title: For loop python - by year and groupby operation
body: <p>New here. I have rainfall data that shows seasonal totals from 1990-2011 for 24 rain gauges (i.e.'RG1, 'RG2'). I did a spatial join to associate each rain gauge with the watershed it's in. I'm interested in looking at these yearly - I did one for 2008, where it shows me the season rainfall totals for each watershed. How do I write a for loop that iterates over the data for each year (1990-2011) and puts it all in one dataframe? Any help is appreciated - thank you!</p>
  <pre><code>year2008s = rainfall_watershed.loc[rainfall_watershed['year']=='2008'].groupby('WATERSHED_NAME', as_index=False)['inches'].sum()
  </code></pre>
  <p><img src="https://i.stack.imgur.com/tYgUn.png" alt="screenshot of dataframe" /></p>