Forecast field methods of summer drought and flood during astronomical period and the successful results of 338 forecast tests in 6 countries of the northern hemisphere, where obvious effects of application in disasters reduction and increasing yield in agriculture were obtained, and reported in this paper. The methods and effects of numerical tests on hysteresis of the forecast field using dynamical model were also introduced and the preliminary mechanism knowledge of the hysteresis effects was briefly summarized from the relationships between earth or ocean and atmosphere. KeyWords astronomical period hysteresis effects trend forecast of drought and flood dynamical model

Tab.1 trend forecast tests of summer drought and flood in 6 countries of the north hemisphere Countries Year Range Correct/total China 1958~1999 6 areas in the Yellow River Basin 201 areas times/246 areas times, and the Yangzi River Basin several huge flood were predicted U.S.A 1971~1999 114 stations in 28 years 25 years times/28 years times, big flood in 1993 was predicted German, U.K 1983~1999 Berlin, London, Moscow and 59 years times/64 years times, 2 big Russia, Japan Tokyo floods in Europe were predicted (Data from the Central Meteorological Service and the World Climate Bulletin)