Rainwater harvesting, or similar methods of location specific accumulation. A lot depends on the terrain of the region on what you can engineer.
Most of these systems can be well hidden since you want to reduce evaporation and all that. Tapping a local watershed area into catch basins that feed buried cistern tanks from some for of natural run-off catch basin or artificial pond would be the most practical.
Problem is getting materials to a location and all that jazz. Creating ponds that look like part of the terrain is also a little harder than it sounds if you're not just lining a natural occurrence.
Cistern creation is probably the hardest work aspect. Lots of digging (again assuming nothing natural to line/tap into) and oddly enough maintenance to ensure they are not getting their feeds plugged, sealing some off, etc. Can be done with simple cans or really complex liner approaches to create underground "bags" of water. Key thing with all of them is to avoid collapse as they dry/are used up.
Food for thought. This is usually something you'd want in place well before summer in most locations.