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On the flat open land, 10KW is enough broadcast juice on the AM band to go 1000 miles and still be heard.
Below is a link to a graphic about this from an article appearing in Radio World Engineering Extra (June 8, 2011), and reprinted in the Summer 2011 Edition of the IEEE Broadcast Technology Society Newsletter.
This chart was referenced to the groundwave propagation curves of the FCC for AM broadcast stations of several power levels and very good earth conductivity, over most of the AM broadcast band.
A field of 0.5 mV/m (shown in the chart) is getting close to the limit for useful reception in quiet areas, assuming no interference from other broadcast stations.
I agree with many here who point out that there isn’t much value to them in a high power AM station that broadcasts the same programs in the same time slots that are available on local, lower-power AM stations.
But OTOH, the high-power station may be providing one of few signals that are useful in some parts of its daytime coverage area.
http://i62.photobucket.com/albums/h85/rfry-100/MWCoverage_zps05117c8c.jpg
