Irrigation Scheduling and Water Management for Crops

Irrigation Scheduling and Water Management for Crops

Irrigation is one of the most important management decisions a grower makes. Too little water stresses crops and reduces yield; too much wastes a costly resource, promotes disease, leaches nutrients, and can lead to runoff that harms waterways. Evapotranspiration-based scheduling is the standard professional approach because it ties irrigation decisions to actual plant water use and weather conditions.

What Is Evapotranspiration?

Evapotranspiration (ET) is the combined loss of water from a cropped field through two pathways: evaporation from the soil surface and transpiration through plant leaves. ET is typically expressed in inches per day or inches per week. It is driven by temperature, solar radiation, humidity, and wind speed. Reference ET (ETo) is calculated for a standard reference crop — usually short, well-watered grass — and serves as a baseline. Your state's Cooperative Extension service and USDA publish ETo values by region and by week of the growing season.

Crop Coefficients (Kc)

Different crops use water at different rates depending on their growth stage, canopy size, and physiology. The crop coefficient (Kc) adjusts ETo to reflect a specific crop's actual water use:

Crop ET (ETc) = ETo × Kc

Kc values are published in Cooperative Extension guides and USDA Irrigation and Drainage Paper 56 (FAO-56), which is the primary reference for this topic. A young seedling has a low Kc (often 0.3–0.5). A mature, fully canopied crop at peak growth may have a Kc near 1.0 or above. As the crop matures and senesces, Kc drops again. Always use the Kc that matches the current growth stage.

Field Capacity and Permanent Wilting Point

Soil holds water in pores between particles. Field capacity is the maximum amount of water a well-drained soil can hold after gravity has pulled free water away — usually reached one to three days after heavy rain or irrigation. Permanent wilting point is the soil moisture level at which the water remaining is held so tightly by soil particles that plant roots can no longer extract it, and the plant wilts permanently. The difference between these two values is called plant-available water (PAW). Irrigation should be applied before soil moisture drops to a threshold called the management allowable depletion (MAD) — commonly set at 50% of PAW for most vegetable and field crops — to avoid yield-reducing stress.

Calculating Net Irrigation Need

To find how much irrigation water to apply in a given week:

1. Multiply ETo by Kc to get ETc for the week.

2. Subtract any effective rainfall (rainfall that actually enters the soil and is available to roots — not runoff).

3. The result is the net irrigation need for that period.

Net Irrigation Need = ETc − Effective Rainfall = (ETo × Kc) − Effective Rainfall

All values should be in the same unit, typically inches per week.

Converting Water Depth to System Run-Time

Once you know how many inches of water to apply, you need to convert that to minutes of run-time for your irrigation system. For a drip system, the key piece of information is the emitter flow rate (usually gallons per hour, GPH) and the spacing of emitters, which together determine how many inches of water the system applies per hour.

A common conversion: 1 inch of water per 1,000 square feet equals approximately 623 gallons. Knowing your total irrigated area and total system flow rate, you can calculate run-time:

Run-Time (hours) = Water Volume Needed (gallons) ÷ System Flow Rate (gallons per hour)

Cooperative Extension worksheets provide these formulas for specific emitter spacings and system configurations.

Irrigation Methods and Efficiency

Drip irrigation delivers water directly to the root zone through emitters. It is the most efficient method, with application efficiencies commonly cited at 85–95%. It reduces foliar disease by keeping leaves dry and minimizes evaporation.

Overhead (sprinkler) irrigation wets the entire canopy and soil surface. Application efficiencies are generally lower than drip, commonly 70–85%, depending on wind and design. It is practical for large fields and can be used for frost protection.

Furrow irrigation delivers water in channels between crop rows by gravity flow. It is low-cost but has the lowest efficiency of the three methods due to deep percolation and runoff at the end of rows. It is often used for row crops on flat or slightly sloped land.

Choosing a method involves matching crop type, field topography, water availability, and cost. High-value vegetable crops often justify the capital cost of drip; grain crops on large, flat fields may be served well by furrow or center-pivot sprinklers.

Water Conservation and Environmental Responsibility

Over-irrigation contributes to nitrate leaching into groundwater, salinity buildup, and erosion. In many states, water use for agriculture is regulated, and users must hold water rights or permits. Maintaining accurate irrigation records helps demonstrate compliance and supports crop management decisions year to year.

Safety Notes

When installing or inspecting buried drip tape or overhead risers, locate all underground utilities before digging. Pumps and pressurized lines store significant energy — follow lockout/tagout (LOTO) procedures before servicing any pump, valve, or filter assembly. Wear appropriate PPE (gloves, safety glasses) when handling pipe cement, lubricants, or chemical injectors (fertilizer or chemigation systems). Chemigation — injecting pesticides or fertilizers through an irrigation system — requires a properly functioning anti-siphon device to prevent backflow into the water supply, and in most states requires a pesticide applicator license for pesticide injection.