Potential nutrient removal function of naturally existed ditches and ponds in paddy regions: Prospect of enhancing water quality by irrigation and drainage management.

Conventionally, paddy fields are regarded as important non-point sources of nutrient pollution, while ecological ditches and ponds are developed to reduce or retain nutrient export from agricultural fields. To quantify the potential nutrient removal function of ditches and ponds that naturally existed in rice growing regions, a representative paddy irrigation and drainage unit (IDU) composed of fields, ditches and a pond in the one-season rice region of the middle Changjiang River basin, China was monitored for two years. With data and knowledge gained, a Water Quantity and Quality Model for Paddy IDUs (WQQM-PIDU) is developed and applied for 30 years simulation to produce a general view. The monitored and modelled results showed that nutrient concentration peaks after fertilization was delayed and lowered in ditches and ponds, compared to those in paddy fields. Concentrations of runoff from the IDU outlet were generally lower than from the field during the whole rice growing season except the transplanting period. If fully utilized as temporary reservoirs, ditches and ponds naturally existed in a typical paddy IDU would reduce 39% nitrogen loads from field edges with a range of 17%-93% and 28% phosphorus loads with a range of 12%-92%. Although typical paddy IDUs discharge fewer nutrient loads than the content input into them, the discharge concentrations may be risky to surface waters. For their nutrient removal function, natural ditches and ponds are recommended to be included into irrigation and drainage management with accurate water level management during drainage, which is a promising and cost-effective approach to enhance surface water quality in rice growing regions.