Aerial photographs are used to develop a relationship between the number of debris slides generated during a hydrologic event and the size of the event, and the long-term average debris-slide frequency is calculated from climate records using the relation. For a site in California with an average of 8.3 slides km-2 yr-1, a sequence of four photo sets (representing 10--15 years, 35--50 observed slides, and 4--6 large storms) is needed to estimate the long-term debris-slide frequency to within 30% of the actual value (p=0.90). If climatic records are used, a record length of 5--10 years (17--35 observed slides and 2--4 significant storms) is sufficient to provide the same accuracy. The climate-based model suggests that debris-slide frequency changed from approximately 1.6 to 8.3 slides km-2 yr-1 during the late 1930s owing to an increased frequency of high-intensity storms. The model accurately predicts the change in slide-scar density observed on sequential aerial photographs following the climatic shift.