Investigation of Subsurface Drainage Water Quality and Quantity of Drains With out Envelopein Comparison with Mineral Envelope in Shadegan Plain

In on-farm development projects, implementation of subsurface drainage
system is one of the most important issues. Considering the high costs of
preparing mineral envelope for subsurface drain pipes, continued execution
of such projects, especially in Khuzestan province, has faced some
difficulties. To overcome such a problem, use of synthetic envelopes or,
where soil characteristics allow, subsurface drainage without using envelope
may also be an option. Thus, in this research, in an experimental field of 20
ha under wheat in Shadegan region (Khuzestan province), the performance of
subsurface drainage without using envelope was compared with the
conventional drainage by mineral envelope. Each experimental unit had five
subsurface drain pipes (laterals) spaced at 50 m and installed at depth of
1.7m, with a length of 220 m. To study water table condition, some
piezometers were installed perpendicular to the middle laterals of each unit.
Also, the rates of drains discharge and drainage water quality of the middle
laterals in each unit were measured in manholes constructed on them. The
maximum salinity of drainage water from drains with mineral envelope
(replicates 1 and 2) and drains without envelope were 36.9, 52.1, and 47
dS/m, respectively, and the minimum salinities were 25.7, 23.6 and 12.6
dS/m. The maximum drain discharge in the first replication of mineral
envelope was 0.59 lit/sec, and for the second one was 0.28 lit/sec and in the
drains with no envelope was 0.23 lit/sec. Investigation of Salt Exit Index
(SEI) of soil showed that salt leaching continued during the wheat growing
season. Results of drains discharge showed the acceptable operation of drains
without envelope and proper performance of drains with mineral envelope
during the growing season. No sediment discharge was observed in any
treatment.