Drought evolution at various time scales in the lowland regions and their impact on vegetable crops in the Czech Republic

Two multi-scalar drought indices, the Standardized Precipitation Index (SPI) and the Standardized Precipitation–Evapotranspiration Index (SPEI), were used to study secular drought evolution from 1901 to 2010 in the lowland regions of the Czech Republic. To assess the temporal patterns of droughts as multi-scalar events, the SPEI and SPI were calculated for short-term (1–2 months), medium-term (3–12 months) and long-term droughts (13–24 months). In this study, daily and monthly potential evapotranspiration was integrated to estimate the evaporative power of the atmosphere and to explain its effect on drought conditions in the Czech Republic. For the lowland regions in the Czech Republic, long winter droughts occurred frequently at the beginning of the 20th century, while spring and summer droughts prevailed toward the end of the century. The mean durations of the drought episodes calculated from both indices were the same in the short time scales, between 2.2 and 2.5 months, whereas the mid- and long-term droughts that were determined based on the SPEI were longer than those that were identified by the SPI. We found relatively significant negative correlations between the SPEI from April to September and the detrended yields of Root vegetables (r = −0.68), and a linear regression model based on the SPEI series explained 59.1% of the variability of the annual detrended yield. The results showed that more frequent occurrences of dry episodes during the growing periods of Brassica vegetables produced damaging effects, especially during the planting and formation of the stem bulbs in kohlrabi, setting of the heads in cauliflower, and head formation in cabbage (late spring and earlier summer droughts). During sowing, the Bulb vegetables were less affected by even the longest dry spell (early spring droughts). Fruit vegetables were exposed to the risk of impact from a longer dry spell during their flowering and formation of the first fruits (summer drought episodes, i.e., SPEI ≤ −1.5 at July–August). However, the greatest fraction of decreases in Fruit vegetable yields occurred during the growing seasons with extreme wet spells in June (SPEI ≥ 1.5).