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Processing tomato is an important crop in California, where ≈ 100,000 ha is grown annually. In the past, processing tomatoes have been irrigated mostly by sprinkler and furrow irrigation, although several tests have been conducted with drip irrigation, and a few growers are using subsurface drip irrigation. Yields of tomato have been shown to be sensitive to water management when the amount of irrigation water closely matches plant water use. Tomatoes have been identified as susceptible to drought stress and waterlogging at both ends of the furrow irrigation cycle. Subsurface drip irrigation is a relatively new method in which drip irrigation laterals are buried permanently 20 to 60 cm below the soil surface. This method has provided the control and uniformity of water and fertilizer distribution necessary to maximize the yield of processing tomatoes. A computerized control system maintains nearly constant soil water and nutrient concentration in the root zone by irrigating and fertilizing frequently, thus avoiding small water and nutrient stresses, especially during the critical period between first and peak bloom. During the maturation and ripening stage, irrigation and nutrient concentrations can be adjusted to increase soluble solids and to adjust the maturation rate to coincide with the harvest schedule. Maximum yield levels can be obtained when nearly all the fertilizers (N, P, and K) are injected precisely in time and space through the drip irrigation system to meet the crop nutrient requirement. Water-use efficiency (WUE), defined as the ratio of yield: unit of water used by the plant, can be maximized by using this precise irrigation and fertilization technique. Yields >200 t·ha^-1 of red tomatoes were achieved in large field plot research, and commercial yields of 150 t·ha^-1 were achieved in large-scale field applications with a lesser degree of control. Therefore, we predict that with further fine-tuning, commercial yields of 200 tons of processing tomatoes/ha could be achieved using a subsurface drip irrigation system with accurate water and fertility management.

The influence of irrigation frequency and the severity and rate of development of soil water deficits on the vegetative growth and water status of carrots (Daucus carota L. var. sativa DC.) grown for seed were investigated in a fine sandy loam soil. Beginning with the period of rapid development of primary umbels, various irrigation frequencies [daily vs. intervals corresponding to 30 mm of accumulated crop evapotranspiration (ET_c)] were investigated at irrigation rates ranging from 40% to 120% of estimated ETC. The magnitude and rate of development of soil water deficits markedly influenced carrot responses to developing water deficits. Stomata] conductance and leaf water potential (LWP) measurements exhibited some potential for use in irrigation scheduling and were the most sensitive and consistent indicators of plant water status. Under low-frequency continuous-deficit irrigation, a combination of moderate reductions in stomatal conductance and major reductions in peak leaf area and late-season maintenance of viable leaf area occurred. These responses were effective water-conserving mechanisms, allowing growth at a reduced rate and continued development of viable seed. In contrast, rapid development of soil water deficits resulted in nearly complete stomatal closure, cessation of growth, and rapid reductions in leaf area.

Seed yield and quality of carrot (Daucus carota var. sativa DC.) were influenced by a wide range of water application regimes and levels. Irrigation treatments were imposed beginning at the time of extension of the primary umbel and extending throughout the reproductive development period. The three application regimes used were: 1) a high-frequency, low water deficit treatment [100% of daily accumulated crop evapotranspiration (ET_c)]; 2) a series of five low-frequency (irrigated after 30 mm of accumulated ET,) application treatments with a range of water deficits from moderate to minimal (40% to 120% of ET_c applied); and 3) a series of three treatments that had rapidly developing water deficits applied by terminating irrigation at 7, 5, and 2 weeks before harvest after being grown under low-stress conditions. Pure live seed (PLS) yield was optimized by different treatments within each of the three application regimes. Maximum yields were achieved with short-term (2-week) rapidly developing water deficits near harvest, moderate deficit irrigation with 60% to 80% of ET_c applied after 30 mm of ET_c, or with a low water deficit, high-frequency application. Seed germination percentage decreased as the amount of applied water increased. This effect was more pronounced in the later-developing umbel orders. However, seed quality measured as seedling root length was improved with increasing water application.