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<br />FORECAST COMPARISON <br /> <br />· Potential Evapotranspiration (PET): RTi calculated monthly estimates of PET using the Penman- <br />Monteith method for a grass reference crop (Allen et al 1998). Land cover data from the National <br />Land Cover Dataset (NLCD) was used to select single time-averaged crop coefficients to modify the <br />PET estimates (FAO 2008). According to the NLCD, coniferous forest covers approximately half of <br />the RIODELCO basin. The remainder of the land cover includes grassland, deciduous forest, and <br />shrubland. <br /> <br />· Actual evapotranspiration (AET): AET values were calculated from PET by assuming an AET/PET <br />ratio based on expected soil moisture conditions resulting from recent precipitation and snowmelt. <br /> <br />· Runoff: Monthly runoff values were calculated from observed flow data. Water balance results did <br />not change appreciably based on whether natural or observed flows were used. Therefore, observed <br />flows were used because the available natural flow data ended in 2005. <br /> <br />· Runoff coefficient (ROC): A monthly ROC value was calculated that represents the fraction of rain <br />plus melt that converted to streamflow at the RIODELCO gage. On a monthly time scale, the values <br />are sometimes unrealistic (e.g., greater than one) because of the lag time between rain plus melt and <br />streamflow. <br /> <br />Figure 4-2 presents the monthly water balance with important terms highlighted in yellow. The monthly <br />water balance terms are aggregated to a seasonal (April-September) basis for each year. The water <br />balance enor (WB Error) was calculated as the difference between the rain plus melt reaching the soil and <br />the loss from AET and runoff. This error represents terms that are not accounted for in the water balance <br />(e.g., soil moisture changes) as well as uncertainties in the individual estimates. Although the soil <br />moisture change was not estimated, the relatively constant seasonal water balance enors may indicate that <br />the soil moisture change over the April-September period was not significant for the years included in the <br />current analysis. Overall, the seasonal water balance enors were 10-15% of the seasonal runoff values. <br /> <br />5 <br /> <br />~Riverside Technology, inc. <br />