Flood Irrigation

Part of Irrigation

Flood irrigation is the oldest and most widely used irrigation method in human history. Water is released onto a field and allowed to flow freely across the surface, soaking into the soil as it spreads. In its simplest form it requires no infrastructure beyond a water source, a channel, and a way to open and close the water supply. Despite its simplicity, flood irrigation fed civilizations for millennia. For communities rebuilding after collapse, it is the easiest irrigation system to establish quickly, requiring only earthmoving tools and labor.

Types of Flood Irrigation

Flood irrigation covers a range of methods from completely uncontrolled to carefully managed. Understanding the spectrum helps choose the right approach for available resources.

Method	Control Level	Water Efficiency	Infrastructure Needed
Wild flooding	None	Very low (20–40%)	None — open gate and walk away
Basin flooding	Medium	Moderate (40–60%)	Level basin with bunds
Furrow flooding	Higher	Moderate–good (50–70%)	Furrows between crop rows
Border strip flooding	Good	Good (50–70%)	Leveled strips with borders

This article focuses on basin flooding — the most practical form for initial establishment — and improvements to raise efficiency.

Basin Flooding

A basin is a flat, level area of ground enclosed by earthen bunds (low ridges). Water is released into the basin and held until it soaks into the soil. Because the basin is level, water distributes evenly across the surface.

Basin flooding is ideal for:

• Flood-tolerant crops: rice, taro, lotus
• Orchards and trees (basin around each tree)
• Field crops that can tolerate brief inundation: wheat, barley, sorghum
• Pasture and hay fields

Layout and Construction

1. Choose a relatively flat area. A slope of less than 0.2% is ideal; slopes up to 0.5% are manageable with careful construction.
2. Measure and stake out the basin perimeter. Basin size depends on water supply and leveling precision.
3. Excavate the interior slightly and use the soil to build perimeter bunds 20–40 cm high and 30–50 cm wide at the base.
4. Level the basin interior by moving soil from high spots to low spots. Check using a water-filled hose as a level: place both ends on stakes of the same height and compare water levels across the basin.
5. Compact all bunds by treading or tamping to prevent erosion when water is applied.

Basin Sizing Guide

Water Supply	Recommended Basin Size	Notes
Low (1–5 L/s)	0.05–0.1 ha (500–1000 m²)	Small basins fill faster, less risk of uneven wetting
Moderate (5–20 L/s)	0.1–0.5 ha	Standard farm basin
High (20–50 L/s)	0.5–2.0 ha	Large-scale grain production

Larger basins require more precise leveling. Without precise leveling, large basins develop deep water in low spots and dry areas at high spots.

Level First, Irrigate Second

The single most important factor in basin flooding performance is level soil. An unlevel basin wastes water, drowns part of the crop, and leaves another part dry. Spend extra time on leveling before the first irrigation. For every hour spent leveling, expect several seasons of improved water distribution.

Filling a Basin

Water enters the basin through a break or gate in one bund, typically at the highest point or from the head of a supply ditch. Fill rapidly — a fast fill ensures water reaches all parts of the basin before significant infiltration occurs.

Target fill time: cover the basin surface within 10–30 minutes for most crops. Slow filling over several hours means the first areas wetted receive far more water than the last.

Required inflow rate to fill a 1,000 m² basin in 20 minutes to 10 cm depth:

• Volume = 1,000 m² × 0.1 m = 100 m³ = 100,000 L
• Rate = 100,000 L / 1,200 seconds = ~83 L/s (too fast for most sources)
• Practical approach: apply 5–8 cm depth (50,000–80,000 L), fill over 30–60 minutes with 15–25 L/s

Adjust depth based on crop water requirement and soil type.

Water Depth Guide by Crop

Crop	Application Depth	Frequency (dry season)
Rice (paddy)	5–10 cm maintained	Continuous or every 3–5 days
Wheat, barley	5–8 cm applied, drained	Every 10–20 days
Sorghum, millet	4–7 cm applied, drained	Every 10–18 days
Orchards (tree basins)	10–20 cm applied, soaks in	Every 14–28 days
Pasture and hay	5–10 cm applied	Every 7–14 days

After filling, close the inlet and allow water to soak in completely before the next irrigation event.

Inlet and Outlet Structures

Simple Earthen Cut

The simplest inlet: dig a notch through the bund at the inlet point. Block with a bundle of straw wrapped in burlap, a cut sod block, or a tightly packed wad of clay. Remove to irrigate, replace when done.

Earthen cuts erode with repeated use. Reinforce the sides of frequently used cuts with flat stones or wooden boards.

Wooden Gate (Check Gate)

A wooden plank with soil packed on both sides serves as a gate. Drop the plank into a notch cut in the bund; lift it to irrigate. For a permanent installation, construct two parallel timber guides sunk into the bund and slide the gate plank between them.

Concrete or Fired-Clay Pipe Inlet

For permanent systems, install a short section of pipe (10–20 cm diameter) through the bund at inlet level. Cap with a wooden plug. Remove the plug to irrigate. This eliminates bund erosion at the inlet point entirely.

Efficiency Improvements

Unimproved basin flooding typically achieves 40–60% efficiency. Practical improvements:

1. Level the Basin Precisely

Each centimeter of leveling improvement reduces over-application in low spots. Hand leveling with a board drag (a 2–3 m wide plank pulled across the wet soil surface) achieves reasonable results without machinery.

2. Rapid Fill, Then Cut Off

Fill the basin as fast as your water supply allows. Cut off inflow as soon as the desired depth is reached. Water continues soaking in (the “opportunity time” effect) after inflow stops. Waiting until all visible water disappears before closing the inlet wastes water.

3. Manage by Soil Feel

Check soil moisture at 20 cm depth before each irrigation. Irrigate when this layer is dry or barely moist. If it still has moisture, wait — over-irrigation is more common than under-irrigation in flood systems.

4. Night Irrigation

Irrigating at night reduces evaporation loss by 15–25% compared to midday irrigation in hot climates. In a community with multiple basins to fill, night irrigation also spreads labor across a longer period.

5. Short, Frequent Irrigations vs. Long, Infrequent

Applying 5 cm twice as often as needed is more efficient than applying 10 cm half as often, because less water drains below the root zone on each pass. Adjust based on your water delivery schedule and labor availability.

Common Problems

Problem	Cause	Solution
Dry patches in the basin	Unlevel surface	Re-level with soil drag when soil is moist
Bund erosion at inlet	High inflow velocity	Line inlet area with stones; slow fill rate
Waterlogging after irrigation	Insufficient drain time; poor soil drainage	Install outlet plug or drain; wait longer between events
Bunds breached by children or animals	Low bunds, heavy traffic	Raise and compact bunds; fence if possible
Crops drowned in low spots	Unlevel basin; too much water applied	Reduce application depth; re-level

Rice Paddy as a Special Case

Flooded rice fields are a specialized form of basin irrigation. Puddled (deliberately compacted) soil forms an impermeable layer that slows drainage and holds standing water. Paddy basins are kept flooded to 5–10 cm for most of the growing season.

Puddling process:

1. Flood the basin to 5–10 cm.
2. Work the wet soil thoroughly by foot, animal, or a hand puddler (a weighted disc dragged across the surface). This collapses soil structure and creates a dense, nearly impermeable plow pan at 15–25 cm depth.
3. Transplant rice seedlings into the puddled mud.
4. Maintain a 5 cm water depth throughout the vegetative phase.

Puddled paddy soils hold water for 10–30 times longer than unpuddled soil, dramatically reducing irrigation demand after the initial flood.

Puddling Destroys Soil Structure

Puddling the soil for rice severely damages its structure for subsequent upland crops. If you rotate rice with vegetables or grains, plan for one or more seasons of organic matter addition and tillage to restore soil structure after rice. Do not puddle fields intended for long-term mixed cropping.

Flood Irrigation Summary

Flood irrigation releases water onto a level basin enclosed by earthen bunds, relying on gravity to distribute water across the field surface. Basin size should match water supply — 500–1,000 m² for low-flow sources, up to 2 ha for large flows. Precise leveling is the single most important factor in uniform water distribution. Fill basins rapidly to ensure even coverage, then cut off inflow. Unimproved efficiency is 40–60%; night irrigation, precise leveling, and soil-moisture-based scheduling raise this to 60–75%. Rice paddy is a specialized form with deliberately puddled soil to retain standing water, but puddling degrades soil structure for subsequent non-rice crops.