I am looking for county irrigation data for the US.  This data should
measure the number of acres that are irrigated in each county in a
given year.  I want data that ranges at least from 1975 to 1999 for
the entire continental U.S.  The census of agriculture and the
National Resources Inventory provide data for some years (which I
already have).  I would like all the data to come from the same source
to assure consistency.  I would also like to have supporting
information about the methodology behind the data.  It would be best
if I could download the data from the internet for free, but I am also
willing to consider paying a small amount for it.

Since 1985, the USGS has published a report on water use in the US
every 5 years.  The latest report is for 2000.  The data you want (and
much more) are available (free) for the years 1985, 1990, 1995, and
2000 at http://water.usgs.gov/watuse/.  Just click on the "Download
2000 data for counties" link for the 2000 data, etc. The methodologies
followed for compiling these data are fairly well documented in the
associated reports, which are also available from the above URL.

Thanks hfshaw,

If I don't find the data for all the range of years that I wanted from
the a unique source, the USGS link you added as a comment can be very
useful.  I can use their data to complement what I already have.  The
documentation provided in the site will help me with that.

I appreciate your help.