3.1.5 Synthesis of long-term discharge records

Where possible, long-term flow records from USGS gage data combined with spill and flow diversion records from project facilities can be used to create synthetic long-term regulated and unregulated flow records. However, study reaches are often located in ungaged basins and the USGS gage period of record often does not encompass the complete period of interest (i.e., either the unregulated or regulated periods are not part of the period of record). Therefore, it is often necessary to construct synthetic long-term flow records for regulated and unregulated periods of record in ungaged basins. A common approach to constructing synthetic flow records is to install temporary discharge monitoring stations and correlate these flow records to discharge records from nearby USGS gaging stations. When temporary discharge monitoring stations are not installed, a combination of proration (drainage area relationship between the point of interest and the drainage area of a nearby USGS gaging station) and water budget techniques can be used to construct a synthetic discharge record. Proration techniques between ungaged basins with few or no discharge records and gaged basins with long-term discharge records are appropriate as long as the correlated sites drain similar geologic/geomorphic terrain with similar climate (temperature and precipitation) and land use patterns. Selected reference

Tague, C., and G. E. Grant. 2004. A geological framework for interpreting the low-flow regimes of Cascade streams, Willamette River basin, Oregon. Water Resources Research 40: doi 10.1029/2003WR002629.