Variability of water availability is important both for the functioning of aquatic species and also for the availability of water for human use: water that is only available in a few wet years must not be considered renewable. Because most global runoff comes from areas of very low climatic variability, the total global runoff is generally of low variability. Indeed, even in most arid zones, there tends to be few problems with variability of runoff because most usable sources of water come from high mountain regions which provide highly reliable glacier melt as the chief source of water, which also comes in the summer peak period of high demand for water. This historically aided the development of many of the great
civilizations of ancient history, and even today allows for agriculture in such productive areas as the
San Joaquin Valley. However, in Australia and
Southern Africa, the story is different. Here, runoff variability is much higher than in other continental regions of the world with similar climates. Typically temperate (
Köppen climate classification C) and arid (Köppen climate classification B) climate rivers in Australia and Southern Africa have as much as three times the coefficient of variation of runoff of those in other continental regions. The reason for this is that, whereas all other continents have had their soils largely shaped by
Quaternary glaciation and
mountain building, soils of Australia and Southern Africa have been largely unaltered since at least the early
Cretaceous and generally since the previous
ice age in the
Carboniferous. Consequently, available nutrient levels in Australian and Southern African soils tend to be orders of magnitude lower than those of similar climates in other continents, and native flora compensate for this through much higher rooting densities (e.g.
proteoid roots) to absorb minimal
phosphorus and other nutrients. Because these roots absorb so much water, runoff in typical Australian and Southern African rivers does not occur until about or more of rainfall has occurred. In other continents, runoff will occur after quite light rainfall due to the low rooting densities. The consequence of this is that many rivers in Australia and Southern Africa (as compared to
extremely few in other continents) are theoretically impossible to regulate because rates of evaporation from dams mean a storage sufficiently large to theoretically regulate the river to a given level would actually allow very little draft to be used. Examples of such rivers include those in the
Lake Eyre Basin. Even for other Australian rivers, a storage three times as large is needed to provide a third the supply of a comparable climate in southeastern North America or southern China. It also affects aquatic life, favouring strongly those species able to reproduce rapidly after high
floods so that some will survive the next drought. Tropical (Köppen climate classification A) climate rivers in Australia and Southern Africa do not, in contrast, have markedly lower runoff ratios than those of similar climates in other regions of the world. Although soils in tropical Australia and southern Africa are even poorer than those of the arid and temperate parts of these continents, vegetation can use organic phosphorus or phosphate dissolved in rainwater as a source of the nutrient. In cooler and drier climates these two related sources tend to be virtually useless, which is why such specialized means are needed to extract the most minimal phosphorus. There are other isolated areas of high runoff variability, though these are basically due to erratic rainfall rather than different hydrology. These include: • Southwest Asia • The Brazilian
Nordeste • The
Great Plains of the United States == Possible water reservoirs inside Earth ==