Maximum daily rainfall
Maximum daily precipitation is an indicator of the highest volume of water (in the form of rain, snow, sleet or hail) recorded over the course of a day, measured in millimetres (mm) or its equivalent litres per square metre (l/m²). This indicator describes the trends for maximum daily precipitation in the Basque Country between 1971 and 2016 and plays an important role when applied to agriculture and water resource management.
- Between 1971 and 2016, there was no significant trend in the maximum daily precipitation for the Basque Country as a whole.
- The decadal rates of change for maximum daily precipitation at a spatial level are not uniform and only show a few small areas with a significant increase or decrease in maximum daily precipitation.
Relationship of the indicator to climate change
As the climate changes, so does the intensity of precipitation. Very heavy precipitation refers to cases where the amount of rain or snowfall experienced at a location substantially exceeds the norm. Exactly what constitutes a period of heavy precipitation varies from place to place and season to season.
Climate change may affect the intensity of precipitation . Ocean warming increases the amount of water that evaporates into the air. The more moisture-laden the air is, the heavier the precipitation may be. Potential impacts of heavy precipitation include crop damage, soil erosion and increased risk of flash floods. Most flash floods are the result of very heavy rainfall over a short period of time.
Figure 1. Time series for maximum daily precipitation in the period 1971-2016 for the Basque Country as a whole (correlation coefficient R = 0.043, value of p = 0.68.
In this section, we will analyse the evolution of the maximum daily precipitation in the Basque Country between 1971 and 2016.
As can be seen in Figure 1, the time series for maximum daily precipitation in the Basque Country as a whole does not show any significant trend for the period of data analysed, meaning that the slight positive trend observed may be due to the internal variability of the series itself.
Figure 2. Rate of change of maximum daily precipitation in the Basque Country (mm per decade), 1971-2016.
Figure 2 shows the decadal rate of change in maximum daily precipitation at a spatial level, featuring small areas with significant decadal trends, with increases and decreases of between 5 and 10 mm per decade. For example, an area with a significant increase in maximum daily precipitation of between 10 and 15 mm per decade was identified in the extreme east of Gipuzkoa, in the Bidasoa basin. The most important aspect is that changes in most of the region are less than 5 mm per decade and are neither uniform nor significant.
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 air temperature.
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 maximum daily precipitation for each year, defined as the highest precipitation recorded over one day.
We can also calculate the decadal trend (Sen's slope), i.e. the increase/decrease in the magnitude of maximum daily precipitation 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).
Gaztelumendi S., Otxoa de Alda K., R. Hernández, M. Maruri, J.A. Aranda, P. Anitua (2018) “The Basque Country Automatic Weather Station Mesonetwork in perspective”. WMO/CIMO Technical Conference on Meteorological and Environmental Instruments and Methods of Observation (CIMO TECO-2018). 8 - 11 October 2018. Amsterdam, The Netherlands.
Hernández R., M. Maruri, K. Otxoa de Alda, J. Egaña, S. Gaztelumendi. (2012) “Quality Control Procedures at Euskalmet Data Center”. Advances in Science and Research – Topical Library, Volume 8, 2012, pp. 129-134.
Herrera, S., J. M. Gutiérrez, R. Ancell, M. R. Pons, M. D. Frías and J. Fernández. 2012. Development and analysis of a 50-year high-resolution daily gridded precipitation dataset over Spain (Spain02). Int. J. Climatology, 32, 74-85, doi: 10.1002/joc.2256.
Herrera, S., J. Fernández and J. M. Gutiérrez. 2016. Update of the Spain02 Gridded Observational Dataset for Euro-CORDEX evaluation: Assessing the Effect of the Interpolation Methodology. Int. J. Climatology, 36, 900-908, doi: 10.1002/joc.4391.
Klein Tank, A.M.G., Zwiers, F.W., Zhang, X. 2009. Guidelines on analysis of extremes in a changing climate in support of informed decisions for adaptation, climate data and monitoring WCDMP-No 72, WMO-TD No 1500, p 5.
Proyecto Klimatek 2016. Elaboración de escenarios regionales de cambio climático de alta resolución sobre el País Vasco. IHOBE. Gobierno Vasco.
