APES 5.5 Irrigation Methods

Enduring Understanding:

  • When humans use natural resources, they alter natural systems. 

Learning Objective:

  • Describe different methods of irrigation.
  • Describe the benefits and drawbacks of different methods of irrigation.

Essential Knowledge:

  • The largest human use of freshwater is for irrigation (70%)
  • Types of irrigation include drip irrigation, flood irrigation, furrow irrigation, and spray irrigation.
  • Waterlogging occurs when too much water is left to sit in the soil, which raises the water table of groundwater and inhibits plants’ ability to absorb oxygen through their roots.
  • Furrow irrigation involves cutting furrows between crop rows and filling them with water.  The system is inexpensive, but about 1/3 of this water is lost to evaporation or runoff.
  • Flood irrigation involves flooding an agricultural field with water.  This system sees about 20% of the water lost to evaporation and runoff.  This can also lead to waterlogging of the soil.
  • Spray irrigation involves pumping ground water into spray nozzles across an agricultural field.  This system is more efficient than flood and furrow irrigation, with only ¼ or less of the water lost to evaporation or runoff.  However, spray systems are more expensive than flood abd furrow irrigation, and also requires energy to run.
  • Drip irrigation uses perforated hoses to release small amounts of water to plant roots.  This system is the most efficient, with only about 5% of water lost to evaporation and runoff.  However, this system is expensive and so is not often used.
  • Salinization occurs when the salts in groundwater remain in the soil after the water evaporates.  Over time, salinization can make soil toxic to plants.
  • Aquifers can be severely depleted if overused for agricultural irrigation, as has happened to the Ogallala Aquifer in the central United States.
  • 70% of freshwater is used for watering crops
    • 20% goes to industrial purposes
    • The last 10% typically goes to residential zones

Types of Irrigation

Flood

  • A body of water or river/stream is diverted toward a dug-out plot of land
  • The land is flooded, which waters the crops being grown there
    • This is typically used for water-intensive crops like rice

Pros

  • Easy
  • Inexpensive
  • Mechanization is not required

Cons

  • Water must be close by in considerable amounts
  • Depletes nearly freshwater
  • Not suited for all crops
  • Land must have a gradient for water to run past
  • Levees needed
  • 20% of water is lost to evaporation
    • Not efficient
  • Waterlogging/salinization occurs

Furrow

  • Troughs are dug between rows of crops and water is allowed to run through them

Pros

  • Low investment
  • High-sediment water can be used
  • Allows for some precision of application

Cons

  • Not efficient on sandy soil
  • Difficult to apply in small amounts
  • 33% of water is lost to evaporation
  • Soil erosion

Spray

  • Large pumps send out water and spray it directly over crops

Pros

  • Precision application
  • Supplements can be introduced into the water
  • Efficient
    • 25% or less evaporates
  • Can be programmed to run at certain times of the day

Cons

  • Larger up-front cost
  • Can include machinery run with electricity/fossil fuel use
  • Nozzles can clog
  • Pivot systems can wear ruts in soil

Drip

  • Pipes run through the entire plot, dripping water on each crop individually

Pros

  • Very low evaporation rate (5%)
  • Reduced nutrient leaching
  • No land grading needed
  • Incredibly precise distribution of water

Cons

  • Very expensive
  • Clogs easily
  • Requires mechanization
    • Remember that with construction comes fossil-fuel powered machines
  • Placement makes any other processes very difficult
  • Pipes may be damaged by pests

Impacts

Waterlogging

  • Healthy plants need some air pockets near their roots
  • Plants need oxygen too
  • Also needs room to grow
  • When you water too much, the soil becomes so saturated that there is no oxygen
  • The plant can no longer grow and is deprived of nutrients and oxygen
  • The roots can rot from too much water

Fixes

  • The best way is just to let the field dry out
  • Try to not let it happen again
  • If waterlogging is unavoidable, select plants that can tolerate constant exposure to water
  • Working sand into the soil improved drainage
  • Choose a more precise irrigation method

Salinization

  • Salt buildup in the soil
  • Freshwater does have a little salt in it
  • When water is repeatedly put into the soil then evaporates, it leaves behind that salt, which can build up
  • Salt inhibits plant growth and leaves the soil unusable
  • This causes desertification

Fixes

  • Ironically, flushing the field with more freshwater is the only way to get rid of the salt
  • This is very expensive to often times land is just left and abandoned
  • Choose salt tolerant plants
  • The best way to fix this is to prevent it from happening in the first place

Aquifer Overuse

  • The Ogallala Aquifer is a huge reserve a groundwater right in the middle of the US
  • The overuse of this and other aquifers is an example of tragedy of the commons
  • Agriculturalists draw a completely unsustainble amount of groundwater from this aquifer
    • It is not able to recharge by infiltration anywhere close to how fast we take water out

Fixes

  • The only way to protect our aquifers is to use water sustainably
  • Once we’re out, we can only wait for it to replenish, which may take a very long time