Vapour pressure deficit, in addition to light and temperature, is the main meteorological element that influences the degree of transpiration rate, which in turn determines the impact of drought stress on the yield of onions. In this study we attempt to assess the effect of vapour pressure deficit on onion cultivation in Poland and present a temporal and spatial distribution of vapour pressure deficit for the years 1966–2005. During the period “end of emergence–harvest” (Ee–H), vapour pressure deficit had the greatest impact on onion yield in central western Poland and in the Sandomierz Basin (south east). In these regions the average total yield of onion was 15% lower than the long-term average for the entire country, and yields lower than the long-term average occurred every 1–2 years. A statistically significant year-to-year increase in vapour pressure deficit during the Ee-H period indicates the possibility of a further increases in the risk to onion cultivation.

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