Lychee trees really like some level of humidity. If the air is dry (which it is in some climates) watering becomes all the more important. It's important to keep the soil moist, and make sure it does not dry out too much any time, but it's also important to make sure the soil does not become waterlogged, because the roots don't like to sit in standing water. So consistent watering is very important.
Also, if a lychee is growing in a container, consistent watering is even more important. Soil in a container is more prone to drying out, and also the root system is more limited by the smaller area, which makes it more difficult for the roots to supply enough water when soil moisture levels are very low.
Lychee can tolerate full sun in hot climates but it takes more water and the soil must be moist. The new little leaves growing on the plant are vulnerable to wind, and if not protected from the wind the leaf shape can become deformed as it grows.
Longan is more drought and cold tolerant than lychee.
