Maximum length of consecutive wet days
The maximum length of consecutive wet days is an indicator of the maximum number of consecutive days where precipitation is equal to or more than 1 mm, where precipitation is water in the form of rain, snow, sleet or hail. This indicator describes the trends for the longest wet periods in the Basque Country between 1971 and 2016 and plays an important role when applied to agriculture and water resource management.
- Since 1971, there has been a negative trend in the maximum length of consecutive wet days for the Basque Country as a whole, but with a low level of statistical significance.
- There appeared to be some areas of significant decadal variation, albeit anecdotally. The highest positive rates of change were in the east of Gipuzkoa and in the southwest of Alava.
Relationship of the indicator to climate change
Precipitation is an important component of the hydrological cycle. The amount and spatial distribution of precipitation in the Basque Country is strongly influenced by atmospheric circulation, mainly related to the advection of maritime air masses from the Atlantic. The combination of changes in precipitation patterns due to climate change can lead to extreme hydrological events such as floods and droughts. Some systems that are closely linked to the hydrological cycle are very sensitive to the effects of changes in the characteristics of precipitation. However, climate projections of precipitation indicate a high uncertainty for regional results, so it is important to analyse past changes in precipitation in the Basque Country, particularly changes in the longest wet periods.
Figure 1. Time series for the maximum length of consecutive wet days in the period 1971-2016 for the Basque Country as a whole (correlation coefficient R = -0.1, p-value = 0.33.
This section details the evolution and changes in the maximum length of consecutive wet days in the Basque Country from 1971 to 2016.
Figure 1 shows the time series for the maximum length of consecutive wet days, defined as days where the total daily precipitation is equal to or greater than 1 mm. The results show a slight negative trend of 0.1 days/decade for the Basque Country as a whole, but with a low level of statistical significance (p=0.33), meaning that changes may be due to the variability of the precipitation data series itself.
Figure 2. Rate of change from the maximum length of consecutive wet days in the Basque Country (no. of days per decade), 1971-2016.
The decadal rate of change for the longest wet period represented in figure 2 shows that there is no trend in a large part of the Basque Country. Where there is a trend, it is not uniform and is generally not significant.
There are some areas with statistically significant results. More specifically, areas with a positive rate of change in terms of longest wet periods were detected in the east of Gipuzkoa, in Tolosaldea, and in the southwest of Álava, with a significant increase of between 0.1 and 2 days every 10 years.
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 the longest wet periods, i.e. the maximum length of consecutive wet days (≥ 1 mm).
We can also calculate the decadal trend (Sen's slope), i.e. the increase/decrease in the number of consecutive wet 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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