Water management in Khorezm: Current situation and options for improvement (Hydrological Perspective)

Abstract

The combination of hydrological research findings from field to regional level in the irrigated croplands of Khorezm, Uzbekistan, revealed that water availability at farm and field level fails to meet agricultural requirements in parts of the region, although water withdrawal from the Amudarya river is huge. In 2004 and 2005, the seasonal gross water input to a sub-unit of the Khorezm irrigation and drainage system - a hydro-unit investigated as a case study - constituted 2,630 mm and 2,810 mm, respectively, in which the share of pre-season leaching amounted to 700 mm each year. These findings correspond well with the 2,240 mm water input monitored in the vege-tation period of 2005 at Khorezm-wide level. Reduction of actual evapotranspiration (ET) was observed in the range of 5-10% at sub-unit level, more than 25% in tail-end locations in Water Users Associations and 30-40% in single fields. Technical overall irrigation efficiency in the vegetation periods 2004 and 2005 averaged 27.5% in the sub-unit and 26% for whole Khorezm. These low efficiencies in the sub-unit are plausible as also drainage water output reaches high shares, with 62% and 67% of the irrigation water input in 2004 and 2005, respectively. On the other hand, the local practice of allowing for shallow groundwater tables that feed the crop in a system of 'furrow and sub-irrigation via capillary rise' raises the de-facto efficiency to 38%, but entails the well-known (Section Regional level of Khorezm) salinity problems of the region. The depleted fraction - the ratio between actual evapotranspiration and the sum of water inflow via irrigation and rainfall - in the peak irrigation season 2005 was around 0.3 at Khorezm level. With groundwater tables above 1.4 m and moderately saline groundwater, approx. 65-70% of the irrigated areas are at risk of waterlogging and salinization in April and July each year (major leaching and cropping periods, respectively). The dysfunctional drainage system, together with excessive leaching at field level and huge water losses from the irrigation -(conveyance and distribution) network are major reasons for the observed shallow groundwater in the region. With reference to technical aspects only, current problems with the water -management system in the region are mainly caused by (1) inflexible irrigation scheduling, (2) low efficiencies of water application at field and network level, (3) inappropriate infrastructure and insufficient maintenance, (4) limited options for groundwater management and (5) a general lack of input data to support water -management. These factors are therefore starting-points for interventions towards an improvement of water use. Such measures include among others the advanced determination of net irrigation needs taking the temporal behavior and site-specific dependencies of the field water balance into account, the optimization of the field water application process, and raising network efficiency. Together, these interventions have a water-saving potential of 50% relative to the current water withdrawal from the river.