Maximum length of consecutive dry days
The maximum length of consecutive dry days is an indicator of the maximum number of consecutive days where precipitation is less than 1 mm, where precipitation is water in the form of rain, snow, sleet or hail. This indicator describes the trends for the longest dry periods in the Basque Country between 1971 and 2016 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 a negative trend in the maximum length of consecutive dry days in the Basque Country as a whole, but the results are not significant and therefore not conclusive.
- The decadal rate of change at a spatial level is not uniform and the most significant indications can be seen in Álava. A significant increase in the longest dry period was identified in part of the Rioja Alavesa. A similar situation exists, but in the opposite direction, to the west of the Gorbea foothills.
Relationship of the indicator to climate change
Drought can be defined as a prolonged period of consecutive days of dry weather caused by a lack of precipitation, resulting in a severe water shortage for some activities, populations or ecological systems. Drought can also be viewed as a prolonged imbalance between precipitation and evaporation.
As average temperatures rise due to climate change, the Earth's water cycle is accelerated through an increase in the rate of soil evaporation and transpiration in plants. A higher rate of evapotranspiration makes more water available in the air for precipitation, but contributes towards drying out some land areas, leaving less moisture in the soil. Drought conditions can negatively affect agriculture, water supply, energy production, human health and many other aspects of society. Impacts vary depending on the type, location, intensity and duration of the drought. For example, effects on agriculture can range from slowing plant growth to severe crop failures, while impacts on water supply can range from declining water levels in reservoirs to significant water shortages. Lower flow rates of streams and groundwater can also harm ecosystems in general, by damaging plants and animals and increasing the risk of forest fires.
Figure 1. Time series for the maximum length of consecutive dry days in the period 1971-2016 for the Basque Country as a whole (correlation coefficient R = -0.067, value of p = 0.52.
In this section, we will analyse the evolution of the maximum length of consecutive dry days in the Basque Country between 1971 and 2016.
Figure 1 shows the time series for the maximum length of consecutive dry days, defined as days where the total daily precipitation is less than 1 mm. The results for the Basque Country as a whole show a negative trend of 0.6 days/decade, but the statistical significance is very low (p=0.52), meaning that no conclusive conclusions can be drawn.
Figure 2. Rate of change of the maximum length of consecutive dry days in the Basque Country (no. of days per decade), 1971-2016.
Figure 2 shows the decadal rate of change for the longest dry period between 1971 and 2016. A negative decadal rate of change predominated in the Cantabrian watershed, but the indication was hardly significant. There were some areas with statistically significant variations, particularly in Alava, but the results were not uniform throughout the region.
The western half of Rioja Alavesa showed the greatest decadal variations for the longest dry period, with a significant positive rate of change of between 4 and 6 days per decade. Another large area with a significant negative rate of change was identified in the mountainous area of north-western Alava, with a decrease in the maximum length of dry days of between 2 and 4 days per decade.
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 dry period, i.e. the maximum length of consecutive dry days (< 1 mm).
We can also calculate the decadal trend (Sen's slope), i.e. the increase/decrease in the number of consecutive dry 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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