Sécheresse et eaux souterraines

Droughts and underground water

B.R.G.M., France

Abstract

The effect of droughts on underground water are of two types: there is the effect on supply and flowing conditions of free waters, consequences for exploitation of the resource. The sensitivity of free water to supply deficiency resulting from drought (winter mainly but also summer in relation to the rainy season that it precedes: the reconstitution of the humidity of the soil will reduce efficient infiltrations) depends on the part taken by meteoric contribution in their supply and the flow/stock ratio of each waterway.

Droughts reduce the low water mark flow rates of rivers and can reduce the basic level of the nappes that they drain, while increasing the regulatory role of these nappes: the contribution of underground water to keeping up the low water mark may increase. In other words droughts reduce the amount of water going in and increases the amount going out of waterways. Moreover, underground water can be further exploited to counter shortages of rainfall and water flows, with effects that would be more or less differed. The sensitivity of underground water to drought as well as its role in countering shortages depends to a large extent on the very varied hydrogeological conditions and situations in France.

Most free waterways resist annual droughts and are only sensitive to droughts over several years - whereas the captive nappes are insensitive, but their regulatory capacities are unequally exploited.