To understand the value of soil water to plants, one must first ask how water enters the soil, where it goes and how long it stays. Winter rain or melt water tends to infiltrate more deeply than summer rain and stays around for longer. At the lower winter temperatures, soil water can accumulate between precipitation events and continue to infiltrate downards. Once water reaches a certain "safe" depth between 20 -40 cm, depending on soil type, it is removed chiefly by plant transpiration. Water from summer rainfall rarely accumulates and typically wets only the top 0-20 cm, from where it is rapidly lost by evapotranspiration. Thus, summer precipitation causes brief pulses of soil moisture near the soil surface, while winter precipitation tends to recharge deeper soil layers with an annual cycle of depletion.

Desert plants differ greatly in their uses of summer and winter water. Some deep-rooted woody plants do not take up any summer rain water, but most shrubs, sub-shrubs and perennial grasses take up a mixture of shallow summer and deeper winter water. Annuals and succulent perennials take up the most summer rain water. This diversity in water use suggests that most desert plants are not able to take up water wherever and whenever it becomes available. Physiological and morphological tradeoffs restrict the water use of desert plants, so that every species leaves some water to be taken up by another species with a different water use pattern. By knowing what these tradeoffs are, one can make predictions about how the plant forms and functions should change with changes in the amounts of summer versus winter precipitation. Such changes in the patterns of precipitation may take place in the near future due to global warming and changes in land use.