September 2017 in mainland Portugal was the driest of the last 87 years
2017-10-06 (IPMA)
The month of September 2017 in Continental Portugal was the driest of the last 87 years (Figure 1), classified as extremely dry. Consequently an increase of the area in severe and extreme drought conditions was verified.
According to the PDSI – Palmer Drought Severity Index, at September 30, 81.0% of the territory was in severe drought and 7.4% in extreme drought (Figure 2). It should be noted that the SPI index 6 months (April to September), a scale that reflects the precipitation deficit and the level of the meteorological and agricultural drought, shows on September 30 most of the territory's basins in the severe drought class (Figure 3).
September 30th marked the end of the hydrological year 2016/2017 (October 1, 2016 to September 30, 2017), with a total accumulated precipitation in this period of 621.8 mm (70% of normal), with the 9th lower value since 1931. However, it should be noted that the period from April to September was extremely dry, with monthly values of the amount of precipitation always lower than the mean value, corresponding to the 2nd driest after 2005.
It should also be noted that in this semester the average value of the maximum temperature (27.72°C) was the highest since 1931 and the average value of the average temperature was the 2nd highest (after 2005).
The combination of much lower than normal precipitation values and very high temperature values, in particular the maximum temperature, resulted in high evapotranspiration values and significant values of soil moisture deficits.
The water soil content, on September 30, in most of the interior regions and southern Portugal, shows values below 20%, even in some places equal to or less than the point of wilting. In the regions of the north and central coast the values varied between 20 to 40%.
It should be noted that in the south-west of Europe, particularly in Spain and in some regions of central and southern France, soil water values were at or below the wilting point. This situation is due to the synoptic conditions that occur at these latitudes (intense anticyclone, almost stationary, that extends from the Azores to the Western Mediterranean).
Regarding temperatures, the average value of maximum air temperature, 27.49°C, was higher than normal with an anomaly of +1.20°C, but the mean value of minimum air temperature, 12.42°C, was lower at 1.74°C to the normal value, being the 5th lowest value since 1931.
The period from 1st to 8th September was the hottest of the month, with 6th being the warmest day with an average temperature of 24.1°C (+ 3.9°C compared to normal). The highest value of maximum air temperature occurred on 7th, 33.1°C (+ 6.8°C compared to normal).
Imagens associadas
Figure 1 - Anomalies of the amount of precipitation regarding average values in the period 1971-2000, in September, in mainland Portugal.
Figure 2 - Spatial distribution of the meteorological dry index as of September 30, 2017.
Figure 4 - Percentage of water in the soil (mean 0-100 cm depth), in relation to the water capacity usable by the plants, 30 September 2017, 00 UTC t + 0, ECMWF-HRES (16 km resolution).
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