Global water reserves continue to decline, with serious social and economic implications

Is the report of good quality? Are the conclusions backed up by solid data?

“Yes, the report is of good quality. The World Meteorological Organisation is a body that rigorously examines issues relating to the atmosphere and related fields such as hydrology, glaciology and oceanography, as it has effective monitoring systems in place across the globe. The report’s conclusions are supported by solid data.

There are areas of Spain (the west and north-west) where rainfall this summer has fallen by 75 per cent (below average). What does this mean in climatological and human terms? We have experienced one of the driest and hottest summers in recent decades in Spain, and this has had significant effects on water levels. Curiously, the areas most vulnerable to drought in Spain are the regions in the north of the peninsula, as they are not prepared for short but intense droughts (one or two months without rain). Consequently, drought alarms are sounding and supply problems are arising in towns that have not adapted their urban water management plans to the new climatic conditions that are emerging – conditions characterised by greater irregularity in rainfall”.

What do drought episodes such as this summer’s reveal about the future climate in Europe?

“That the rainfall pattern is changing. So-called ‘express droughts’ are becoming increasingly frequent; that is, short-lived (2–3 months) but very intense droughts accompanied by high temperatures that increase water evaporation. This is what has happened across much of Europe this summer and has also affected the north of our country. These are areas accustomed to regular and continuous rainfall throughout the year. Consequently, when there are two or three months of rainfall well below normal, they begin to experience problems with agricultural and, in particular, urban water supplies.”

How can it be that Spain’s water reserves are at 62 per cent of their capacity whilst there is widespread concern about drought?

“Because the overall figure for the country as a whole masks very diverse regional and local realities. As has been pointed out, the rainiest areas of Spain (the Cantabrian regions and the Pyrenees) are the least prepared to withstand short-term droughts such as the one we have been experiencing in our country since last May. Their urban water supply systems are not equipped to cope with this new climatic reality. They rely exclusively on water from rainfall, against a backdrop of increasingly erratic rainfall, and this heightens their vulnerability to drought. By contrast, the south-east of the Iberian Peninsula – where the lack of rainfall and the impact of severe droughts are greatest – has a water supply system organised around the use of multiple sources (rivers, aquifers, desalination, reuse), which enables it to cope with several months of rainfall well below normal levels.”

What are the implications of the report’s findings?

“All of this compels us to review our water planning to adapt it to the new climatic reality we are experiencing, with higher temperatures, very hot summers that increase water evaporation, and more irregular rainfall. Spain must begin to plan its water resources and usage within the new climate change scenario by prioritising water reuse and relying less on direct rainfall. Cities must expand their water storage reservoirs to be able to cope with weeks of little or no rain. If this is not done, we are doomed to live in a permanent state of drought alert, especially in the rainiest areas of Spain in the north of the peninsula. We need to develop multi-source systems that are not exclusively dependent on rainfall. As I say, the reuse of treated water and the exchange of water flows between the countryside and the city are solutions that are easy to implement and would reduce our country’s vulnerability to droughts.”

The report is sound because it covers various relevant variables relating to the hydrological cycle. Some of these involve considerable uncertainty, such as the study of evaporation, which is based on global models that have limitations and problems. It also provides useful observational data from specific locations, such as the soil moisture anomalies in the network available in the United States or the general assessment of global river flows. Although the latter has significant spatial gaps, given the limited availability of data in some regions of the world —such as Africa or much of Asia—, it represents a valuable contribution on a global scale.

It should be borne in mind that this assessment analyses a specific year and does not provide an overview of long-term trends. We may experience a very wet year, such as this winter in Spain, but this forms part of a very different context. In other words, this report does not record interannual variability, potential trends or possible future scenarios. Its purpose is to provide an overview of the situation in 2025, both in terms of the magnitude of the various variables used to measure water resources and in terms of any anomalies.

Drought is a consistent feature of our Mediterranean climate and, by extension, also in areas situated on the boundary between regions with subtropical characteristics and those with more temperate climates. Both geological and biological records, as well as documentary sources, demonstrate that drought is an inherent feature of our climate.

The current focus of interest is: are these droughts becoming more frequent? Are they becoming increasingly severe? How is climate change affecting these droughts? In this regard, we must distinguish between dynamic and thermodynamic mechanisms. Dynamic mechanisms are those related to atmospheric circulation. For example, Azores high-pressure systems that are longer-lasting, more persistent and more intense, preventing the entry of storms associated with the polar front and leading to more widespread dry conditions. On the other hand, there are the thermodynamic processes, which relate to temperature. Currently, what we are observing is that rainfall patterns are not changing substantially, but increasingly higher temperatures are leading to ever-greater atmospheric demand.

When we have a situation of precipitation deficit, such as a dry year or season, this increases stress on both water resources and vegetation. Above all, in the case of water resources, there are times when every drop of rain counts, and there is greater evaporation from reservoirs and river systems. As for vegetation, if the soil is already low in moisture due to a rainfall deficit, a temperature of 42 °C is not the same as one of 35 °C. Vegetation suffers much more because the atmosphere demands more water from it, and physiological processes occur that cause plants to assimilate less carbon or even die or wither due to this lack of moisture.

In this report, the World Meteorological Organisation presents the state of water bodies in 2025 throughout their entire life cycle. The data presented, gathered from reliable sources, show that river flow in that year was generally extremely low, which would lead to an extremely dry period. Furthermore, it also reports episodes of extreme rainfall over short periods of time. This pattern prompts us to reflect on the changes we have been witnessing in the water cycle in recent years.

In Spain, the decline in rainfall in some regions should be interpreted as an anomaly compared to the usual pattern, which does not directly affect reservoir capacity. The key is to manage the volume of water stored in these reservoirs effectively to ensure we can cope with extreme events, meet the needs of agriculture and carry out annual hydrological planning. Furthermore, it indicates that slower-replenishing water reserves – comprising groundwater, and water lost from glaciers and snow – have historically acted as a buffer during drier periods; however, we will be able to rely on them less and less, as they too have shown a downward trend in recent years.