IMPROVING IRRIGATION WATER OPERATION IN THE LOWER REACHES OF THE AMU DARYA RIVER - CURRENT STATUS AND SUGGESTIONS

Irrigated agriculture is widespread in the Central Asian drylands and important for food security of the region. However, irrigation practices, based on rules made for cotton production on large units, do not provide adequate guidance for the now widespread small farms that produce cotton, wheat and rice. Excessive, unsustainable water use is the consequence. Land and water resource management practices were analysed in 2006 for the irrigated area (approx. 1885 ha) of a water users' association (WUA) as a case study. The Shomakhulum WUA is situated in the Khorezm region, Uzbekistan, in the Aral Sea Basin. The designed water allowance of 0.40 1 s(-1) ha(-1) was 46% higher than the 0.21 1 s(-1) ha(-1) required in 2006. A ponding experiment conducted at two locations under typically shallow groundwater conditions showed that actual seepage losses in the major distributing canal (Pakhtakiyar) in the WUA were about 2% at one site and negative at the other. This does not support the present blanket recommendation of an additional 29% of water to compensate for anticipated seepage losses. Considering the effects of shallow groundwater, the seasonal leaching requirements were estimated at 200mm versus the 400mm based on the blanket local recommendations for a 14 ha cotton field. However, the actually applied water amounted to 670 mm, which exceeds the calculated requirements by 235% and the local recommendation by 68%. The findings showed (i) excessive irrigation water supply and in turn high wastage owing to an underestimation of the potential shallow groundwater contribution, (ii) overestimation of conveyance losses and (iii) excessive leaching due to outdated knowledge on leaching requirements when compared to standards. We suggest that the outdated water allowance, the inaccurate estimation of the conveyance efficiency and excessive water applications during leaching cause a waste of precious fresh water, and a rise in the shallow groundwater levels which consequently increases secondary soil salinity. The revision of irrigation and leaching norms combined with an improved knowledge on seepage dynamics under shallow groundwater levels is needed before any further recommendations can be made to the farmers in order to reach the objective of sustainable agriculture in the area. Copyright (C) 2011 John Wiley & Sons, Ltd.