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As of early May in Chiang Mai, we are in a water shortage. Water Catchment, aka Rain Water Harvesting (RWH) can virtually eliminate short-term droughts and successfully mitigate even long-term droughts. It all becomes a question of scale, in terms of how much land can be used for catchment and storage.
Everything comes back to water use. If one uses little, then little needs to be catched and stored. Also, water reuse can reduce the need for water. The easiest are to use shower and sink water to flush toilets, and to use laundry water to water the lawn and additional fruit, vegetable, and herb gardens.
The simples approach is to build an underground cistern. While that can have some costs involved, especially doing this properly. In Chiang Mai, most water falls about six months out of the year and the other six water is largely inadequate. This means about 5 months of water is needed in terms of storage. Doing a quick 15,000 liters times 5 months = 75,000 liters needed for storage. Savings from water catchment is probably something like 800 THB/month in drinking water and regular water consumption. Not much, considering the cistern and water filtration need to be built and maintained. That is 75 cubic meters, such as 12m x 2.5m x 2.5m, a slightly deeper than average lap pool (alternatively 12.5m x 2m x3m = a slightly wider swimming lane.
In a sealed or covered cistern, there is little evaporation. If one wants to use a narrow-lane swimming pool for storage, then a simple cover should limit evaporation. The only trick is that unless the pool or pond will remain full, it needs to be designed to handle a drop in water stored (and still be functional). For example, a 10m x 2m x 3.75m (deep) pool would allow for comfortable lap swimming even when half-full, and possibly even when 25% full.
There are really only three parts to catchment: a roof, a diverter (if needed) and dealing with the first 2.5mm of rain runoff, which contains dirt and debris, bird poop, and the like. This goes into the water storage. Also, there is the overflow component, which is important in the event of heavy rains or when there has built up a significant amount of water storage already.
Water filtering for cleanliness, especially to drinking-water quality, is important. Many sources of water have levels of toxins and pathogens, as well as agricultural and industrial contaminants. A bio-char-based, four stage water filtration system should work well for drinking water.