Outflow reduction and salt and nitrogen dynamics at controlled drainage in the YinNan Irrigation District, China

The YinNan Irrigation District in NingXia, China diverts each year about 1.6 × 109 m3 water from the Yellow River for irrigation use. More than half of that water is discharged back to the downstream channel or some low-lying depressions as a result of agricultural drainage. Several studies have indicated that the District is excessively drained, partially caused by the over-dimensioning of the existing drainage system, and proposed to improve the situation by controlled drainage practice. We subsequently carried out a field experiment of controlled drainage in the rice growing area of the District in 2004–2005. Field observations showed that reduction of the drainage depth of field ditches from 1 to 0.4 m resulted in a drainage flow reduction of 50–60%. Drainage water salinity increased only slightly but was still below the salt tolerance level of rice. Measurements of nitrogen concentrations showed no clear trend of changes as the result of irregular fertilization practice in the experimental site.