves. There is a line drawn around all of this, with the sun outside and humidity and temperature flowing in. An arrow from transpiration and evaporation leads to evapotranspiration.

Crop water use varies

If the ET rate of a crop determines the water use of that crop, we could expect water use of a single crop to vary in similar spatial patterns to evaporation rates. For example, if evaporation rates are very high in Arizona because of the hot, dry climate, you would expect ET rates to be higher for a given crop in that climate. ET is measured by the average depth of water that the crop uses, which is a function of the plant and of the weather conditions in the area. In cool, wet conditions, the plant will require less water, but under hot, dry conditions, the same plant will require more water.

Figure 4.1.4 shows a range of typical water use for crops in California. The graph shows how much water needs to be applied as irrigation to grow different crops. Notice how some crops, like alfalfa, almonds, pistachios, rice, and pasture grass can require four feet or more of water application. Other crops, like grapes, beans, and grains only require about one to two feet of water.

If we moved the plants in Figure 4.1.4 to a cooler and more humid climate, the rate of evaporation would be less and the crop water demand would decline as well. In a hot dry climate, you need to apply more water to the plant to keep it healthy and growing because more water is evaporating from both the soil and through the stomata on the plants’ leaves, so the plant is pulling more water out of the soil via its roots to replace the water transpiring from its leaves.

Figure 4.1.4. Water use of California crops, as measured by water application depth. Note: The range shown reflects the first and third quartile of the water application depth for the period 1998-2010, and the white line within that range reflects the median application depth during that period.

Credit: California Agricultural Water Use: Key Background Information [4], by Heather Cooley, Pacific Institute.

Water Sources for Crops

Where do plants get their water?

The source of water for most land plants is precipitation that infiltrates or soaks into the soil, but precipitation varies dramatically geographically. For example, we know that Florida gets a lot more precipitation per year than Arizona. Figure 4.1.5 below shows the average annual precipitation across the United States and around the globe. Notice on the map of the U.S. that the dark orange colors represent areas that get less than ten inches of precipitation per year. And, the darkest green to blue regions receive more than 100 inches or more than eight feet of precipitation per year!

Climate, including the temperature of a region and the amount of precipitation, plays an important role in determining what types of plants can grow in a particular area. Think about what types of plants you might see in a high water resource region versus a low water resource region. A low resource region with respect to water receives lower precipitation, so would have desert-like vegetation, whereas a higher resource region for water would have lusher native vegetation, such as the forests of the eastern US.

Regions that receive enough precipitation to grow crops without irrigation (i.e., those areas shaded green on the map below) would be considered high resource areas with respect to water. A high-resource region is more likely to be a more resilient food production region. In contrast, a low resource region with respect to water would be regions on the map below in the orange-shaded colors. In these regions, extra effort is needed to provide enough water for crops, such as through the development of an irrigation system.

Compare the crop water use values in Figure 4.1.6 with the average annual precipitation in Figure 4.1.5 and you'll see that there are parts of the US where there isn't enough precipitation to grow many crops. In fact, there is a rough line running down the center of the US at about the 100th meridian that separates regions that get more than about 20 inches of rain per year from regions that get less than 20 inches of rain per year. On the map in Figure 4.1.5, this line is evident between the orange-colored areas and the green-colored areas. Generally, west of the 100th meridian there is insufficient precipitation to grow many crops. If a crop's consumptive water use or ET is greater than the amount of precipitation, then irrigation of the crop is necessary to achieve high yields.

Figure 4.1.5. a) Average annual precipitation in the United States 1961-1990 and b) Mean annual Global Precipitation (1961-1990)

Credit a) USGS: The National Map [5] and b)Terrascope: Rainwater Harvesting [6]

Figure 4.1.6. Water use of California crops, as measured by water application depth. Note: The range shown reflects the first and third quartile of the water application depth for the period 1998-2010, and the white line within that range reflects the median application depth during that period.

Credit: California Agriculture Water Use: Key Background Information [4] by Heather Cooley, Pacific Institute.

How can we grow crops when there is insufficient precipitation?

In regions where precipitation is insufficient to grow crops, farmers turn to other sources of water to irrigate their crops. Irrigation is the artificial application of water to the soil to assist in the growth of agricultural crops and other vegetation in dry areas and during periods of inadequate rainfall. These sources of water can be from either surface or groundwater. Surface water sources include rivers and lakes, and diversion of water from surface water sources often requires dams and networks of irrigation canals, ditches, and pipelines. These diversions structures and the resulting depletion in river flow can have significant impacts on our river systems, which will be covered in the next part of this module. The pumping of water for irrigation from aquifers also has impacts, which are also discussed in the next part of this module.

Water use for irrigation comprised about 80-90 percent of U.S. consumptive water use in 2005, with about three-quarters of the irrigated acreage being in the western-most contiguous states (from USDA Economic Research Service [7]). For example, in the state of Colorado, irrigation comprised 89% of total water withdrawals in 2010 (Figure 4.1.7). Irrigated agriculture is also very important economically, accounting for 55 percent of the total value of crop sales in the US in 2007 (from USDA Economic Research Service [7]). Globally only about 18 percent of cropland is irrigated, but that land produces 40 percent of the world's food and about 50 percent by value (Jones 2010).

Figure 4.1.7. Colorado Total water withdrawals by water-use category

Credit: Data from Maupin, M. A., Kenny, J.F., Hutson, S.S., Lovelace, J.K., Barber, N.L., and Linsey, K.S., 2014, Estimated use of water in the United States in 2010: U.S. Geological Survey Circular 1405, 56 pp. [8]

Click for a text description of the Colorado Total water withdrawals image

A pie chart of Colorado total water withdrawals by water-use category in 2010 shows that almost 89% percent of water withdrawals were from irrigation. The remaining percentages are as follows (approximately): 8% public water supply, 1.18% industrial, 1.11% aquaculture, .70% thermoelectric power, .34% domestic fresh, .33% livestock, .26% mining.

Activate Your Learning

In this activity, you will employ geoscience ways of thinking and skills (spatial thinking and interpretation of the spatial data to characterize specific regions for the geographic facility).

Figure 4.1.1. Mean annual lake evaporation in the conterminous United States, 1946-55.

Credit: Data from U.S. Department of Commerce, 1968. From Hanson 1991.

Irrigation Efficiency

The amount of water used for irrigation varies depending on the climate and on the crop being grown, and it also depends on the irrigation technique used. Just like in your garden or home landscaping there are more or less efficient sprinklers. In many parts of the world flood or surface, irrigation is still used where water flows across a field and soaks into the soil.

Surface or flood irrigation is the least efficient manner of irrigation. When a field is flooded, more water than is needed by the plant is applied to the field and water evaporates, seeps into the ground, and percolates down to the groundwater, where it can be out of reach of the plant's roots. Another problem with flood irrigation is that the water is not always applied evenly to all plants. Some plants might get too much water, and others get too little. On the other hand, flood irrigation tends to use the least energy of any irrigation system.

Furrow irrigation (Figure 4.1.8) is another type of surface irrigation in which water is directed through gated pipe or siphon tubes into furrows between rows of plants. When using furrow irrigation, water is lost to surface runoff, groundwater, and evaporation, and it can be challenging to get water evenly to an entire field.

Figure 4.1.8. Furrow irrigation of an onion field in the Uncompahgre Valley, CO.

Credits: Perry Cabot

More efficient methods of irrigation include drip irrigation (Figure 4.1.9) sprinklers (such as center pivots, Figure 4.1.10), and micro-spray (Figure 4.1.11) irrigation. All of these methods, while more efficient, also require significant investments in equipment, pipes, infrastructure (e.g., pumps Figure 4.1.9) and energy. In addition to the high cost, some soil types, irrigation networks, field sizes, and crops pose greater challenges to the implementation of more efficient methods of irrigation. For example, in the Grand Valley of western Colorado, the irrigation network is entirely gravity-fed, meaning that farmers can easily flood and furrow irrigate without the use of pumps. In addition, the fields are small and the soils are very clayey, all of which make using center pivots for row crops particularly challenging and expensive. But, in the same valley, the peach orchards have successfully used micro-spray and drip systems. A major advantage of more efficient irrigation in addition to reduced water consumption is that crop yields are often higher because the water can be applied more directly to the plant when water is needed.

Figure 4.1.9. Filtration and pumps for a drip irrigation system for onion and bean crops in the Uncompahgre Valley, CO.

Credit: Gigi Richard

Figure 4.1.10. a) Center pivot sprinkler irrigation on an alfalfa crop in the San Luis Valley, CO and b) a hay crop for cattle feed in the Uncompahgre Valley, CO.

Credit: Gigi Richard

Figure 4.1.11. Micro-spray irrigation at a peach orchard in the Grand Valley, CO.

Credit: Gigi Richard

Activate Your Learning

Table 4.1.1 presents data on the top 15 irrigated states in the United States. You can see how many acres of land are irrigated in each state, and how much water is used for irrigation of both surface water and groundwater. Consider the relationship between the amount of irrigated land in a state, the type of irrigation used and the amount of water used.

An acre-foot is a unit of measure for large volumes of water and is the volume of water required to cover one acre of land to a depth of one foot (325,851 gallons). Imagine a football field, including the end zones, one foot deep in water.

Virtual Water

How much water do you eat?

Water is essential to growing food and every bite of food we consume required water to grow, process, and transport. The water necessary to grow, process, and transport food is often referred to as virtual water or embedded water. Virtual water is the entire amount of water required to produce all of the products we use, including our mobile phones and cotton t-shirts. But a global assessment of virtual water reveals that the majority of water that we consume is in the food we eat. If we total up all of the virtual water embedded in everything we use and eat, we can estimate our total water footprint. Water footprints can be used to provide insights into how much water is used every day in all of our activities including producing our food. For example, Figure 4.1.12 shows the amount of water used per person around the globe associated with wheat consumption. When you eat food imported from another region, you are eating the water of that region. The apple from New Zealand, grapes from Chile, and lettuce from California all required water to grow and by consuming those products you’re "eating" that virtual water. The concepts of virtual water and water footprints can be powerful tools for businesses and governments to understand their water-related risks and for planning purposes (water footprint network [9]).

Figure 4.1.12. Water footprint per capita related to consumption of wheat products in the period 1996–2005.

Credit: Figure from Hoekstra, A.Y. and M.M. Mekonnen, 2012, The Water Footprint of Humanity, Proceedings of the National Academy of Sciences, vol. 109, no. 9 [10].

Check Your Understanding

Scroll through this infographic [11] explaining virtual water and then answer the questions below.

Formative Assessment: Turning Water into Food

Instructions

Please download the worksheet below for detailed instructions.

• Download Module 4 Formative Assessment Activity Worksheet [12]

You will perform three activities in this assessment:

1. Watch the video below, Turning water into food, and answer the questions on the worksheet as you watch the video
2. Visit the water footprint calculator [13] website, compare how your water footprint changes with varying levels of meat consumption, and answer questions on the worksheet. This portion of the assessment will be included in the weekly discussion and not included in the assessment quiz.
3. Perform a comparison of the virtual water [14] embedded in different food products and answer questions on the worksheet.

Video: Turning water into food, Bruce Bugbee | TEDxUSU (16:32)

Submitting Your Assignment

Please submit your assignment in Module 4 Formative Assessment in Canvas.