The Colorado Basin River Forecast Center uses the NWSRFS 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 1975 through 2005. 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 supplimental will indicate higher total contents. This is not
indicative of the longer term soil moisture influence on ESP.