Salt Leaching Efficiency of Subsurface Drainage Systems at Presence of Diffusing Saline Water Table Boundary: A Case Study in Khuzestan Plains, Iran

To design subsurface drain spacings and depths, the main efforts are usually focused on so-called equivalent depth as well as selecting a steady/unsteady drainage model. These models have been used widely for half a century and some software are developed to design the drainage systems. In spite of recent developments in drainage such as dry-drainage, bio-drainage and controlled drainage, the main concern remains on subsurface water quality and its harmful impacts on environment. In regions were the drainage system is most needed for salt leaching at reclamation stage as well as agricultural reclamation rotations, the main concern is salt concentration of drainage outflows. There are some examples in Iran that demonstrate irrigation and drainage networks that are well equipped with open and subsurface drainage systems. Application of suitable irrigation water provides an overall irrigation efficiency of 35-40%. As a result, in surface irrigation practices, the deep percolation is larger than leaching fraction and leaching requirements of the properly arranged cropping patterns in the crop rotation programs. Such conditions are observed in some projects for which the initial/capital salt leaching was well managed, but still after a long time salt concentration in drainage water is much larger than the applied water salinity of the same soil profile. Detail studies indicate that this can be attributed to existence of Salic/Natric horizons below the field laterals and/or sub main drainage canals, presence of highly saline stagnant or perched water tables below the drains which act as a diffuse boundary and low salt leaching efficiency.