Agro-Meteorological Trends of Recent Climate Development in Khorezm and Implications for Crop Production

Abstract

The extremely continental climate in the inner Aral Sea Basin is characterized by a high annual temperature amplitude, and constantly very low ­precipitation. Climate change in the Khorezm region, a district of Uzbekistan ­dominated by extensive irrigation agriculture, is manifest by temperature increases, especially during the winter period (40-year record). The 10-year average of ­summer temperatures in 1981–1990 was 0.2–0.5°C above the long-term average in 1930–1990. Time series of agro-meteorological parameters based on data (air and soil temperature, relative humidity, wind speed, precipitation, and radiation) of three meteorological stations in Khorezm revealed (a) air temperatures supporting good growing conditions for cotton, rice, wheat, sorghum, and maize; (b) comparatively good air and soil temperature conditions for cotton production; and (c) no negative climate impact on regional water demands for irrigation yet. Moderate spatial variability was observed between climate in the central irrigation system, at its southern ­margin to the desert, and in the desert itself (represented by the stations Urgench, Khiva, and Tuyamuyun, respectively).

For nearly all years between 1970 and 2007, Growing Degree-Days (GDD; based on crop specific baseline temperatures and maximum thresholds) indicated the suitability for all investigated crops. The conditions for winter wheat have over time improved in terms of GDD, especially in the southern margin of the irrigation system (Khiva). Absolute GDD minima, a parameter used for assessing the period of temperatures sufficient for crop growth, were found to be close to the lower boundaries for cotton. Water availability and fertilizer additions are other factors that determine cotton performance. Minimum GDDs only slightly reduced during an observation period of 21 years.

The earliest planting dates for cotton, derived from an analysis of soil temperature thresholds, range between 21 March and 18 April during the observation period at all three sites. In most years, the first suitable cotton planting dates occurred between April 1 and 5. The cotton planting dates at the southern margins of the irrigation system appear more variable, caused by an increased risk of late frosts. In light of this data, recommendations from the Ministry of Agriculture and Water Resources of Uzbekistan to plant cotton in Khorezm from mid- to end-April have become conservative and give room for site-specific adjustments.

The long-term (1970–2007) annual average potential evapotranspiration (PET) at Urgench station reached 1,378 mm year⁻¹. PET of the vegetation periods varied between 1,100 and 1,200 mm. Only a slight PET decline is detectable, i.e. crop water demand in Khorezm remains constant.