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ABSTRACT, indicating a significant potential for improving water productivity through agronomic and irrigation management interventions. The ratio of average ET a divided by average TWA gave an average application efficiency of 78% (some fields under-irrigated and some fields over-irrigated), the remaining 22% of water applied leaving the field. Since more than 60% of the water used for irrigation in Tajikistan and Uzbekistan is pumped from, even if all this 22% of water returns to the stream, substantial energy savings would accrue from improving the average application efficiency at field level. The range of values for TWA indicates the inequity in water distribution/accessibility. Addressing this inequity would also increase water productivity at field and project level.

Poor on-farm irrigation practices and rising groundwater table depths are the major reasons for low cotton yields in the Sardarya province of Uzbekistan. To ensure sustainability of cotton production in the area, precise calculations of irrigation requirements are needed to optimize crop yields and to keep groundwater table depth below the root zone to avoid soil salinization. To determine optimal groundwater table depth and irrigation amounts for the Sardarya province of Uzbekistan, the Soil-WaterAtmosphere-Plant (SWAP) model was used. SWAP was calibrated and validated using measured data from an experimental cotton field during the agricultural year of 2006 and 2007. The calibrated SWAP model was then used to simulate optimal groundwater table depth and irrigation amounts. The simulation results show that for the existing conditions in the study area, a groundwater table depth of approximately 200 cm together with an irrigation application of 2500 m 3 ha À1 will be the most appropriate combination for optimal cotton yields (% 3.0 t ha À1 ). However, to achieve maximum potential yields of cotton (5-6 t ha À1 ), leaching of excessive salts from the root zone through freshwater application would be imperative. This would require rehabilitation of the existing drainage network in the area. Copyright # 2010 John Wiley & Sons, Ltd. RÉ SUMÉDe mauvaises pratiques d'irrigation et l'élévation du niveau de la nappe phréatique sont les raisons majeures des faibles rendements de coton dans la province Sardarya en Ouzbékistan. Pour assurer la durabilité de la production de coton dans la région, des calculs précis des besoins en irrigation sont nécessaires pour optimiser les rendements agricoles et maintenir la profondeur de la nappe phréatique sous la zone des racines et pour éviter la salinisation des sols. Pour déterminer la profondeur optimale de la nappe d'eau souterraine et les doses d'irrigation pour la province Sardarya, un modèle sol-eau-plante-atmosphère ( ). Cependant, pour atteindre le rendement potentiel maximum du coton (5-6 t ha À1 ), le lessivage des sels en excès dans la zone des racines par l'application d'eau douce serait impératif. Cela nécessiterait la réhabilitation du réseau de drainage existant dans la région.

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