| Based on the monthly precipitation,yearly precipitation of 7 weather stations in Tianshui 1961～2005(represent station 1936～2005),frequency of drought disaster and the yield reduction caused by drought disaster,the frequency of drought disaster in temporal distribution and degree of loss had been analyzed.The results showed that the frequency of drought had been increased from 1961 to 2005;the highest frequency of drought disaster was in hot summer while the lowest was in spring in a year.The effective coefficient of drought disaster to main crop has been given and the assessment of degree of risk to planting crop also has been made. |