Salt behavior in soil affected by groundwater management and reuse of drainage water using subsurface drainage system

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A series of research can be summarized as follows :1)Based on the field survey of subsurface drainage projects in Punjab Province, Pakistan, including Fourth Drainage Project, the characteristics of drainage in the Punjab Plane were made clear. A unit of subsurface drainage system can be characterized by the utilization of a pump to drain collected drainage water in a sump through underdrain and collecting drain pipes off to a drainage canal due to flat topography. 2)In a calm experiment for a kind of sub-irrigation utilizing drainage water, high efficiency of fresh water use was displayed, however, salinity problem arose. Thus necessity of measures, such as shortening and shifting of the duration for drainage water application, was suggested. 3)Based on the on-site experiment in Punjab, it was ascertained that the continuous irrigation with drainage water significantly decreased the wheat grain yield and accelerated the farmland salinization. During the three year continuous experimen … More t, the ordinary soil degraded to saline soil and increased its sodicity. However, the reduction of yield was limited and soil ECe showed little increase in the first year experiment, so it is suggested that the above problems can be controlled by the careful and proper water management practices, such as the effective *eaching of salt every year, application of cyclic irrigation with canal water and drainage water, and utilizing blended water of canal water and drainage water. 4)Based on the on-site studies in Aral Sea Basin, a remarkable finding was obtained on the salt behavior in saturated soil layers of rice fields. Salts from the deeper layer diffused upward in percolated water and thus increased soil water salinity in the shallow layer after the subsoil was saturated. This finding refutes the hypothesis that rice cultivation practiced in arid areas is effective for leaching salts already accumulated in the soil. Because of the movement of percolated water from rice fields through the zone below the field-drain bed, the rise of the groundwater table and salt accumulation were accelerated in the adjacent upland fields. Thus mixed cropping with rice and upland crops based on the crop rotation system in an irrigation block exacerbated waterlogging and salt accumulation in upland fields. Less