The proper way to water trees or grape vines is tens or hundreds of gallons at a time then let the soil dry down. Otherwise you don't get the root development that takes up nutrients and anchors the plant. Trees that are watered shallowly for their whole lives often blow right over on maturity. And, in watering in California, Arizona and probably most places in the western US you need what is called a leaching fraction. That means you need to water well beyond the root zone to push out and avoid the accumulation of salts. It is far more energy efficient to pipe or flow the water to the plant than carry it. And... we are talking about a significant amount of water.
This is what I mean by scale. People have trouble conceiving it in agriculture for some reason. They tend to believe a few computer controlled hydro gardens can feed the world. Nope. You know all the countless green circles you see on the ground while flying across the country? That's real food production in action and even with all that we may become strained. A change in global diet might help. Improved genetics might help. But farmers aren't stupid. Over the course of generations they have achieved some degree of efficiency. Gadgets might help get a bit more but the easy gains have been largely worked out.
Picture a daddy-longlegs spider, but with four jointed legs about one metre long (three always touching the ground while the fourth moves position). Then a light-weight body with storage and probes/applicators.
If the base station had water supplied, solar charging, etc - I'd find this useful in my vegetable garden. Swarms of them could potentially look after larger gardens.