5.5 - Irrigation Methods

Learning Objectives

The main objective is to describe different irrigation methods and explain the benefits and drawbacks of each. It’s also essential to look at the environmental effects of overwatering to understand how to avoid them. The associated skill is describing the disadvantages, advantages, or unintended consequences of potential solutions.

I. Global Water Use

Globally, agriculture is the number one use of water, accounting for 70% of all water use. That water mostly goes to irrigating crops and providing water to livestock.

After agriculture, global water gets used for industrial purposes (manufacturing or power plants) and municipal settings (people’s homes, showers, and baths).

II. Irrigation Methods: Efficiency and Cost

Irrigation methods vary a lot in efficiency, meaning how much water is lost to runoff or evaporation, and in cost.

Irrigation Method	Description	Advantages	Disadvantages
Furrow Irrigation	Cutting furrows (trenches) between crop rows and filling them with water.	Cheap and easy.	Least efficient method. About 1/3 (33%) of the water is lost to evaporation and runoff.
Flood Irrigation	Flooding the whole field with water.	Very cheap and easy.	About 20% of the water is lost to evaporation and runoff. Can waterlog the soil, which only some crops (like rice and cranberries) tolerate.
Spray Irrigation	Pumps groundwater through sprinklers or spray nozzles across the field.	More efficient than furrow and flood: only about 1/4 (25%) or less of the water is lost to evaporation or runoff.	A lot more expensive because of the equipment, and it needs energy (usually fossil fuels) to run the pumps.
Drip Irrigation	Uses perforated hoses or lines to release small amounts of water right at the plants' roots.	Most efficient: only about 5% of the water is lost, so it's about 95% efficient. Conserves water and helps prevent waterlogging and salinization.	Very expensive, so it isn't used often. It's impractical for large commodity crops like corn or wheat, so it's mostly used for vegetables, orchards, and smaller areas.

Ranking for the exam: drip is the most efficient, then spray, then flood, then furrow (the CED's order). The loss numbers in the table are the CED's rough figures. Real losses vary a lot with the system, the weather, and the soil, which is why spray's "1/4 or less" can overlap flood's 20%.

Example calculation: A 10-hectare cornfield needs 500 mm of water to reach the crop's roots over one growing season. How much water does the farmer have to apply with furrow irrigation (about 1/3 lost) compared with drip irrigation (about 5% lost)?

Water the crop needs: $10 \text{ ha} \times \frac{10{,}000 \text{ m}^2}{1 \text{ ha}} \times 0.5 \text{ m} = 50{,}000 \text{ m}^3$ (500 mm is 0.5 m)

Furrow: only 2/3 of the water reaches the roots, so the farmer must apply $\frac{50{,}000 \text{ m}^3}{2/3} = 75{,}000 \text{ m}^3$

Drip: 95% of the water reaches the roots, so the farmer must apply $\frac{50{,}000 \text{ m}^3}{0.95} \approx 52{,}600 \text{ m}^3$

Water saved by switching to drip: $75{,}000 \text{ m}^3 - 52{,}600 \text{ m}^3 \approx 22{,}400 \text{ m}^3$ , about 30% less water.

III. Environmental Consequences of Overwatering

Overwatering crops can lead to two major forms of soil degradation: waterlogging and soil salinization.

A. Waterlogging

Waterlogging happens when too much water is left sitting in the soil, filling up all of its pore space.

Impact: When the pores are filled with water, air gets displaced. Plant roots can't take in the oxygen they need, which can stunt plant growth or kill the plants outright if they aren't naturally tolerant of fully saturated soils.

Rising water table: Extra water keeps soaking down and raises the water table (the top of the groundwater). Once the water table sits close to the surface, the root zone stays soggy all the time.

Solutions:

Use drip irrigation to minimize the water added.
Perform soil aeration, either stabbing the soil with a pitchfork or using a machine to pull small cores out. Aeration creates channels for water to drain and air to reach the roots, which leads to deeper root growth over time.

B. Soil Salinization

Salinization is the excessive buildup of salt in the soil over time.

Mechanism: This usually happens when farmers use groundwater for irrigation. Groundwater naturally contains small amounts of salt. As the pumped water goes onto the crops, the water evaporates and leaves the salt behind. Over years, the salt concentration gets so high that it keeps plants from getting the water they need and can make the soil toxic to plants.

Where it's worst: Hot, dry places with heavy irrigation and poor drainage, where a lot of water evaporates. Parts of California's San Joaquin Valley have had to stop farming some fields because of salty, poorly drained soil.

Solutions:

Use drip irrigation to minimize water input.
Switch to a fresh water source like streams or lakes instead of groundwater.
Aerate the soil so water drains deeper, washing the salt down away from the plant roots.
Flood or rinse the soil with a lot of fresh water to carry the salt completely out.

IV. Aquifers and Groundwater

Groundwater is water stored in the pore space of rock and sediment layers below the water table. Aquifers are groundwater deposits that humans can actually use.

Recharge: Aquifers are replenished by groundwater recharge, which is rainwater that trickles down through the soil and gets stored underground.

Unconfined Aquifers: Sit just below the surface with no confining rock layer above them. They recharge quickly.

Confined Aquifers: Have a layer of impermeable rock both above and below.

Recharge Rate: They only get recharged in very small recharge areas, so they recharge very slowly.
Risk: Confined aquifers are longer-term water storage, but they’re far more prone to depletion or overuse because humans can pull water out of them much faster than nature can replenish it.

Aquifer depletion (the Ogallala Aquifer): The Ogallala Aquifer (also called the High Plains Aquifer) lies under parts of eight states in the central US, from South Dakota to Texas. According to the USGS, about 30% of the groundwater used for irrigation in the US has come from it.

Heavy pumping for irrigation started around 1950. Farmers pump far faster than the aquifer recharges, because very little rain soaks down that deep in this dry region. Water levels have dropped the most in Kansas and Texas.
By 2017 (latest USGS report), the water level had dropped an average of 16.8 feet since pumping began, and as much as 262 feet in some wells. About 292 million acre-feet of water were gone, roughly 9% of what the aquifer held before pumping.
Where wells run dry, farmers have to switch to dryland (rain-fed) crops, which yield much less. It's also a tragedy of the commons (5.1): each farmer pumps for their own fields, but everyone draws from the same shared aquifer.

Big picture

Irrigation methods are a trade-off between cost and conservation. Cheap methods like furrow and flood irrigation are like leaving a garden hose running constantly. The plants get water, but huge amounts go to waste. Expensive methods like drip irrigation are like a high-precision water dispenser, delivering about 95% of the water to the roots. But excessive use of any method can cause waterlogging, which drowns the roots by filling their air pockets, or salinization, which poisons the plants by building up residual salts and turning the soil into unusable brine.

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