what is the appropriate formula for the PredTrend

Detrending-The data below represent the annual rainfall at a weather station in the Sacramento area of the Uncoin National forest (Cioudcroft, NM). 1. Plot the data and insert a linear trend in the data (and have it display the equation for the trend). 2. Using the equation for the trend, detrend the rainfall data and produce a new rainfall index value as shown in the video I posted on Blackboard. 3. Using the new rainfall index, construct a 3-year moving standard deviation of the index AND the original rainfall data. 4. Plot both the moving standard deviations ON THE SAME GRAPH, and insert trendlines for both series. Questions: 1. What, If any, trend is there in the rainfall near Cloudcroft over time? 2. Based solely on the coefficent of variation for the orginal and detrended serles, does the trend seem to have any effect on that measure of risk? 3. Based on the trends in the standard deviations, does the trend seem to mask/hide the increase in rainfall risk