EVALUATING DRAINAGE DESIGN PARAMETERS FOR WASTEWATER IRRIGATION APPLICATIONS TO MINIMIZE IMPACT ON SURFACE WATERS

Abstract

DRAINMOD, an agricultural water management computer model, was used to evaluate the hydrology for anirrigation scenario for an Ohio community using treated wastewater as the water supply. This model was used to simulatethe runoff potential of the proposed site under drainage and wastewater irrigation, to evaluate subsurface drainagesystem design parameters and irrigation interval, and to evaluate the effect of wastewater irrigation on potential cropyield. Constituent concentrations of the wastewater were within normal recommendations for agricultural irrigation, andwere not the focus of the study. A 40-year climatic record and USDA Soils5 data were used as inputs to the model toevaluate hydrologic response over a range of input values of irrigation rate and interval, and drain spacing and depth.Simulation results indicate that an irrigation interval of 1 or 2 days could meet the annual irrigation application criteriaof 67 cm (26 in.). Frequency analysis of runoff over the 40-year period for the 2-day irrigation interval revealed thatapproximately 99% of all days produced zero runoff. A response surface of average annual irrigation depth for the 2-dayirrigation interval indicated that the target depth of 67 cm could be attained with a range of drain spacing (7.5 to 12.5 m;25 to 50 ft) and depth (85 to 150 cm; 33 to 60 in.) combinations. Within the range of drain spacing and depths evaluatedwith wastewater irrigation at the 2-day irrigation interval, relative crop yield increased when an increase in drain depthwas coupled with a decrease in drain spacing.