Irrigation Systems And Water Management

The amount of water that needs to be applied to the crop is called the irrigation requirement (IR). Water use efficiency or application efficiency (AE) is the fraction of the water applied that is available to the plant. Water losses from evaporation or wind drift, surface runoff, deep percolation out of the root zone, and leaks in irrigation pipes lower the AE of the system.² Surface (furrow, flood, seepage) irrigation systems have AEs between 20 and 70% (over 50% if tailwater is recovered). Overhead sprinkler systems have AE values of 60 to 80%, and drip systems have AE values of 80 to 95%.² The amount of water that a crop needs for proper growth and development is referred to as the evapotranspiration requirement of the crop (ETc). The ETc varies with the type of crop, the growth stage of the crop, and the evaporative demand of the crop, which is affected by temperature, humidity, and wind. The crop canopy size, plant population, method of irrigation, and soil type also influence the ETc level.³ The ETc for a crop is calculated by multiplying the base evapotranspiration value (ETo) by a crop coefficient (Kc) using the formula ETc =ETo*Kc. The ETo value is based on the amount of water that evaporates from a uniform green surface. Kc values vary with the crop, stage of growth, and the cropping system (bare ground vs. mulched) and region. The Kc value repersents the fraction of the ET of a fully-irrigated reference crop that is used by the actual crop. So if the Kc of a specific crop at a specific growth stage in a specific region is 0.45, then the crop will use about 45% of the ET of the fully irrigated reference crop. Typical ETo and Kc values (Tables 1 and 2) are vailable in some vegetable growing regions from agencies such as UF-IFAS and the California Department of Water Resources.^2,3 Estimates of ETo can also be made directly in the field using monitoring equipment. The irrigation requirement (IR) is then calculated by dividing the ETc value by the application efficiency (AE) value for the irrigation system, IR=ETc*AE. If a crop required 1 inch of water per week to grow, the grower would need to apply 2 inches of water using a furrow irrigation system with an AE of 50%, 1.4 inches of water using a sprinkler system with an AE of 70%, and 1.1 inches of water using a drip system with an AE of 90%. The goal of water management is to apply the required amount of water at the appropriate time.

Applying too little water when it is needed leads to plant stress and potentially lower yields. Early-season water deficits can delay plant maturity and reduce production. Late-season deficits can reduce yield and product quality. Applying too much water is a waste of water and money. Too much water also leaches nutrients out of the root zone, can increase the severity of root diseases, and can result in reduced yield and product quality.^2,4,3

Irrigation should be applied to keep water levels in the root zone between the soil’s field capacity (the maximum amount of water that the soil can hold) and the point at which plants start to be stressed from a lack of water, the range of soil moisture levels called the “safe irrigation zone” (Figure 2).² The frequency and amount of irrigation needed to keep moisture levels in the safe irrigation zone depend on the water holding capacity and infiltration rates of the soil, weather conditions, and crop demands.² Rainfall levels should also be factored into determining the need for irrigation in non-mulch systems. In systems using plastic mulch, the mulch can prevent most of the rain-water from reaching the root zone, and rainfall contributes little to meeting the water requirements of the crop, especially on sandy soils. In soils with low water holding capacities, the calculated volume of water needed per day may be more than the soil can hold. In this case, irrigation events should be split over the day so that the amount of water added per application does not exceed the maximum amount that the soil can hold. If conditions require the application of 5,000 gallons of water per acre per day, but the maximum water application for the soil is 2,000 gallons, then the water will need to be applied over at least three applications.

¹ _{Ozores-Hampton, M., McAvoy, E., Muchovej, R., Hanlon, E., Shukla, S., and Roka, F. 2017. Management of soil and water for vegetable production in southwest Florida. University of Florida IFAS Extension Publication #SL-233. https://edis.ifas.ufl.edu/ss452.}

² _{Zotarelli, L., Dukes, M., Liu, G., Simonne, E., and Agehara, S. 2018. Principles and practices of irrigation management for vegetables. 2018 Vegetable Production Handbook. UF-IFAS.}

³ _{Snyder, R. and Pruitt, W. 1992. Evapotranspiration data management in California. In Irrigation and Drainage: Saving a Threatened Resource – In Search of Solutions, Proceedings of the Irrigation and Drainage Sessions at Water Forum ‘92.}

⁴ _{University of Massachusetts. 2013. Irrigating vegetable crops. https://ag.umass.edu/vegetable/fact-sheets/irrigating-vegetable-crops.}

_{Websites verified 2/14/2019}

For additional agronomic information, please contact your local seed representative.

Performance may vary from location to location and from year to year, as local growing, soil and weather conditions may vary. Growers should evaluate data from multiple locations and years whenever possible and should consider the impacts of these conditions on the grower’s fields. The recommendations in this article are based upon information obtained from the cited sources and should be used as a quick reference for information about irrigation and water management. The content of this article should not be substituted for the professional opinion of a producer, grower, agronomist, pathologist and similar professionals dealing with vegetable crops.

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