Pai has 3 distinct seasons. The rain, the the dry (or cold season) and the hot season.
In order to keep the water flowing year round to irrigate the crops in the Pai River valley, engineers have constructed a series of weirs to dam up the streams flowing out of the mountains because the dry can last up 6 months.
There is an architectural marvel located on the way to Hua Chang waterfall on the Pai Beverly Hills road and another interesting one located at Happy Mountain (I wonder why they call it that) or the old jungle party rave site.
The below photograph was taken from the weir looking back at Happy Mountain early in the morning.
A weir is basically a huge barrier that dams up water till it reaches the top. Once it reaches the top water flows over providing a trickling stream. What the weir does is allow pressure to build up and the water redirected to an irrigation channel to serve the crops in the area.