Long-term drought can negatively impact the water balance of plants, causing unstable crop physiological conditions and low crop yields, threaten natural ecosystems and water supplies. Soil drought occurs during long-term periods without precipitation. Drought in Bulgaria is characterized by different features during different seasons in the year. Spring droughts are especially typical in northwestern Bulgaria and in the coastal region of the Black Sea. Summer droughts are most intense and of longest duration on the coast of the Black Sea and in the Thracian Lowland of the Maritsa River basin. M. Sabeva investigated droughts between 1896 and 1960. She assumed drought to be periods in which 10 or more days passed having less than 5mm of precipitation. Bulgaria experienced several summer drought episodes during the 20 century, most notably in the 1940s, 1950s, 1980s and 1990s. Usually drought in Bulgaria occurs over most of the national territory. That is why severe droughts greatly damage agricultural crop production.