APES 5.3-5.4: Irrigation and Pest Control

Irrigation

The controlled application of water to agricultural land, landscapes, lawns, or disturbed soils to support plant growth. It can supplement rainfall or provide nearly all of a crop’s water.

Why can irrigation be important even when its primary purpose is supplying water for plant growth?

Irrigation can help protect crops from frost, suppress weeds, prevent soil consolidation, cool livestock, reduce dust, dispose of sewage, and support mining. Irrigation is often paired with drainage to maintain productive soil conditions.

Supplementary irrigation versus full irrigation

Supplementary irrigation adds water when rainfall is insufficient. Full irrigation supplies nearly all crop water needs and is most common in very arid areas or during dry seasons.

Groundwater and surface water as irrigation sources

Groundwater is obtained from springs or wells, whereas surface water is withdrawn from rivers, lakes, or reservoirs. Excessive groundwater pumping can deplete aquifers.

Nonconventional irrigation-water sources

Possible sources include treated wastewater, desalinated water, agricultural drainage water, fog or atmospheric condensate, and harvested floodwater.

How does rainwater harvesting differ from floodwater harvesting for irrigation?

Rainwater harvesting commonly collects and concentrates runoff from roofs or unused land. Floodwater harvesting, or spate irrigation, diverts temporary flood flows into normally dry channels and fields.

Why can treated wastewater be useful for agricultural irrigation?

It can provide a relatively consistent water supply and may cost less than some alternatives. Dissolved nitrogen, phosphorus, and potassium can also act as fertilizer.

What chemical and biological hazards may accompany agricultural use of untreated or inadequately treated wastewater?

Wastewater may contain pathogens, heavy metals, pharmaceuticals, pesticides, and other organic or inorganic pollutants. These contaminants can harm human health, plants, and soil depending on their concentrations and environmental fate.

Why does increasing competition for water make irrigation management more important?

Agriculture competes with cities, industry, and biofuel production for limited water. Population growth, higher food consumption, and water-intensive animal agriculture increase pressure to produce more food per unit of water.

Surface irrigation

An irrigation method in which water flows over the land surface and infiltrates the soil primarily under gravity. Furrow, border-strip, basin, and flood irrigation are forms of surface irrigation.

Furrow irrigation

A surface-irrigation method in which water flows through shallow channels, or furrows, between rows of crops. Water infiltrates laterally and downward into the soil around the plants' roots.

Flood or basin irrigation

A surface-irrigation method in which water is released onto and spreads across a relatively level field or enclosed basin. It requires little specialized equipment but can waste water through runoff, evaporation, or uneven infiltration.

Why does surface irrigation generally require less capital and energy than pressurized irrigation?

It uses gravity and the slope of the land to move water, so it needs less pumping and specialized equipment. However, it usually provides less control over application depth and therefore has lower water-application efficiency.

Micro-irrigation

A low-pressure, low-flow system that distributes water through pipes and applies small discharges near individual plants. Drip emitters, subsurface drip systems, micro-sprays, micro-sprinklers, and mini-bubblers are examples.

Drip irrigation

A micro-irrigation method that releases water slowly at or near each plant’s root zone. It can minimize evaporation and runoff and typically has a field water efficiency of about $80\%-90\%$ when properly managed.

How can drip irrigation cause water loss despite being highly efficient?

If emitters release water too rapidly or operate too long, water can move below the root zone by deep percolation. This water is generally unavailable to the crop and may contribute to groundwater contamination or rising water tables.

Fertigation

The application of dissolved fertilizer through an irrigation system, commonly through drip irrigation. It can deliver nutrients directly to the root zone while reducing some losses.

Sprinkler or overhead irrigation

Water is transported through pipes and distributed above the field by pressurized sprinklers, sprays, or guns. Because water is exposed to the atmosphere, evaporation and wind-related losses can be important.

Center-pivot irrigation

A sprinkler system made of connected pipe sections supported by wheeled towers that rotate around a central water-supply point. The rotating structure irrigates a roughly circular field; drop nozzles positioned close to crops can reduce evaporative losses.

Low Energy Precision Application (LEPA)

A center-pivot configuration that uses drop tubes or hoses to place water close to the soil surface or between crop rows. Reducing the distance water travels through the air lowers evaporation and can improve application efficiency.

Lateral-move or wheel-line irrigation

A series of sprinkler-bearing pipe sections mounted on wheels is moved across a field in strips. It is less expensive than a center pivot but requires more labor because the line must be drained, repositioned, and reconnected.

Subirrigation

A method that artificially raises the water table so soil is moistened from below the plants’ root zone. Ditches, canals, pumps, weirs, and gates may regulate the water table; greenhouse versions can recycle water and nutrients.

Field water efficiency

The fraction of applied irrigation water that is transpired by the crop, expressed as a percentage: $\text{Field water efficiency}=\frac{\text{water transpired by crop}}{\text{water applied \to field}}\times100$.

What does a low field water-efficiency value indicate?

Much of the applied water is not transpired by the crop; it may be lost through evaporation, runoff, leakage, or deep percolation. Improving uniformity and applying only the crop’s required amount can increase production per unit of water.

Why can improved irrigation efficiency reduce both environmental impacts and farm costs?

Using less water or applying it more precisely reduces water waste, runoff, pollution, and deep drainage. It also decreases the energy and fuel required to pump water or maintain operating pressure.

How can over-irrigation lead to soil salinity?

Excess irrigation can raise the water table through deep drainage. As water moves upward and evaporates, dissolved salts can become concentrated in the root-zone soil, reducing plant growth unless drainage or water-table control removes the salts.

How can poor irrigation distribution affect water quality?

Uneven or excessive application can waste water and dissolved chemicals, increase runoff, and transport nutrients or pollutants into surface water. Over-irrigation can also promote deep drainage and contamination of groundwater.

What is waterlogging, and how can irrigation cause it?

Waterlogging occurs when excess water saturates the soil and fills air spaces needed by plant roots. Over-irrigation or inadequate drainage can raise the water table, reducing root oxygen and impairing crop growth.

Approximate seasonal water requirement of wheat, barley, or oats

These crops require approximately $450\text{–}650\ \mathrm{mm}$ of water over the growing season. A millimeter of water over a square meter corresponds to approximately one liter.

How do seasonal water requirements compare for sugarcane and onions?

Sugarcane requires approximately $1500\text{–}2500\ \mathrm{mm}$ per growing period, whereas onions require about $350\text{–}550\ \mathrm{mm}$. The large difference illustrates that crop choice strongly affects irrigation demand.

Why is drainage often needed together with irrigation?

Irrigation adds water to the soil, while drainage removes excess surface or subsurface water. Together they help prevent waterlogging, rising water tables, and salt accumulation while maintaining suitable root-zone conditions.