The Wachirathan Waterfall (pronounced "wah-chee-rah-TAHN"; also spelled Vachirathan) was the second major waterfall on the way up to the summit of Doi Inthanon. Of all the waterfalls we saw in Doi Inthanon National Park, we thought this one had the most power and spray, and it was probably the most popular of the lot on the mountain's slopes (probably due to its ease of access).
Even though our guide told us that this was 80m in cumulative height, I'm thinking it was more like 40m or so. The falls was also wide, but it was segmented when we saw it during our trip, where the far right section of the falls were stringier than the left. I reckon during the wet season, this would be both wide and raging (with all the segments we saw consolidated into a monster).
But even though we visited during the start of the Dry Season, the falls had enough volume to produce plenty of mist. And from this mist, we did see rainbows though it made photography a bit of a challenge. There were also parts of the loop walk around the waterfall (especially just downstream of it in its lower reaches) that was quite muddy and slippery as the path was drenched in its mist. So I have no doubt that this was a year-round waterfall though I'd imagine the flow becomes lighter as the Dry Season progresses.
Directions: Since we were escorted, we can't give exact directions. However, we can say that it took us about 25 minutes to drive downhill from the Siriphum Waterfall or about 20 minutes uphill from the Wachirathan Waterfall.