The strong association between savanna vegetation and climates with a hot, wet summer and a warm, dry winter provides the first clue that water availability is a key factor of savanna ecology. The dominance of water availability as a determinant of savanna structure and function is particularly strong at the dry end of the savanna spectrum. The pattern of water supply in relation to the water requirements of the plants influences both the physical vegetation structure of dry savannas and their ecological composition. A traveller passing down the aridity gradient from a moist savanna, receiving perhaps 1000 mm rainfall per annum, into a desert shrubland or grassland receiving 300 mm per annum will be struck by the progressive decrease in the height and density of the trees, and the consequent change in the proportion of trees to grasses. A similar change can be noted when passing from a sandy to a clayey soil under the same climate and is due, in part, to the different hydrological characteristics of the two soils. The obviousness of the importance of water in savannas can sometimes be a hindrance to understanding their ecology, since it conceals the importance of other more subtle factors.

Water availability determines savanna function by controlling the duration of the period for which processes such as primary production and nutrient mineralisation can occur.