In the writing that we present below, our purpose is to carry out a temporal comparative analysis of the precipitation in the Andorran Pyrenees. Specifically, we want to compare, for the meteorological station of the FEDA hydroelectric plant, located in the southern half of Andorra at 1,135 meters of altitude, the characteristics of the annual and seasonal precipitation of the thirty years 1996-2025 with those of the periods, also of thirty years, immediately preceding, that is, 1936-1965 and 1966-1995. In a complementary way, we will provide data from the weather station of Ransol, located at 1,641 meters of altitude in the northern third of Andorra, obtained from a rainfall series reconstructed by us simultaneously with the FEDA central.
Here we have used the series of precipitation that goes from December 1935 to November 2025, obtained from three sources: the meteorological archive of FEDA (FHASA before 1988), the data of the National Meteorological Service (web of the Andorran Government meteo.ad and values validated by Météo-France) and self-developed materials. We take December of one year to November of the following year as the annual period due to the fact that the precipitation in December is usually more connected to the following January and February than to the preceding autumn.
We will focus on the FEDA and Ransol central stations
The average annual rainfall for the 1936-1965 and 1966-1995 trentenis was, at the FEDA headquarters, 910 and 848 millimeters, respectively. In the period 1996-2025, this average has been 822 mm. There has therefore been a reduction in annual precipitation of 3.1% compared to 1966-1995 and 9.7% compared to 1936-1965 in the last thirty years. However, when comparing the period 1996-2025 with the previous thirty years using Welch’s t-test, and also using the Mann-Whitney non-parametric test, we come to the conclusion that this reduction is not statistically significant, at the usual significance level of 0.05. At the Ransol station, the average annual total for the period 1996-2025, of 973 mm, represents a decrease of 9.2% compared to the period 1966-1995, which, with the application of these two statistical tests, gives, from a statistical point of view, a signal of a slight reduction.
If we take seasonal periods, so that winter is the quarter made up of December, January and February, spring the quarter made up of March, April and May, summer the one made up of June, July and August and autumn the one made up of September, October and November, we have at the FEDA central, for the thirty-year period 1996-2025, an average precipitation in winter, spring, summer and autumn of 140, 217, 230 and 235 mm, respectively. That of the winter quarter represents 91.2% of that of the winter of the period 1936-1965 and 90.1% of the winter of 1966-1995. In the winter season, there has therefore been practically no variation. The precipitation of the equinox seasons, spring and autumn, has also not changed.
Although the summer precipitation has decreased by 23.6% compared to the quarterly June-July-August of the period 1936-1965, it represents up to 93.1% of that of the summer of the period 1966-1995.
While in 1936-1965 summer precipitation represented 33.1% of total annual precipitation, in 1966-1995 it constituted 29.2% of this annual total, and in 1996-2025 it has been 28.0%. As for Ransol, there is a summer reduction in the last thirty years of only 8.6% compared to 1966-1995. If we apply the Welch t-test and, additionally, the Mann-Whitney test, this summer decrease is not statistically significant.
If we interpret the seasonal rainfall regime in terms of an indicator formed by the first letters of the names of the stations ordered according to the decreasing seasonal amounts of precipitation, and considering the seasons as the quarters mentioned above, we have that, at the FEDA central, in the 1936-1965 and 1966-1995, this indicator was ETPH, while in the 30 1996-2025 has been TEPH. In Ransol we have, in the 30-year period 1996-2025, a seasonal regime with the code ETPH, a code that was of the TEPH type in the period 1966-1995, and which was also of the ETPH type in 1936-1965. In the case of Ransol, we can speak of a seasonal rainfall regime of a balanced type (in which all four seasons have a slightly disparate rainfall weight), as, for example, that of the Aran Valley, since the precipitation of the summer quarter, which constitutes, as we have said, the seasonal maximum in the 30-year period 1996-2025, represents only in this same 30-year period 27.1% of the total annual rainfall.
Returning to the annual precipitation, we must highlight, within the 30-year period 1996-2025, the existence in Andorra of a very long persistent period of marked pluviometric deficit. We are referring to the negative rainfall anomaly that, with only a short break of more important rainfall, affected the country from 2004 to 2012, a period in which there were also the two years with the lowest rainfall amounts in the entire annual rainfall series of the FEDA central, in 2006 and 2007, with only 546.4 and 450.2 mm total, respectively. The average of these two years (498.3 mm) represents only 54.1% of the average of the thirty-year period 1976-2005 and 51.1% of the average value of the seventy-year period 1936-2005.
In addition, the 30-year period 1996-2025 has towards its end another period of severe meteorological drought that began in 2021, after the year of the famous Gloria storm, and which has lasted in the Andorran Pyrenees and, by extension, in Catalonia as a whole, until the winter of 2024. The average precipitation of the three years from 2021 to 2023 has state, at the central FEDA and Ransol, of only 685.6 and 851.8 mm, respectively.
We have completed this examination by taking into consideration interannual rainfall variability. We will say, in this sense, from the calculated values of the coefficient of variation (abbreviated, CV), or quotient, in percentage, of the standard deviation and the arithmetic mean, that the interannual variability of precipitation, with the data always from the FEDA central, is relatively high in Andorra. It is 23.1% in the last thirty years 1996-2025. In the periods 1936-1965 and 1966-1995, the CV was 21.7 and 21.2%, respectively. The CV of the FEDA plant in the 30-year period 1996-2025 is higher than the CV that occurs in maritime climates of mid-latitudes, such as the Cantabrian strip or Galicia, where it does not exceed the 20-22% range. The CV of the plant is close to the pluviometric patterns of the Mediterranean subtropical world, although it is still relatively far from what characterizes Mediterranean Spain, where the CV is higher, in many of its spaces, at 25-30%. Ransol, on the other hand, as is most likely the case, as a whole, in the northern lands of Andorra, closer to the area of Atlantic influence, clearly presents CVs specific to the oceanic temperate world, not at all Mediterranean, since they are only 17.9 and 15.6% in the thirty years 1966-1995 and 1996-2025, respectively.
