Modeling and assessing the impact of reclaimed wastewater irrigation on the nutrient loads from an agricultural watershed containing rice paddy fields.

Two models were used in concert to predict nutrient loads in a waterbody receiving irrigation return flows from a rice paddy production system. Two irrigation scenarios were simulated, one using reclaimed wastewater as the irrigation water source, the other using water from a surface reservoir designed to supply irrigation water. Total nitrogen (TN) and total phosphorus (TP) loads in irrigation return flows from the rice paddy fields were simulated using the field-scale water quality model Chemical, Runoff and Erosion from Agricultural Management System model for rice paddy fields (CREAMS-PADDY). The output from CREAMS-PADDY was then used as input data for Hydrological Simulation Program-FORTRAN (HSPF) model. HSPF was used to evaluate TN and TP loads in the receiving waterbody at the watershed-scale. CREAMS-PADDY and HSPF were calibrated for both hydrology and water quality using observed data. Both CREAMS-PADDY and HSPF showed good agreement between the observed and simulated data during the calibration and validation periods. Simulation indicated that TN and TP loads from the study paddy fields increased by 207% and 1022% when reclaimed wastewater was used for irrigation compared to conventional irrigation. Irrigating paddy fields (18.8% of the 385 ha study watershed) with reclaimed wastewater increased the TN load at the watershed outlet by 10.3% and TP by 14.0%. The increase in nutrient loads was the result of the high nutrient concentration in the reclaimed wastewater. The procedures used in this research can be used to develop wastewater reuse strategies that minimize environmental impacts on watershed water quality.