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sac_sm.cgi NWSRFS Modeled Lower Zone Soil Moisture Images
Image Desciption
The Colorado Basin River Forecast Center uses the Sacramento Soil Moisture Accounting Algorithm (Sac-SMA) to simulate basin runoff from rain and snowmelt driven events. Sac-SMA divides the soil response into fast responding upper zone (approximately the top 20-50 mm of soil) and the longer term lower zone (generally deeper than 50mm.) The lower zone portion of the model helps to determine the volume of runoff during the spring and summer months by indicating a basin's antecedent condition prior to melt. Images linked below are a snap shot of the date's lower zone contents as a percentage of the average or median contents on that date. The daily average and median values for each elevational sub-area within the river basin are derived from the calibration process during which the model parameters are adjusted to match the historical observed stream flow. In the Upper Colorado and Lower Colorado this period is water years 1981 through 2010. Forecasters may use these images to help explain atypical basin responses from ESP.

It should be noted that once the melt has commenced, lower zone free supplemental will indicate higher total contents. This is not indicative of the longer term soil moisture influence on ESP.

More information about the Sac-SMA Model.
Daily Model Inches to Saturation Images

Virgin River Basin - Operational - Percent of Average

Upper Colorado - Reforecast - Percent of Average
The reforecast images are derived from the calibration time series during which historical precipitation and temperature records are used to generate a historical simulation of streamflow. In these images only regions where the lower zone total average is greater than 1 inch of liquid contents are plotted. These are typically the higher elevation regions and account for the majority of snowmelt runoff. While the other regions can contribute to in channel flow during liquid precipitation events, they do not normally accumulate sufficient snow to make up a significant portion of the Spring melt.