“Prolonged drought” towards a common understanding of the phenomenon and of its impacts

Document produced by the expert network on water scarcity and drought

Current state of play in April 2007

Preambule

The following paper represents the current state of play of the reflections and “findings”, within the CIS, of the expert network on scarcity and drought (for the activity on “prolonged drought” and associated “temporary derogations”). For the expert network, this paper represents an intermediate document on this topic, in the short time period (4 months since the endorsement of the mandate of the group by the water directors). The current work shows that it will be impossible to provide one unique european wide indicator to define prolonged drought, the one produced by EEA today (on water stress) doesn’t reflect the situation. Due to the complexity of the phenomenon, to its variability according to climatic and geographic conditions it is better to work on different parameters to be included in local indicators. The presence or not of these parameters in local indicators will depend on their local relevance. The current paper establishes a first list potential parameters. This paper highlights also the importance of the time dimension when trying to understand “prolonged drought” and associated “temporary derogations”. The impacts, environmental, social and economics need also to be taken on board. Even if prolonged droughts are unpredictable natural phenomena, preventive mitigation measures could be proposed in sensible areas.

Finally, in the month to come, the expert network will still need to work on parameters, time dimension and impacts, in close contact with EEA and JRC in order to obtain a common understanding of “prolonged drought” and associated “temporary derogation” of the WFD.

Introduction

The environmental objectives of the Water Framework Directive (WFD) are the core of this EU legislation providing for a long-term sustainable water management on the basis of a high level of protection of the aquatic environment. The stated goal in the WFD is the achievement of environmental objectives by 2015, including the achievement of “good ecological status”(GES) in all European water bodies. Nevertheless, it is clear that for some water bodies it will be difficult to reach this “good ecological status” by the deadline provided. This is why the WFD, when setting the environmental objectives in its article 4, also makes provision for some possible exemptions from achieving the environmental objectives at local scale, when and where necessary. These exemptions are to be submitted to a transparent and open process of justification (technical, ecological, economic and social). It is also agreed that there should be a stepwise thinking process for considering what sort of exemption may be most appropriate: first consider extending the deadline and only if this fails consider setting “less stringent objectives”.

The article 4.6 allows for temporary deterioration in the status of water bodies as a result of “prolonged drought” among others. The purpose of the current document is to sum up general principles in order to prepare an adequate decision-making on the “prolonged drought” exemptions.

Another purpose is also to propose different indicators helping to identify prolonged drought in different geographic and climatic contexts. It aims also to integrate the duration the event and of its associated impacts. And finally it aims at providing an overview of both environmental and socio economic impacts associated to “prolonged drought” events. The Current paper is only dealing with “prolonged drought” and associated exemptions, at the light of article 4.6 and not tackling scarcity issues.

However, and as a guiding general common understanding, it could be considered that a “prolonged drought” occurs when the impacts this extreme phenomenon produces are high enough to affect the ecological status of a water body during a specific period of time, and important additional measures need to be taken into account during its duration to avoid permanent deterioration. Specific indicators need to be selected and defined to identify “prolonged droughts”, prevent and mitigate its effects on the water status.

As the WFD indicates in article 4.6., the river basin authority may declare a “temporary derogation” to GES, after the following conditions have taken place:

-  Practical steps are taken to avoid further deterioration

-  Exceptional circumstances could not reasonable be foreseen

-  Measures are included in the program of measures (and/or drought management plan)

-  Exceptional circumstances are reviewed periodically (e.g. through a follow up of the POM and/or drought management plan)

-  Summary of effects of circumstances is included within the RBMP.

In order to avoid drought effects, it is recommended that river basin authorities establish an appropriate indicator system that allows identifying the different extreme phenomenon phases, predicts possible impacts, and establishes associated measures to apply. Drought status phases can be considered for example: normal, pre-alert, alert and emergency. Once the river basin or affected area reaches the emergency status, the phenomenon could be considered as “prolonged drought”. Its severity is directly related to its duration, additional climatologically conditions, specificities of the river basin, measures applied throughout the previous phases, and impacts to environmental social and economic uses.

The effects of a meteorological drought could greatly vary depending on the existing scenarios: basins with storing aquifers directly linked to the water body system, and/or regulating infrastructures (e.g. reservoirs) could be less vulnerable to impacts, while basins without storing capacity could be more rapidly affected. Other factors will undoubtedly influence drought impacts, such as demands and uses of the area.


Following, there are some examples of indicators, which river basin authorities might use, separately or aggregated, to develop an appropriate indicator system and determine the severity of a drought and needed measures to avoid impacts.

I Which indicators can be used to identify a “prolonged drought”?

1.1General points

The document on scarcity and drought, already validated by the water directors in June 2006, provides different definitions for both scarcity and drought. It shows that it is difficult to have one “Europe wide” definition of these events. The document presented to the water directors shows that there is a clear distinction between drought which is due to natural factors and water scarcity which is influenced by human activities. On a longer term both issues are understood as aridity (natural) and desertification (man made). It will be the same for the concept of “prolonged drought” which may largely vary from one region to another. The purpose of this paper is to draw a list of some technical criteria which could be helpful to identify a “prolonged drought” when occurring:

-  the river flow

-  precipitation values ( in mm (under usual values))

-  rainfall minus evapotranspiration

-  the size of the permanent bed of the river (problems to support water dependant ecosystems)

-  the level of water within the river

-  the level of water in the aquifer

-  level of water in reservoirs

-  the flow (or not) between the river and the aquifer

-  environmental indicators (e.g associated plant and fish species of the river ecosystem)

-  the number of rainfalls (days) ….

-  Soil humidity

-  Snow reserves levels and glaciers (where present)

-  the unability of water management measures to cope with the usual water use ….

These criteria could be addressed in terms of thresholds at different levels (district, sub-basin, water body) in order to identify the “prolonged drought” phenomena and to take the appropriate measures to protect both environmental and economic uses.

1.2 Defining drought

A ‘drought’ is a natural abnormality, caused by natural forces or ‘force majeur’. Precipitation values are the only indicator that does reflect in a sensible way those natural modifications of the water regime. All other possible water-related indicators, such as levels of water in reservoirs, aquifers or rivers can significantly be influenced by human uses, and reflect a mixture of drought and water scarcity situations.

The drought definition regarding precipitation values should be updated regularly, in order to include climate change trends and should be seen in the context of long-term trends.

The precipitation values indicator is related to the relevant geographical units, on the scale of sub-basins or basins. The current indicator of water stress established by the EEA is not able to reflect the situation of drought in Europe as it is elaborated at national scale, which is not the relevant scale to reflect drought issues.

A drought occurs when there is a temporary and localised lack of water. Statistically, it is a rare phenomenon or out of the normal pattern. Otherwise, we are confronted with water scarcity or dryness phenomena (long term disequilibrium).

Drought occurs when there is a lack of rainfall over a river basin and a resource shortage due to natural often increased by man made conditions.

The main effects are:

·  reduced flow, even dried-out waterways

·  Environmental impacts:

o  effects on river fauna and flora being under threat

·  Impacts on crops (P-EPT).

·  A drinking water supply shortage.

·  Sea water intrusion,

·  Water management intervention measures

·  Shortages for other water users, such as industry, navigation, irrigation, energy production plants,…

The artificial recharge of aquifers and storage reservoirs could postpone, reduce or cancel out these adverse effects depending on the severity of the drought episode.

Within the context of the Water Framework Directive, the focus is on drought in the sense of the effect it can have on environment and water uses. When defining drought, we have to focus on both impacts and causes (rainfall deficit). These causes are often linked to the following parameters which should be included in a common understanding of “prolonged drought”.

1.3 trying to define a “prolonged drought

The fact that time dimension has to be taken on board, when identifying a “prolonged drought” is explicitly shown by the word “prolonged”. In order to identify and justify prolonged droughts, the different River Basin Districts (RBD) or bio-geographic zones should develop statistical analysis of irregular situations of rainfalls. It seems that for some regions, the criteria will be only seasonal and for other multi-seasonal or even inter-annual, as for example in the Mediterranean. In any case, the ‘rarity’ of rainfall levels in comparison to historical data needs to be statistically shown.

Furthermore, WFD relates to the fact that those droughts “are exceptional or could not reasonably have been foreseen”. If a drought is exceptional, this should be related to its unusual duration or intensity, though all RBD should develop a historical analysis of drought recurrence.

Precipitations

Indicators are mainly focused in the precipitation which actually affects river flow and recharges waters: effective precipitation (precipitation minus evapotranspiration, P-ETP).

When monitoring rainfall, the relevant hydrological year (eg September of Year 1 to August of the following year) should be used instead of the calendar year.

Basic premise: there is no storage (except artificial storage), precipitations can only be considered as flux. However, on the major aquifers, weak kinetics causes the flux to be used artificially as storage.

The annual distribution of precipitation-linked inflow to environments is a very important factor to define a drought. The basic annual average is quite simplistic and must therefore not be used as it conceals temporal distribution. Indeed, if there is heavy rainfall in winter and low rainfall in summer, and there are no reserves, the environment is put under stress despite posting a correct annual average. Water availability must be in line with demands (drinking water, crops,…). Precipitation must be transformed in available waters (rainfall transformed in flow for surface waters and rainfall transformed in stored volume for groundwater). The topography, the natural storage capacity of soils and the length of rivers are important factors to be taken on board when evaluating water availability. At the end this availability must be compared with a so-called “normal” situation.

Rivers react very quickly to precipitation unlike groundwater (reaction time ranging from several hours for rivers to several months for the latter – without even broaching the subject of “fossil” groundwater!). The time lapse between precipitation and the observation of the consequences of the inflow or non-inflow of water must therefore be taken into account. This time lapse must be determined on a case-by-case basis (see the BRGM study for France). Mapping of response times needs to be produced, so that predictive models can be implemented. The predicted water levels must be compared with predetermined thresholds (flow and piezometry of objectives, minimum biological flows, historic thresholds etc.)

There is also the problem of connections between groundwater and surface water, with them draining each other. In a given year, low groundwater levels mean that the aquifer can no longer supply surface bodies, although rainfall is normal. The expected theoretical water flow and draught levels would be distorted due to the groundwater being recharged. Flux monitoring requires the implementation of a management model. This can only be done within the framework of a management plan which will need a few years to be finalised.

Flow in rivers

The presence of water in rivers is directly linked to rainfall, as response times are short. This statement must be qualified in the case of surface water which has been re-supplied. It is important to focus on the average flow, at the return period. In other systems, control through reservoirs or channels might reduce lack of precipitation effects, but in any case, the lack of water will produce overall impacts in the management units.

Dried-out riverbeds might be a direct consequence of droughts. This can be observed visually and can be measured using the km of dried-out waterway or be related to the total dried-out area in a given river basin. Qualitative indicators can be graded: reduced but visible flow, non-visible flow, dried-out.

In France, the average flow over several days is compared to the QMNA5 flow (low water flow occurring only once every five years – with a 1/5 probability of it not being reached each year). The number of days during which the flow is lower than the reference level (for example QMNA5) must be counted. This corresponds to the number of days during which there is insufficient dilution.

For instance, in France, in the frame of master plans (SDAGE of 1992 water act), flows «of dry season» have been established, above this value there is no «competition» between environmental functions and economic uses related to water. A «crisis flow» has also been established under this value drinking water service can’t be provided safely and aquatic species are in danger. These flows are established at a local level throughout a participative process involving local actors (water agencies, industrialists, ecologists, farmers,…).