Since the1980s,because of scarcy precipitation in northern China,the level of farm-land groundwater has been decreasing and the storage capacity of soil risen accordingly.Ac cord-ing to the information observed from the experimental site of water resources to the south-east plain of Beijing,the0～3m layer soil was determined as the defined depth of soil reservoir and its pondage capacity was analyzed.Through computation it was found out that,before the rainy season during1991～1995,the normal annual available pondage capacity of soil reservoir was137.4mm(0～2m depth)and188.8mm(0～3m depth)respectively.The results showed that soil reservoir had very good regulation capacity and85%of the normal annual precipitation was stored in the3m deep soil(the defined depth of soil reservoir).So if some available measures are taken before an extraordinary rainstorm happens,then there will be no or less surface runoff ap-pearing and most of the precipitation will be stored in soils.