Drought: contrasting forecasts for summer 2020

Despite a preoccupying situation at the end of April, the perspective of a major drought for summer 2020 has become less ominous for France. However, for Poland and the United States, the situation is particularly worrying.

At the end of April, the American journal Science published the conclusions of research carried out by the Earth Institute of Columbia University. This study by American climatologists shows that several American States are already subject to a megadrought: a period of hydric stress that covers several decades. Since the 2000s, the water reserves of 9 States in the American west have not recovered sufficiently to avert a drought. Based on hydrological models and analyses of tree rings, the study covers more than 1,000 years and shows that the American west has undergone four megadrought episodes. The current drought stands out due to the area covered, larger than the previous episodes. Lastly, the study shows that the current drought is mostly due to climate change and anthropic activities.

Summer soil-moisture reconstruction for SWNA / Source : Science, 17 April 2020

Since 2015, Poland has been struck by chronic drought that has exceeded all known records this year. The winter of 2019-2020 was the warmest recorded in Poland since 1850, when meteorological records began. Farmers have witnessed the damage done as from the month of April, with forecasts of as much as 70% of harvests lost, leading to fears in the short term of rising prices for farm produce. And the future holds little scope for hope as the World Meteorological Organization (WMO) estimates that Poland will have no more than two or three seasons a year, since winter will vanish.

On the occasion of Earth Day, on 22 April, the WMO published its final report on the world’s climate in 2015–2019. Its conclusions are clear:

The period 2015–2019 was the hottest ever observed. The average temperature of the planet has increased by 1.1 °C since the pre-industrial era and by 0.2 °C in comparison to the average for the period 2011–2015. Since the 1980s, each decade has been hotter than the previous one. Furthermore, the WMO declared that “the average global temperature is liable to exceed records during the next five-year period (2020–2024).

Regarding drought, the forecasts for France give far less cause for concern this year, compared to last summer: the bulletin published by the Geological and Mining Research Agency (BRGM) in June shows a large number of areas with excess groundwater recharging, where groundwater levels are very high. However, certain areas are still suffering from shortages from previous years, with low groundwater levels: the plain of Alsace, the corridor of the Saone, the upper and lower Rhone, and the east of the Massif Central remain under surveillance.

Groundwater levels have an impact on the health of surface waters: those of rivers and lakes. “In a low water period, 60 to 80% of the water flowing is groundwater. If the aquifers are too low, they cannot feed the rivers”, explained Anne-Valérie Hau-Barras, regional director of BRGM for the Occitanie region.

The water height of rivers and that of the alluvial groundwater are interdependent. During periods of high water levels, when rivers are in flood, their waters feed the groundwater. On the contrary, during low water levels, it’s the groundwater that feeds the river. The groundwater is also recharged by rain that falls on the watershed in winter and spring. In summer, the rain is consumed directly by plants and evapotranspiration.

Studies have shown that the feeding of rivers by groundwater permits avoiding several types of degradation, such as the fragmentation of rivers, increases in their temperatures, and changes in vegetation.

We are working on nature-based solutions, according to the natural water cycle, such as guided groundwater recharge experiments. The aim is for example to favor the re-infiltration of excess water in winter and spring via systems of canals, so as to recharge the aquifers and thus support river flows during low-water periods. These measures must be combined with broader measures to de-impermeabilize soil.

Here again the issue of water withdrawals and reconciliation of water uses is brought to the fore. Published at the beginning of June, a French parliamentary report of the fact-finding mission on the management of conflicts between users of water due to shortage, wrote the following in its introduction:

We have already been struck by the impact of climate change. We can longer ignore the evidence: if we must anticipate the adaptation of our water management to the upheavals occurring at present, then it is necessary to accept as fact that the impacts of climate change are already in motion. The modifications of volumes of precipitations, their concentration during extreme events, the modification of their geographic and seasonal distributions, and the increase in the frequency and duration of drought episodes jeopardize our social and economic organization, which finds itself disarmed in the face of these major and lasting changes.

After carrying out a detailed inventory of the causes and consequences of water shortages, the report’s authors list the levers to be used to avoid a water war: maintain and develop the means assigned to collect data on water, encourage sobriety for water use, and define collective regulations for it, as a matter of urgency.