Topic 5.5 Notes – Irrigation Methods

What Irrigation Is

Irrigation means humans apply water to farmland to help crops grow when precipitation is too low, seasonal, or unpredictable. That makes farming possible in drier places and helps crops survive dry periods.

The big idea is a trade-off you’ll see all through this topic:

• More irrigation can increase crop productivity and reliability.
• More irrigation also increases human demand on freshwater.

A number you should know is that about 70% of human freshwater consumption is used for agriculture. So when APES talks about water use, farming is a huge part of the story.

The four methods you need are furrow, flood, spray, and drip. They mainly differ in:

• how water reaches the roots
• how much water is lost
• how much the system costs
• how much energy it needs

This comparison helps you preview one of the biggest differences among irrigation methods, which is how much water is lost before plants can use it.

Irrigation water losses by method. The figure groups furrow and flood together as surface irrigation; spray appears as sprinkler irrigation.

Irrigation Methods and Their Trade-Offs

These methods are easiest to remember by comparing them side by side.

Spray irrigation

Center-pivot irrigation is the classic spray system. It is the one that creates circular fields when viewed from above.

Center-pivot irrigation

One thing students miss is that spray can lose more water in hot, dry, windy conditions because droplets evaporate before reaching the soil.

Comparing Efficiency and Choosing a Method

Water-use efficiency means how much of the applied water actually stays available for crops instead of being lost to evaporation or runoff.

The ranking to memorize is:

1. Drip most efficient
2. Spray intermediate
3. Flood less efficient
4. Furrow least efficient

There’s one APES nuance here. Spray is taught as more efficient overall than flood and furrow, even though spray and flood percentages can sometimes look close. If a question gives you actual numbers, use those numbers.

The calculation is:

$$\text{Water remaining} = \text{Water applied} \times (1 - \text{fraction lost})$$

So if 1000 L is applied by drip, water remaining is $1000 \times (1 - 0.05) = 950$ L.

Choice usually follows the goal:

• Lowest cost often points to flood or furrow
• Saving water points to spray, especially drip
• Lower energy use points away from spray, because pumping and pressurizing take energy

Soil Problems Caused by Irrigation

Waterlogging

Waterlogging happens when too much water stays in the soil. The water table rises toward the roots, and soil pores fill with water instead of air. Roots need oxygen for cellular respiration, so plant growth drops and plants may die.

It is strongly linked to flood irrigation, but any method can cause it if too much water is applied.

Salinization

Salinization is salt buildup in soil. Here’s the sequence:

1. Irrigation water enters soil carrying dissolved salts
2. Water evaporates or is taken up by plants
3. Salts stay behind
4. More irrigation adds more salts
5. Salt concentration builds up over time

This is most common in dry climates and poorly drained soils. High salt levels can become toxic to plants and lower crop yields.

Waterlogging vs. salinization

• Waterlogging = too much water, too little oxygen
• Salinization = too much salt left after evaporation

They are different problems, but they can happen together.

Aquifer Depletion and the Big Environmental Consequence

A lot of irrigation water comes from aquifers, which are underground layers that store groundwater. Aquifer depletion happens when water is pumped out faster than natural recharge replaces it.

Effects include:

• falling water table
• deeper wells
• more energy needed for pumping
• less water available later

The Ogallala Aquifer in the central United States shows all of these. This map shows water-level change across the aquifer, with many areas showing substantial declines.

Water-level change in the Ogallala Aquifer

More efficient irrigation can reduce stress on aquifers, but it does not automatically solve depletion. If total pumping still exceeds recharge, or farmers use the saved water to irrigate more land, the aquifer can still decline.