Hydrograph recession limbs - a case of deja vu

The upper Columbia River Basin is snowmelt-dominated and characterized by a relatively dry precipitation regime in summer and early fall. Streamflows in the Basin peak in May or June, followed by a very constant recession decay from July to October. A method was developed to forecast monthly river flows during the latter part of the recession period. This method takes advantage of the uniformity in recession-period decay and the hydrologic momentum of large watersheds. A model-year approach is used where the target year's monthly flows are compared to the same recession period of all the years in the historical record. The period of record is searched for the model year which best matches, based on an appropriate objective function, the target year's hydrograph. Using regression analysis, a relationship is developed between the model year and the target year and then employed to predict future monthly flows in the latter portion of the recession period. Nearly every snowmelt hydrograph in the historical record has at least one other snowmelt hydrograph that very closely resembles it. This 'hydrograph similitude' can be used as a forecasting tool.