Water must be provided to replace that lost by evapotranspiration (ET), the combination of soil surface evaporation and water loss from plant leaves. Drip irrigation is an efficient way to deliver water and nutrients in a tunnel, while keeping the foliage dry, which reduces disease pressure. Drip tape is usually 8-10 mil thickness and is laid on the surface or buried an inch or two. Flow rates of drip tapes vary, a medium–flow tape provide 0.5 gpm per 100 feet. High-flow tape with 1.0 gpm flow is useful to prevent clogging and reduce irrigation time. Drip lines should be spaced on or under the soil surface to assure that the entire bed or row is wetted. Light-textured soils have less capillary movement of water than heavier soils, so in these soils, more drip lines may be needed to prevent dry areas in the bed. Many systems are available to add soluble fertilizer to irrigation water. Fertigation is a good way to provide plants with the nutrients they need over the season, or to supplement fertilizers applied at planting. Irrigation water quality should be tested. Water with high pH and high alkalinity may lead to increased soil pH over time.

A mature crop of tomatoes may require 2.5 quarts of water per plant per day, whereas winter greens may grow well only on existing soil moisture, and if irrigated, may develop disease. Determining the optimal amount and timing of irrigation is complicated since it depends on the crop, its stage of growth, soil texture, sunlight, temperature and humidity. Use of soil moisture sensors placed at several locations and depths in each tunnel, combined with irrigation and crop performance records, can help determine best practices on your farm.