Keeping water in your garden depends on a couple of things. While sunlight and temperature are the main factors when it comes to evaporation, others include soil type, plant type and ground cover.
The amount of sand and clay in the soil define the soil type. The more clay soil contains, the longer it holds water. Soil with high clay content has a much lower evaporation rate than sandy soil.
A smooth surface on the soil reduces its evaporation rate. A rough soil surface has a greater surface area, allowing for increased contact with environmental conditions. Strong winds increase the evaporation rate.
Soil with anything growing on it has a lower evaporation rate than bare soil. The denser the plants, the lower the evaporation rate. You can lower it even more by adding mulch, wood chips or grass clippings. This helps hold moisture in the soil and lower its evaporation rate by protecting it from sunlight, temperature and wind.
But the main trick is getting enough rain to make the soil moist.