Hello @hkarimi -
We have used historical data from USGS Water Data for the Nation - https://waterdata.usgs.gov/nwis. It’s a bit time-consuming to work with, you need to review the data to see how consistent and complete the data is on a monthly (or yearly) basis, and you’ll need to process the values to calculate average monthly (or yearly) values for comparison with the model. But it’s a good central spot to look for data, especially if you want to consider sites across a large area.
If anyone knows of observed US flow data that’s easier to work with, please share it!
~ Stacie