Causes of farmland salinization and remedial measures in the Aral Sea basin—Research on water management to prevent secondary salinization in rice-based cropping system in arid land

In the Lower Syr Darya region of the Aral Sea basin, secondary salinization of irrigated lands has been a crucial problem. To clarify the mechanism of secondary salinization, studies on water and salt behavior were conducted in an irrigation block where a rice-based cropping system has been practiced. Results of on-site studies are summarized as follows: (1) since the performance of land-leveling for rice cultivation was extremely poor, the water level was maintained high enough to submerge the highest portion of each plot, and this causes wastage of irrigation water and salt accumulation. (2) A large portion of water introduced to rice plots tends to be released into field drains. (3) Due to excessive irrigation of rice plots with slightly saline river water, dissolved salts were mainly deposited in upland plots in the block and its periphery. Changes in salt accumulation rates were dependent upon the scale of annual changes in the farmland areas that were converted from the upland condition to submerged condition in a process of crop rotation. (4) A remarkable finding was obtained on salt behavior in saturated shallow soil layers of rice plots. An initial decrease in soil water salinity in the shallow layer is due to the leaching effects of infiltration during the initial stage, and the subsequent gradual increase is more likely a consequence of mixing with the saline water that remains in the finer soil pores combined with the concentration effects of crop water uptake, and the upward flow from the lower layers due to occasional interruption and resumption of irrigation water supply. (5) Because seepage water from rice plots flows underneath the field drain, the rise of the groundwater table and salt accumulation were accelerated in the adjacent upland plots. Thus, mixed cropping with rice and upland crops based on a crop rotation system in an irrigation block accelerates waterlogging and salt accumulation in upland plots. Based on these results, several remedial measures were recommended to overcome problems on secondary salinization as follows: (1) avoid mixed cropping with rice and upland crops, and unify either upland crops or rice in an irrigation block to control groundwater table; (2) decrease conveyance and field application losses through improved canal construction and management performance, introduction of canal lining, and improved land-leveling performance; (3) maintain and operate drainage canals to enhance function, particularly installation of subsurface tile drainage for enhancing subsurface drainage function and management of drainage outfall for minimizing environmental degradation caused by saline drainage water in the downstream area; (4) develop a design and management technique of evaporation pond for better effluent management and reuse of drainage water at the outfall of each irrigation block; (5) reduce the water supplied for rice and its use for other crops, or returning the saved water to the river for downstream users including returning to the environment; (6) conclude international water and/or drainage rights agreements among riparian countries and enactment of a basin-wide management regulation to control water withdrawal and drainage.