Droughts

What are droughts?

When asked to define drought, there isn't just one right answer. Many different definitions exist. Drought implies a lack of moisture for an extended period of time which in turns causes a deficit of moisture in the soil. This can mean different things for different areas. For areas which receive high amounts of precipitation (rain, snow), a condition of drought can develop more rapidly than in an area which doesn't.

Many problems can arise due to droughts, including crop damage and water supply shortage. How severe a drought is depends mostly on the degree of the deficiency, the time period, and the size of the area affected. The timing is also a significant factor with the duration of droughts.

Droughts result from a combination of meteorological, physical and human factors. Their primary cause is a deficiency in rainfall and the timing, distribution and intensity of this deficiency in relation to existing storage, demand and water use. Temperature and evapotranspiration may act in combination with insufficient rainfall to magnify the severity and duration of droughts. Moreover, due to changes in land use, water demand and climate the droughts in future may become more frequent and more severe.

Examples in the Danube River Basin

When

Where

Why

1992/1993

Bulgaria, Hungary

very hot and dry summer (1992), contuinued with below average rainfall until October 1993; very low soil moisture in Bulgaria caused a severe loss of acricultural production.

1996

Bulgaria

hot and dry summer accross the entire country

2003

Danube Basin

below-averge rainfall in combination with above-average temperature throughout the Danube Basin. In September the discharge level in the lower Danube basin reached the absolute minimum since 1840.

Case Study: The 2003 Droughts

Contrary to 2002 a remarkable deficit in rain and snow was reported by the Danube countries in 2003. In Germany a long-term drought between February and September was caused by the accumulation and stability of anticyclone weather conditions. Hence the air temperature and sunshine duration exceeded the annual mean values and altogether ten months of the year were drier than average.

Many gauging stations in Germany recorded the hottest summer ever observed. In the other upstream countries low precipitation (rain, snow) and excessive temperatures were reported in 2003 as well.

The overall water deficit in the upper Danube was partly compensated by increased glacial water due to excessive melting in the alpine regions.

But in general, in 2003 the Danube was withering away. At the end of the summer, the water level fell to the lowest levels over a century, stranding ships and barges from Southern Germany to the Romanian lowlands. Romania's Cernavoda Nuclear Power plant, which draws coolant water from the Danube, was forced to shut down for nearly a month.

In the lower part of the Danube Basin the weather was also warmer when compared to the average climatic pattern as it can be seen from the example showing monthly average temperatures in Romania.

Average monthly air temperature variation in Romania in 2003 compared to the long term average pattern 1961-1990.

This climatic phenomenon influenced also the temperature of the surface waters. In the summer period, the monthly average water temperature of the Danube at Bezdan (Serbia and Montenegro) exceeded the maximum-recorded values. In Hungary, the dry season caused even a significant decrease of water levels of the two biggest natural lakes - Lake Balaton and Lake Velence.

The values of the total precipitation (rain, snow) in 2003 as well as the relative precipitation (compared to a long-term annual average) in some countries are shown below:

Country Total precipitation in 2003 [mm] Relative precipitation in 2003 [%]
Germany 714 71
Austria 760 74
Czech Republic 547 76.5
Slovakia 573 75
Hungary 570 95
Serbia and Montenegro 587 88
Romania 522 88.7

What is done to cope with droughts?

The Joint Research Center of the European Commission (being actively involved in the work of the ICPDR) is developing a set of droughts indicators, incorporating the impact of water stress on the natural vegetation and on agriculture. This also includes the production of a soil moisture and plant water stress map (using the so called LISFLOOD model). The LISFLOOD model has been specifically developed to simulate floods in large European drainage basins. Full basin-scale simulations can be carried out in such a way that influences of land use, spatial variations of soil properties and spatial precipitation differences, e.g. by increased flood frequency through climatic change, are taken into account .

The objective is to carry out a feasibility study on drought modelling for Europe, using a test region within the Danube catchment area as an example. This study will draw recommendations for the future development of a European Drought Alert System.

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