HYDRO FORECASTING

The relevant timeframes for hydro forecasting range from hours to even decades into the future, with different priorities and needs as detailed below.

Short-term Streamflow Forecasting

This type of forecasting usually extends between hours to days. The forecasting system is composed of hourly weather forecasts that are bias-corrected and fed into a runoff model. The initial watershed state is also fed into this runoff model. Many types of runoff models exist. 3TIER employs a class of runoff model that is referred to as physically-based, distributed hydrology model. The advantages of such a model are that the important physical processes, such as energy balance and evapo-transpiration are explicitly represented (not parameterized); the meteorological parameters critical in basins with complex terrain are spatially distributed; and, finally, the model lends itself to additional analyses such as the effects of land-use and climate change on watershed hydrology.

Short-term probabilistic forecasts are also developed upon the premise that any forecast contains uncertainty. Regulators and policy-makers are interested in quantifying the underlying uncertainty of the forecasts. This technique allows clients to specify confidence limits based on his/her perception of risk.

Long-term Streamflow Forecasting

This type of forecasting usually extends between months to years. The main difference between long-term and short-term forecasting systems is that climate forecasts are used as input to the hydrology models as opposed to numerical weather forecasts.

Climate forecasts are primarily based on the periodic oscillation of two major worldwide climatology phenomena:

• The Pacific Decadal Oscillation, with a recurring timeframe of about ten years and a strong gradient in the Northern Pacific.
• El Niño Southern Oscillation, with a recurring timeframe of four to six years and a strong warming in the Equatorial Pacific.

To create a long-term hydrologic forecast, the effects of these two climate oscillations on expected weather patterns in the region are fed into the hydrology model. The output is a seasonal streamflow forecast that can be compared to historic average values.

Watershed Planning

Analyses of this type consists of forecasts that range from years to decades in the future. The main building blocks of these watershed planning studies include: a forecast scenario (including climate change for instance), a watershed initial state (modeling saturated soil and record snowpack, for instance) and a runoff model scenario (considering urbanization and clear-cutting, for instance).