1　Introduction

A vast number of hydraulic projects have been built in China in the past few decades for the purpose of flood control，irrigation，generation of electricity，navigation etc.These projects have changed the hydrological regime in basins quite a lot，especially in runoff fomation and routing.It is impossible to obtain satisfactory hydrological forecasting results if the changed condition is not fully considered.For example，a runoff yield forecasting scheme was constructed for the Fengshuba basin in 1982.Its qualified rate of scheme calibration was 85.1%.Here.the qualifled rate means the percentage of the number of forecast flood events within an allowed error with respect to the total number of flood events.However.in the period 1985—1987 a large number of hydraulic projects were bulit.which systematically caused the forecast runoff volume to be overestimated.In particular three events of very large flooding，which occurred after a long drought in summer，were overestimated by more than 70%.Hence.the authors began to study the effect of hydraulic projects on floods and herein propose a runoff yield model which takes into account the influence of a group of reservoirs.