Flood irrigation is one of the oldest ways to water crops, using gravity to spread water across a field. It can be simple and low-tech, but efficiency losses and soil impacts shape its sustainability.

Overview of flood irrigation

Flood irrigation applies water by flooding all or part of a field so water flows over the soil surface and infiltrates to plant roots.

Flood irrigation is common where water can be delivered by canals or ditches and where fields can be leveled to help water spread evenly.

Field photograph showing basin-style surface irrigation, where water is intentionally ponded behind low earthen berms so it can infiltrate into the soil. The visible standing water helps students connect the method to evaporation exposure (sun/wind over open water) and to waterlogging risk when ponding persists and drainage is limited. It also reinforces why field leveling and containment structures matter for uniform distribution. Source

How the method works (field-scale process)

Water delivery and movement

• Water is released at the upper edge of a field or into a basin.

• Gravity drives water across the surface; some infiltrates downward to the root zone.

• Excess water may leave the field as runoff (sometimes called “tailwater” in agricultural settings).

• Standing water exposed to sun and wind increases evaporation losses.

Diagram illustrating how inflow rate affects where water goes during surface irrigation. Large inflows can push water across the field quickly but increase tailwater (surface runoff), while small inflows can reduce runoff but increase deep percolation below the root zone. This visual supports the idea that poor timing and uneven intake can convert applied irrigation water into off-site losses rather than root-zone storage. Source

Why losses occur

• Uneven field slope or roughness causes ponding in some areas and under-watering in others.

• If application continues after the soil’s intake rate drops, more water becomes runoff or remains ponded.

Benefits (why it is used)

Practical and economic advantages

• Often low capital cost compared with pressurised systems (few moving parts; minimal equipment).

• Can operate with limited energy input when gravity-fed (less pumping in some locations).

• Works well at large scales where water is readily available and fields can be managed as basins.

Agronomic advantages in some contexts

• Surface flooding can wet a broad soil area, which may help establish certain crops where uniform germination moisture is needed.

• When well-managed, it can provide adequate root-zone water without complex technology.

Drawbacks and environmental/agronomic impacts

Water inefficiency: evaporation and runoff

The syllabus emphasises that roughly 20% of the water can be lost to evaporation and runoff.

• Evaporation increases with high temperatures, wind, low humidity, and long ponding times.

• Runoff increases when water is applied faster than it can infiltrate or when the field is not evenly leveled.

• These losses reduce irrigation efficiency, increasing withdrawals from rivers, reservoirs, or groundwater supplies.

Waterlogging risk

Flood irrigation can lead to waterlogging, especially when drainage is poor or application is excessive.

• Saturated soil slows gas exchange; oxygen depletion stresses roots.

• Root stress can reduce nutrient uptake and crop productivity.

• Persistently saturated conditions can alter soil structure, increasing compaction and reducing future infiltration, which can further worsen ponding.

Field management challenges

• Achieving uniform water depth is difficult on uneven terrain, which can create:

  • Under-irrigated zones (water stress)

  • Over-irrigated zones (greater evaporation, runoff, and waterlogging risk)

• Timing is sensitive: longer flooding durations generally increase both evaporation and the likelihood of standing water.

Off-site impacts (linked to runoff)

• Runoff leaving fields can transport sediment and any dissolved substances present in field water, contributing to downstream water-quality concerns.

Photograph of agricultural runoff (tailwater/field drainage) being measured as it exits an experimental plot through a weir. It demonstrates how water that does not infiltrate into the soil can leave the field as surface flow, carrying suspended sediment and dissolved constituents to downstream waters. This is the physical pathway behind irrigation-related runoff losses and water-quality impacts. Source

• Even when not heavily contaminated, runoff represents “lost” water that could otherwise support crop growth.

Key takeaways for AP Environmental Science

• Flood irrigation is simple and can be economical, but its sustainability is constrained by predictable efficiency losses.

• The AP-relevant drawbacks to remember are:

  • About 20% water loss to evaporation and runoff

  • Potential for waterlogging with prolonged saturation and poor drainage