How hot do you run your res? If you run it hot I would make sure to compost the water. Compost water...sounds funny eh? Your hydro nutes will leech out of the soil pretty quickly if just straight added to it - however if you use your nutes to break down a natural carbon source you'll end up with a bunch of nutrient rich humus which will stick around for a bit longer. I would recommend making a huge compost pile of browns (dried grass, dried leaves, chipped wood, cardboard, paper, any carbon high nitrogen low material) and then pour the res water over the pile. As long as you hit the ideal 30:1 C:N ratio your pile will break down real fast to produce some killer organic matter with a bit of N and high in P and K - perfect for your garden.
Of course if you are lazy you can straight add it to your outdoor soil. Try to disperse it as much as possible and keep it away from water sources.