The Role of Irrigation and Wastewater: Comparison of Subsurface Drip Irrigation and Furrow Irrigation

This study was conducted to compare two different irrigation systems, subsurface drip irrigation (SDI) and furrow irrigation (FI) in terms of water use efficiency as well as viral contamination and survival when tertiary effluent is used for irrigation water. The effluent was injected with bacteriophages of PRD1 and MS2. The water use efficiency (WUE) of the SDI system in 2001 was significantly higher than that of the FI system (p-value < 0.05), which is consistent with previous research results. Since the SDI system supplies water close to the root zone, the crop can use water more efficiently, while furrowed irrigated water has a greater chance of evaporation. A greater number of PRD1 and MS2 were recovered from the lettuce in the SDI plots as compared to those in the FI plots due to the direct contact of the lettuce leaves with the irrigation water. The loose soil structure was caused by plowing the fields in the first year and by incomplete displacement of the preferential water paths during the second year. The cumulative degree days required for 99.9 % viral die-off were calculated in combination with inactivation rates of viruses and average degree days. Greater cumulative heat units were required in the subsurface while fewer were necessary for the same viral reduction on the surface of the lettuce. MS2 seemed to be more persistent in humid conditions than in dry conditions, in contrast to PRD1. PRD1 was more tolerant at higher temperatures than MS2 and showed overall greater heat unit requirements to realize 99.9 % die-off. Results from this study suggest that deeper installation of drip tapes and/or frequent irrigations as alternative practices are appropriate in order to minimize soil surface wetting in SDI plots and reduce potential contamination. Although temperature alone is not able to sufficiently explain virus survival patterns, the average degree days can reflect the effect of daily temperature changes on viral survival.