International Commission on Irrigation & Drainage Commission Interationale des Irrigation et du Drainage



Management of Drainage Water

Drainage water can be disposed directly to open surface water bodies, e.g., rivers, lakes, outfall drains, seas or oceans. If drainage water is disposed off to large, open surface water systems with significant dilution or assimilative capacity, then water quality problems are minimized.

 

Ultimate disposal of drainage water to a river or sea is not always possible. Closed drainage basins present a unique environmental or water quality challenge. In such situations, evaporation ponds may be an appropriate means for disposing of drainage water. However, toxic substances could accumulate in the ponds. The health of waterfowl, fish and other aquatic biota which use the pond could be negatively affected. Furthermore, these ponds could become habitats for snails and mosquitoes, thereby causing malaria and schistosomiasis epidemics. In addition, if not properly managed, new waterlogged and saline areas will develop adjacent to the ponds.

 

There is now increasing interest in the utilization of natural and constructed wetlands to manage drainage water. Wetlands are particularly effective for removing sediment, N and P. Plant, soil and hydrologic parameters interact in a complex way to filter and trap pollutants, and to recycle nutrients. Certain tree and plant species have the potential to absorb pollutants. Residence time, flow rate, hydraulic roughness and wetland size and shape are some of the factors which influence treatment efficiency. The water supply to the wetland must be sufficient to provide an excess to discharge and prevent salt accumulation.

 

In areas where soils, geologic and hydrologic conditions do not permit constructed wetlands, the saline agriculture/agriculture-forestry system may be appropriate for the disposal of drainage water. The operating principle is to successively re-use saline drainage water to irrigate crops and trees of increasing salt tolerance, and to discharge the final much reduced volume of water into a solar evaporator for salt crystallization. The depth of water ponded in the evaporator is regulated to match the daily evaporation rate. The goal is to make the crystallization pond unattractive to flora and fauna. Another challenge of evaporation ponds is that evaporation is reduced as the ponds become more concentrated. This necessitates the use of additional land to maintain disposal (evaporation) capacity.

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