Evolution of the Hydrological Cycle over France: Processes and Uncertainties – ECHO

Given data availability, impacts studies trying to correctly sample uncertainties due to global climate projections -which is critical-
have been mainly based on statistical downscaling. Thanks to new data availability, it is now possible to use a dynamical
downscaling approach within a multi-GCM framework. We will use both a dynamical and a statistical approach for downscaling, to better estimate the impact of the downscaling method on simulated hydrological changes.
Given the large spread of global climate projections, they
remain a major source of uncertainties in hydrological impacts. An important part of this spread is due to limitations in the understanding of critical processes playing in climate change. A critical aspect of climate change over France is summer drying. Yet, the magnitude of summer drying remains
very uncertain, as it depends to a large extent of land-atmosphere interactions that are poorly
understood. In this project, we will study land-atmosphere interactions based on observations and simulations. It will allow the realism of climate
projections over France to be assessed and therefore to give more weight to results from more
realistic models. It will lead to more reliable ensemble regional climate and continental hydrological projections.
The last central question of this project is the role of intrinsic climate variability as it is also an important source of
uncertainties in climate projections over the next decades. The very recent field of decadal climate predictions
aims at reducing those uncertainties through the initialization of the ocean component of global climate projections.
Given the novelty of this field, little is known on the skill of decadal predictions at the regional scale, and concerning the
continental hydrological cycle. Those are questions we will try to answer in this project, focusing on Europe and France.

To be completed

In summary, this research project will lead to more accurate hydrological projections over France, it
will provide methodological insights useful for impacts studies in many other fields and, more fundamentally, it will
contribute to a better understanding of the evolution of the hydrological cycle over France in a changing and variable
climate.

Boé J., 2012: Modulation of soil moisture–precipitation interactions over France by large scale circulation. Clim. Dyn, published online, doi:10.1007/s00382-012-1380-6

If some past studies have shown that serious impacts on the continental hydrological cycle over France are to be expected in the future climate, some major uncertainties still remain and current knowledge is therefore far from sufficient to allow the design of efficient adaptation strategies. Causes of uncertainties in hydrological projections -regarding the evolution of river discharges, soil moisture, snow-pack etc.- are manifold and their importance differs. Our main objective in this project is to improve the understanding of the processes playing on the evolution of the hydrological cycle over France in order to reduce some of those uncertainties, that have been recently shown to be dominant: the ones associated to downscaling, to climate model imperfections, and to intrinsic climate variability.
Downscaling -i.e. deriving from coarse global climate model (GCM) projections the high resolution forcing necessary for impact modeling- is an important source of uncertainties in the projected impacts of climate change. Given the lack of availability of ensembles of high resolution regional climate projections forced by an ensemble of global climate projections until very recently, impacts studies trying to correctly sample uncertainties due to global climate projections -which is critical as they are dominant- have been mainly based on statistical downscaling. Thanks to new data availability, it is now possible to use a dynamical downscaling approach within a multi-GCM framework. As statistical and dynamical downscaling do not share the same advantages and shortcomings, there is an high interest in conducting a complete hydrological impact study based on dynamical downscaling, which will be done in this project.
Given the large spread of global climate projections, they remain the major source of uncertainties in continental hydrological impacts over France. An important part of those uncertainties is due to limitations in the understanding of critical processes playing in regional climate change. The most critical aspect of climate change over France is certainly summer drying. Yet, the magnitude of summer drying remains very uncertain in current projections, as it depends to a large extent of land-atmosphere interactions that are poorly understood. In this project, we will study land-atmosphere interactions and in particular the soil-moisture precipitation feedbacks based on observations and very high resolution mesoscale simulations. It will allow the realism of climate projections over France from different models to be assessed and therefore to give more weight to results from more realistic models. It will lead to more reliable ensemble regional climate and continental hydrological projections.
The last central question of this research project is the role of intrinsic climate variability as it is also an important source of uncertainties in climate and impacts projections over the next decades. The very recent field of decadal climate predictions aims at reducing those uncertainties through a correct initialization of the ocean component of global climate projections. Given the novelty of this field, little is known on the skill of decadal predictions at the regional scale, and concerning the continental hydrological cycle. Those are questions we will try to answer in this project, focusing on Europe and France through an analysis of nee climate decadal predictions and realization of decadal hydrological predictions for the main French river basins.
In summary, this research project will lead to more accurate hydrological projections over France, it will provide methodological insights useful for impacts studies in many other fields and, more fundamentally, it will contribute to a better understanding of the evolution of the hydrological cycle over France in a changing and variable climate.