a b s t r a c tThis paper analyzes the effect of the shallow water table on water use of the winter wheat (Triticum aestivum L.) that has replaced alfalfa (Medicago sativa) on the irrigated lands of the Fergana Valley, upstream of the Syrdarya River, in Central Asia. The effect of the shallow water table is investigated using HYDRUS-1D. Numerical simulations show that the contribution of the groundwater to evapotranspiration increases with a rising water table and decreases with increasing irrigation applications. Under irrigation conditions, an increase in the groundwater evapotranspiration is associated mainly with an increase in evaporation loss, causing a buildup of salinity in the crop root zone. Evaporation losses from fields planted with winter wheat after the harvest amount up to 45-47% of total evaporation thus affecting soil salinity and ecosystem health. Promoting the use of groundwater for irrigation in order to lower the groundwater table is suggested to achieve water savings from the change in the cropping pattern. Unlocking the potential of groundwater for irrigation in the Fergana Valley can also contribute toward managing soil salinity and improving the health and resilience of water, land and ecosystems of water, land and ecosystems (WLE).

Recent evidences from some irrigated areas worldwide, such as Central Asia, suggest that water used for irrigation contains magnesium (Mg 2þ ) at levels higher than calcium (Ca 2þ ). Excess levels of Mg 2þ in irrigation water and/or in soil, in combination with sodium (Na þ ) or alone, result in soil degradation because of Mg 2þ effects on the soil's physical properties. More than 30 per cent of irrigated lands in Southern Kazakhstan having excess levels of Mg 2þ are characterized by low infiltration rates and hydraulic conductivities. The consequence has been a gradual decline in the yield of cotton (Gossypium hirsutum L.), which is commonly grown in the region. These soils require adequate quantities of Ca 2þ to mitigate the effects of excess Mg 2þ . As a source of Ca 2þ , phosphogypsum-a byproduct of the phosphorous fertilizer industry-is available in some parts of Central Asia. In participation with the local farming community, we carried out a 4-year field experiment in Southern Kazakhstan to evaluate the effects of soil application of phosphogypsum-0, 4Á5, and 8Á0 metric ton per hectare (t ha À1 )-on chemical changes in a soil containing excess levels of Mg 2þ , and on cotton yield and economics. The canal water had Mg 2þ to Ca 2þ ratio ranging from 1Á30 to 1Á66 during irrigation period. The application of phosphogypsum increased Ca 2þ concentration in the soil and triggered the replacement of excess Mg 2þ from the cation exchange complex. After harvesting the first crop, there was 18 per cent decrease in exchangeable magnesium percentage (EMP) of the surface 0Á2 m soil over the pre-experiment EMP level in the plots where phosphogypsum was applied at 4Á5 t ha À1 , and a 31 per cent decrease in EMP in plots treated with phosphogypsum at 8 t ha À1. Additional beneficial effect of the amendment was an increase in the soil phosphorus content. The 4-year average cotton yields were 2Á6 t ha À1 with 8 t ha À1 phosphogypsum, 2Á4 t ha À1 with 4Á5 t ha À1 phosphogypsum, and 1Á4 t ha À1 with the control. Since the amendment was applied once at the beginning, exchangeable Mg 2þ levels tended to increase 4 years after its application, particularly in the treatment with 4Á5 t ha À1 phosphogypsum. Thus, there would be a need for phosphogypsum application to such soils after every 4-5 years to optimize the ionic balance and sustain higher levels of cotton production. The economic benefits from the phosphogypsum treatments were almost twice those from the control.

Expansion of irrigated agriculture in the Aral Sea Basin in the second half of the twentieth century led to the conversion of vast tracks of virgin land into productive agricultural systems resulting in significant increases in employment opportunities and income generation. The positive effects of the development of irrigated agriculture were replete with serious environmental implications. Excessive use of irrigation water coupled with inadequate drainage systems has caused large‐scale land degradation and water quality deterioration in downstream parts of the basin, which is fed by two main rivers, the Amu‐Darya and Syr‐Darya. Recent estimates suggest that more than 50% of irrigated soils are salt‐affected and/or waterlogged in Central Asia. Considering the availability of natural and human resources in the Aral Sea Basin as well as the recent research addressing soil and water management, there is cause for cautious optimism. Research‐based interventions that have shown significant promise in addressing this impasse include: (1) rehabilitation of abandoned salt‐affected lands through halophytic plant species; (2) introduction of 35‐day‐old early maturing rice varieties to withstand ambient soil and irrigation water salinity; (3) productivity enhancement of high‐magnesium soils and water resources through calcium‐based soil amendments; (4) use of certain tree species as biological pumps to lower elevated groundwater levels in waterlogged areas; (5) optimal use of fertilizers, particularly those supplying nitrogen, to mitigate the adverse effects of soil and irrigation water salinity; (6) mulching of furrows under saline conditions to reduce evaporation and salinity buildup in the root zone; and (7) establishment of multipurpose tree and shrub species for biomass and renewable energy production. Because of water withdrawals for agriculture from two main transboundary rivers in the Aral Sea Basin, there would be a need for policy level interventions conducive for enhancing interstate cooperation to transform salt‐affected soil and saline water resources from an environmental and productivity constraint into an economic asset.

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This paper tells the story of trade-off between hydropower and irrigation and its implications for groundwater use in Syrdarya basin in Central Asia. With the independence of the central Asian republics, this trade-off has become a transboundary issue. Efforts to coordinate bilateral action using integrated water resources management (IWRM) principles of basin-wide cooperation have not yet yielded the hoped for results. This paper shows that there could be a 'second best' option of solving at least part of this transboundary problem by 'banking' winter flows released for hydropower production in Kyrgyzstan in the underground aquifers of Uzbekistan's Fergana Valley and extracting it for irrigation in the summer months

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The publications in this series cover a wide range of subjects-from computer modeling to experience with water user associations-and vary in content from directly applicable research to more basic studies, on which applied work ultimately depends. Some research reports are narrowly focused, analytical and detailed empirical studies; others are wide-ranging and synthetic overviews of generic problems.Although most of the reports are published by IWMI staff and their collaborators, we welcome contributions from others. Each report is reviewed internally by IWMI staff, and by external reviewers. The reports are published and distributed both in hard copy and electronically (www.iwmi.org) and where possible all data and analyses will be available as separate downloadable files. Reports may be copied freely and cited with due acknowledgment. About IWMIIWMI's mission is to improve the management of land and water resources for food, livelihoods and the environment. In serving this mission, IWMI concentrates on the integration of policies, technologies and management systems to achieve workable solutions to real problems-practical, relevant results in the field of irrigation and water and land resources.

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