Evaluation of irrigation and drainage water cation composition and salt leaching requirement in Hetao Irrigation District

To determine where the salts go, variations in cation compositions of soil, irrigation water and drainage water in Hetao Irrigation District were analyzed. Field investigations and simulations of soil column leaching experiments were conducted to determine the changes in cation concentrations in irrigation water, drainage water and in soil profile. The study also discussed the use of water soluble Na⁺ concentration to determine irrigation/drainage ratio for salt leaching (LR). The results showed that the average ratio of Na⁺ to total salts in different drainage canal waters was about 87%, which was higher than that in irrigation water. The average ratio of Ca²⁺ was about 7%, which was lower than that in irrigation water. A significant correlation was noted between EC and Na⁺ concentration. This indicated that Na⁺ significantly influenced total salt concentration in drainage water. There existed an annual balance in Na⁺ input-output in irrigated areas. However, about 2/3 of irrigation water Ca²⁺ accumulated in soils with less water insoluble cation discharge. LR determined via Na⁺ proved to be better than using EC. Based on the results, a balanced Na⁺ input-output required an LR of 0.12~0.15. It meant that an annual drainage of 0.6~0.7 billion tons was reasonable for the annual irrigation of 5 billion tons in Hetao Irrigation District.