Seasonal streamflow forecasts
Overview
Seasonal streamflow forecasts, also called seasonal water supply forecasts, are widely consulted by water managers and water users throughout the Colorado River Basin to inform general expectations for the coming runoff season and to inform specific operational decisions such as reservoir release schedules and water allocations. Forecasts are issued from mid-winter (Dec/Jan) through July for a several month forecast period: April-July (most common), March-June, or May-August, depending on the usual runoff peak.
There are two primary federal providers of operational seasonal streamflow forecasts for the basin:
- Colorado Basin River Forecast Center (CBRFC), one of the 11 NOAA National Weather Service River Forecast Centers
- Natural Resource Conservation Service (NRCS) National Water and Climate Center
CBRFC and NRCS use different modeling approaches and their respective forecasted streamflow volumes for the same forecast point may differ. Before 2012, they coordinated their forecasts so that their forecasted streamflow volumes were the same. CBRFC issues forecasts for 143 points within the Colorado River Basin, in addition to 33 points outside the basin (eastern Great Basin). NRCS issues forecasts
Relevance
CBRFC seasonal streamflow forecasts are key inputs into Reclamation's operational models, 24-Month Study and MTOM. The forecasted streamflow volumes for the coming season and water year influence the modeled reservoir conditions and thus operational decisions and planning by Reclamation and many other agencies.
Some basin water agencies produce their own seasonal streamflow forecasts for gages not covered by CBRFC or NRCS, typically using methods similar to those of NRCS (see below).
Methods
In the snowmelt-dominated headwaters of the Colorado River Basin, predictability of streamflow on seasonal timescales in the basin arises from the watershed's initial moisture conditions: the observed snowpack and soil moisture conditions. The skill of the forecasts increases through the winter and spring months as the accumulating snowpack provides more information about the upcoming runoff. Soil moisture deficits (or surpluses) in the fall, before snow begins to accumulate, indicate whether an an unusually low (or high) proportion of the snowpack will become runoff in the stream.
Lower-elevation watersheds which have little or no snowmelt contributing to the annual hydrology cannot be forecast as skillfully as higher-elevation watersheds that are snowmelt-dominated. In all cases, the primary source of forecast uncertainty and error is due to lack of knowledge of how the weather and climate will evolve between the date the forecast is issued (e.g., April 1st) and the end of the forecast period (e.g., July 31st). Other uncertainty and error comes from inaccurate depictions of the snowpack and/or soil moisture as input into the forecast model, and biases in the model itself.
The CBRFC uses a conceptual (or simple dynamical) modeling framework for its seasonal forecasts, coupling a snow model (SNOW-17) with a rainfall-runoff model (Sacramento Soil-Moisture Accounting; SAC-SMA). Conceptual models draw from our understanding of the physics of the real-world watershed, but have very simplified representations of watershed attributes and processes, with 5-20 components and adjustable parameters to relate them to each other. CBRFC runs their models throughout the forecast season (December-July), using historical daily precipitation and temperatures from each year of 1981-2015 to stand in for the unknown future weather. This creates an ensemble of 35 forecast traces, each starting with the same observed initial moisture conditions but individually evolving the modeled snowpack and runoff through the end of the forecast period given the winter/spring weather of 1981, 1982,...2014, 2015. These ESP (Ensemble Streamflow Prediction) forecasts are issued daily, and then become the primary input to the official forecasts issued monthly and semi-monthly.
NRCS uses a statistical modeling framework, employing separate regression models for each gage that are calibrated on the historical relationships between streamflow and several watershed predictors, typically snowpack (SWE) and accumulated water-year precipitation at 2-5 SNOTEL sites. Statistical modeling has been the standard-bearer for seasonal streamflow forecasting in the western U.S. for the past 50 years, and only very recently has its skill in the Colorado River Basin been exceeded by dynamical forecasts from CBRFC--which itself previously relied more heavily on statistical models similar to the NRCS framework.
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Recent and Ongoing Research
CBRFC maintains an active research program to improve
Data and Tools
CBRFC
CBRFC has multiple ways to access and view their seasonal water supply forecasts and related data:
CBRFC Conditions Map - Water Supply Forecasts
The Conditions Map can display several different types of hydroclimate information, depending on the tab selected. The Water Supply Forecasts tab, when opened, defaults to the most recent official 1st of month forecasts, but can also display the latest daily ESP forecasts ("Latest Model Guidance"). Clicking on the triangles (forecast points) brings up a thumbnail forecast evolution plot for that point; clicking within the plot opens a page specific to that forecast point, with an interactive forecast evolution plot and links to plots for previous years, calibration and validation data, and historical streamflow volumes for that point.
CBRFC Upper Colorado Situational Awareness
This page shows the most recent official forecasts for Lake Powell inflows (~all Upper Basin runoff) for April-July and the water year; maps of observed/modeled soil moisture, snowpack, and precipitation used to inform the forecasts; and graphs showing how the sub-basins' contributions and overall forecasted volume compares with average conditions.
CBRFC Water Supply Official Forecast List
This page features an interactive table with the official 1st of month forecasts for all forecast points. Like the forecast map, it links to the pages for each point.
CBRFC Water Supply Forecast Discussion
This document (PDF) discusses provides a general overview of the official (1st and 15th of month) CBRFC forecasts, describing the weather/watershed conditions that have led to the forecasts, as well as forecasts and outlooks for upcoming weather.
NRCS
Additional Resources
State of the Science Report
Chapter XX of the State of the Science report (XXXXX
New and Notable Research (2020-present)
[<URL FOR PAPER>]
Summary