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Senior High School Lesson Plans

by Rich Engel, M.A.

Irrigation Efficiencies

The purpose of this lesson is to help students understand that a farmer or rancher must consider multiple factors when choosing a form of irrigation for crops or pasture. Through these activities, students will investigate the economic factors influencing the choice of an irrigation system that is appropriate for the selected crop and field.

The students will:
  • Describe and simulate the three main types of irrigation systems.
  • Calculate irrigation efficiencies for different fields.
  • Develop cost estimates for irrigating with different systems.

Key Terms:

Basin irrigation, pressurized sprinkler irrigation, micro-irrigation, water holding capacity, saturation, runoff, water conservation, acre foot, precipitated.

Subjects:

Science - Hypothesizing, critical thinking, analyzing, comparing similarities and differences, describing, demonstrating, observing, listening, recording, discussing

History/Social Science/Geography - Comparing past to present, studying geographical and topographical regions, weighing science against social perceptions, economics

Mathematics - Ratios, percentages, arithmetic, measuring

Preparation:

One Hour

Estimated Teaching Time:

Two 1-hour sessions

Materials:
  • Sample problems (below) for student practice
  • 4 potted plants
  • nearby outdoor weather station
  • moderate to hot climate
Background Information:

In 1940 the average farmer in the United States could produce enough food for 19 people. Today, an American farmer can produce enough food to feed 129 people -- 101 in the United States and 28 abroad. Technological advances have increased the productivity of farmers, particularly by improving their ability to provide water to their crops through irrigation. Irrigation is defined as the managed application of water to soil for the purpose of increasing crop production.

Irrigated agriculture has helped American farmers produce the most abundant and diverse supply of food, fiber and foliage products in the world. Irrigation plays an especially important role in the Western United States where growing seasons are longer but there is not enough rainfall to supply an optimum amount of water to commercial crops. California alone produces over 250 agricultural commodities, most of which could not be grown there without irrigation.

The following AgriWater information and activities will focus on the three main types of irrigation - surface gravity flow, pressurized sprinkler, and micro-irrigation. In order to select the appropriate type of irrigation, the farmer must make informed decisions. Water availability, economics, soil types and plant biology factors must all be studied before choosing an irrigation technique. Some of the major considerations for the farmer or rancher include:
  • proximity of the field or pasture to a water source
  • adequate distribution system to the field (pumps, canals or pipes)
  • amount of water required by selected crop
  • quality of available water
  • cost of water
  • topography of the land
  • soil type
  • annual precipitation
  • cost of irrigation supplies
  • availability of labor to set-up and maintain irrigation system
  • fertilization methods
  • methods for recycling or handling excess irrigation water

The complexity of choosing an appropriate irrigation system and utilizing the available water productively has helped to insure that good farmers are true water managers, very aware of conservation principles. It is in their own best economic interest to use water wisely.

Activity 1: Calculating Irrigation Amounts and Efficiencies

Activity Background:

Water managers typically measure water in acre feet, which is the amount that would cover one acre of land a foot deep. The turf area of a football field including the sidelines amounts to about an acre. Imagine an intense rain storm flooding a football field one foot deep in just a few minutes. But a farmer does not apply waterall at once that way. The "inches of water" measurement for irrigation totals refer to the cumulative amount during the growing season. This measure of application does not depend on the acreage under irrigation. Thirty inches of water means the equivalent to thirty inches in cumulative depth, whether the watered area is 40 acres or only a garden plot.

Plants use water to construct new plant cells, thus growing larger and bearing fruit. When this internal water has done its job, most of it passes out of the plant, a process called transpiration. Only a small fraction of the water takenup by plant roots actually remains in the plant. Transpired water removes excess heat from the plant, and like evaporated water, returns to the air. It has accomplished the irrigator's purpose, making plant growth possible. The amounts of transpiration from plants and evaporation from soil are often added together, as evapotranspiration. The quantity of evapotranspiration depends on wind, air temperature, humidity, and direct sunlight.

Soil water in excess of plant needs either evaporates from the soil, runs off the field, or seeps deep into the ground in a process called percolation. Farmers seek to minimize all these losses but they cannot eliminate them. The closer they come to the ideal, the more efficient the irrigation. Farmers can calculate different kinds of efficiency. The "water application efficiency" compares water available at the plant roots to the total applied upon the field. "Water use efficiency" compares the water needed for crop and soil productivity with the amount applied, factoring in the water used to carry away (leach) any concentration of minerals left by evapotranspiration. In either calculation, technical efficiencies will be balanced against economic, social and environmental efficiencies: the evaluation of benefits versus costs.

Different crops require different amounts of water to reach marketable size and maturity. Perennial crops typically require 36 inches or more of usable water per year. In order to get that much water to the plant roots, farmers might apply somewhat more, or rather less, taking into consideration the amount of rainfall during the year and the amount of irrigation water lost to deep percolation, evaporation and runoff. Water scheduling practices can determine crop needs under given climate conditions and deliver the appropriate amounts with uniform distribution, thus minimizing loses. Perfect efficiency is impossible, but Best Management Practices can attain very high efficiencies.

Quick Activity:

Let's calculate how much water a grower had to apply to a crop.

Farmer A's crop, planted on an acre, required 60 inches of usable water during the growing period. The rainfall total during the growing period of the crop was 18 inches. How much water did the farmer have to purchase?

60" - 18" = 42", assuming for simplicity's sake that all the rain was used by the crop

42"/12"=3.5.

So, 3.5 acre feet of irrigation water was purchased.

At $50 per acre foot, the farmer paid $175 per acre in water costs.

Unfortunately, not all of the water applied to the crop through rainfall or irrigation is utilized by the plant. Much of the water is lost through percolation to the groundwater table, evaporation, or runoff. The amount of water made available to the crops in the root zone, divided by the total amount of water that must be supplied to maintain this availability, yields the ratio called water application efficiency. Common water application efficiencies are 65-75% with the best efficiencies ranging from 85-90%. Because of the leaching requirement in areas with high evaportranspiration, farmers in hot, dry regions will not fare as well with application efficiency as farmers in moderate climates. As you can see by the calculation above, every farmer wants to reduce costs and maximize benefits by increasing the proportion of usable water. It is not economical to apply excessive water just to lose much of it to runoff, to the air, or to the subsurface.

Quick Activity:

Calculate the total irrigation water applied to a crop if the crop requires 4 acre feet of usable water, the precipitation total is 16 inches and the water application efficiency is 75%.

4 X 12"=48"

48"-16"=32" needed at the plant roots in addition to rain, assuming the rain to be 100% efficient

32"=.75 x (total applied water)

32/.75=42.67" of irrigation water

Usually, less than half the rain that falls is available to plant roots. Nature is not 100% efficient either, at least not on a plant by plant basis, and often is less efficient than irrigation. On the other hand, by ecological measure nature is completely efficient, letting nothing go to waste as energy and materials cascade from one user to the next. Cultivated systems and natural systems are evaluated by quite different standards.

Activity: Why aren't plants 100% efficient and 100% predictable?

Questions for discussion:
  • What factors would reduce the water available to a plant?
  • What factors would increase the water used by a plant?
  • Why is it impossible to have the plant take up 100% of the water applied?
  • Why will the water needs of a plant always be variable?
  • What can farmers do to maximize water efficiency?
Procedure:
  1. Water 4 similar size potted plants evenly.
  2. Wait for the excess water to drain. Calculate the pot's water holding capacity.
  3. Weigh each of the plants in their pots.
  4. Place two plants outside for 24 hours in an exposed place near a weather monitoring station which records hourly wind, temperature, humidity and sunlight data. Leave the other two potted plants in the classroom and note the thermostat setting during the day and overnight.
  5. Weigh each of the plants in their pots again.

Observations:

The two potted plants placed outside will lose more water weight than the indoor plants because evapotranspiration is higher in wind, dry air, heat and sunlight. (In chilly or frosty weather, the distinction will not be as clear but those are not common growing season conditions, anyway.) This exercise can be made more elaborate by charting the data and correlating the results over several days.

Follow up:

Sun, heat, dry air, and wind will claim a proportion of water from the soil, and will increase plants' water demand. Micro climate conditions outdoors will change throughout the growing season, keeping water managers on their toes. Weather monitoring stations provide data to farmers so they can calculate the changing water demand of their plants, thus reducing over-deliveries. The type of soil also plays a role in water use efficiencies. Sandy soils allow water to penetrate to a deeper depth than clay soils, thus reducing evaporation. On the other hand, clay soils can retain more water for a longer period of time than sandy soils, thus helping uptake at the roots. (Silty soils fall in between sandy and clay soils.) Farmers try to maximize irrigation efficiencies by matching the most economical irrigation system with the needs of the plant, the type of soil, and the climate. Modern irrigation is very dependent upon data, and not a simple task.

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