Hfam II Scope and Development
The HFAM II Elements in the sketch are all interrelated. Water or energy flow in both directions between almost all of the elements; glacial segments for example experience weather and precipitation observed at meteorological stations and runoff flows to stream reaches and reservoirs. Aquifer elements may receive water from land segments, stream reaches or reservoirs and may release water to stream reaches and reservoirs. The number of elements that can be included in HFAM II is not limited.
Hfam II was designed in 2003 to 2005 and completed in 2007/2008, although additional features continue to be added. Goals of the design were increased flexibility and comprehensiveness. These goals continue a philosophy that began with the Stanford Watershed Model series, where concurrent continuous simulation of hydrologic variables like streamflow, soil moisture, infiltration and snow pack conditions was first accomplished. Prior to the Stanford models hydrologic processes like evapotranspiration, flood flows, or low flow frequency were studied separately, often on an ‘event’ basis independent of the behavior of related processes in a watershed.
As modeling became the ‘standard practice’ in hydrologic engineering, unnecessary compartmentalization of processes has persisted. When the scope of modeling is expanded important benefits are realized:
· Dynamic reservoir operations can be linked to current and future reservoir inflows.
· Reservoirs can be operated to compensate for downstream unregulated streamflows.
· Aquifer storage and discharge are combined with surface water for water management planning.
· The effects of physical changes in watersheds (land use, cover) and changes in climate can be analyzed.
Hfam II includes land segments, reaches, aquifer elements, reservoirs, spillways, low level outlets, diversions, powerhouses, and glacial elements. Simulations run continuously on short time steps (hourly). Comprehensive input time series are used (precipitation, temperature, solar radiation, wind, potential evapotranspiration) are used so that water balance and heat balance is calculated across a watershed.
Design and operational analysis can be done with four types of model runs:
These model runs are typically interrelated – e. g. optimization runs use probabilistic run results.