Very heavy precipitation days
Very heavy precipitation days is an indicator of the number of days where daily precipitation exceeds 50 mm, where precipitation is water in the form of rain, snow, sleet or hail, measured in millimetres (mm) or its equivalent litres per square metre (l/m²). This indicator describes the trends for very heavy precipitation days in the Basque Country between 1971 and 2016 and and plays an important role when applied to agriculture, water resource management and the prevention of very heavy precipitation events.
- Between 1971 and 2016, there was no positive or negative trend in the number of very heavy precipitation days for the Basque Country as a whole, with the trend being almost non-existent and insignificant.
- The decadal rates of change at a spatial level show a reduction in the number of days of very heavy precipitation primarily in small areas in the Cantabrian valleys and an increase in mountain areas in the east of Gipuzkoa, showing a statistically significant indication.
Relationship of the indicator to climate change
Climate change may cause very heavy precipitation events to increase in frequency. One consequence of global warming is an increase in evaporation, and as precipitation comes mainly from weather systems that are fed by water vapour stored in the atmosphere, this can increase the intensity of precipitation and the risk of very heavy rain and snowfall. Very heavy precipitation affects river flooding and can determine what types of animals and plants can survive in a given location. It can also cause flooding that can have impacts on human life.
Very heavy precipitation does not necessarily mean that the total amount of precipitation at a location has increased, but rather that precipitation occurs in more intense events. However, changes in precipitation intensity, when combined with changes in the time interval between precipitation events, may also lead to changes in overall precipitation totals. Therefore, it is important to look at the number of days of very heavy precipitation in the Basque Country.
Figure 1. Time series for days of very heavy precipitation in the period 1971-2016 for the Basque Country as a whole (correlation coefficient R = -0.00097, value of p = 1).
This section details the evolution of the number of very heavy precipitation days in the Basque Country between 1971 and Year 2016.
Figure 1 shows the time series for the number of days of very heavy precipitation, defined as those days when the total daily precipitation exceeds 50 mm. The results for the Basque Country as a whole did not show any trend for the period of data observed. The correlation coefficient near 0 shows that there is no linear relationship between days with very heavy precipitation and evolution over time, meaning that there is no significant variation in this indicator for the Basque Country as a whole.
Figure 2. Rate of change of very heavy precipitation days in the Basque Country (number of days per decade), 1971-2016.
Figure 2 shows the decadal rate of change of very heavy precipitation days in the Basque Country since 1971. In line with Figure 1, there is no trend of any kind in a large part of the region. There are some areas with significant decadal trends, but these are small. Moreover, significant spatial rates of change are not uniform across the region.
Some areas have been identified in the Cantabrian watershed where the number of days of very heavy precipitation shows a significant negative trend. However, areas have also been identified in the Cantabrian watershed, especially in the east of Gipuzkoa, where the number of days of very heavy rainfall has increased by approximately 1 day every 10 years. It should be noted that results may contain spatial interpolation errors, particularly in mountainous areas and at the edges of the geographical domain, and, therefore, they need to be treated with caution.
Precipitation measurements in the Basque Country come from meteorological stations, both manual and automatic, managed by different institutions (Basque Government, Provincial Councils, AEMET, URA).
Precipitation is determined at the manual stations, using the rainfall day, counted from 8:00 AM GMT to 8:00 AM GMT, instead of the calendar day, from 00:00 AM GMT to 12:00 PM GMT, which is normally used. In automatic stations, the accumulation of 144 ten-minute records of the calendar day is considered.
Data series have been fed into spatial prediction models to generate a daily resolution cartographic database, which is the starting point for the calculation of this climate change indicator. Static covariates, derived from digital terrain models, have been included in this prediction to explain precipitation.
The cartographic database comes from Phase II of the KLIMATEK project “High Resolution Climate Change Scenarios for the Basque Country”.
These maps are used to calculate very heavy precipitation days, defined as days with daily precipitation exceeding 50 mm.
We can also calculate the decadal trend (Sen's slope), i.e. the increase/decrease in the number of very heavy precipitation days over a decade, and check whether the trend is statistically significant or whether it is really the result of the variability of the thermometric series itself (Mann Kendall test).
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The Basque Country
No changes appreciated
Since 1971