Since the plum tree is mature, it is able to soak up the moisture that falls from the sky. I am assuming that there is a ground cover of grasses and weeds around it. It must already be pretty hardy because it has survived many drought years.
Now if it is not as productive as you wish, getting more moisture to the roots and trapping that moisture could be good. I would not dig a swale on a slope if you do not know what you are doing. If you trap too much moisture on the slope, then the ground could get too saturated and you could have a landslide or new erosion issues with the open ground.
I would simply build up a small berm (with chop & drop material ex. tree branches, weeds (before the seed head forms) a few inches high above the top drip line following contour and maybe at the bottom of the drip line on contour (if needed) to slow and absorb the rainwater when it rains. Also, I would add mulch all around the tree with the chop & drop material you find around the tree area.
Now if you have specific issues with your tree, then let us know. But if it is plenty productive, then I would spend my limited energy instead of digging swales and start planting other fruittrees on contour and do the same mulching methods as described. You'll trap all the moisture running down the slope without a swale and without risking oversaturating the slope and creating a mini landslide or creating erosion issues where you had not before.
What is your annual rainfall in Washington? You must get sufficient rain naturally?
https://en.wikipedia.org/wiki/Kent,_Washington#Climate You get 5 to 6 months of very wet winters and dry summers. With the method I described above, it should be sufficient to trap the limited summer rains. But remember the ground is very wet for the winter months and the land with adequate ground cover and mulching can trap moisture at the root zone to get the tree through the dry season. Plus, I would not start digging a swale and risk disturbing the roots.