Quantifying drainage components and salt and water balance in YinNan Irrigation District, China, based on a controlled drainage experiment

YinNan Irrigation District (YNID) is located in the upper reaches of the Yellow River in NingXia, China. Its irrigated area is about 80000 ha, with one-third of it for rice production. The major part of its drainage system was constructed between the 1950s and 1970s to maintain the salt and water balances of the district. The system, however, has been reported as draining the agricultural lands excessively by several studies. In addition to field, lateral and main drainage ditches, agricultural fields of YNID are also under the influence of the Yellow River channel and some low-lying depressions, thus forming a dual drainage system. Owing to difficulties in irrigation inflow measurement, evaluation of the existing drainage system often appears to be elusive. Based on a dual drainage assumption and an on-site controlled drainage experiment, we present a detailed analysis on drainage components and the salt and water balance of YNID. Results show that, by implementing controlled drainage, shallow drainage from field ditches can be reduced by 60%. Deep drainage from main ditches, the Yellow River channel and low-lying depressions is relatively stable year around, and it neutralized the potential effect of controlled drainage on salinity increase. Drainage water salinity calculated from the dual subsurface drainage model was consistent with field observations, proving that the dual drainage assumption is valid for the study area. Based on this study, field water management practices of the irrigation district can be better targeted and fairly evaluated. Copyright (c) 2006 John Wiley & Sons, Ltd.