Cooler temperatures at higher altitudes cause moisture in the air to condense and fall to the ground. Many mountain ranges receive over 20 inches of precipitation while most basins receive less than 10. Since the temperatures at higher altitudes are cooler, less evaporation occurs, providing a mesic environment. The lower basins maintain warmer temperatures with high evaporation and thus are considered semi-arid to arid. Rain from the mountains forms streams and rivers that flow into the basins.
Many basins remain undrained with no surface outlet to the sea.
Most playas hold water for only a short period of time after rain. Eventually, all the water that reaches the basin floor sinks into the unconsolidated sediments found there. This water fuels reservoirs of groundwater buried under the desert. Some basins, however are not closed and contain rivers that eventually make their way to the seas. Biologically speaking, much of the wildlands we are talking about is considered to be Chihuahuan Desert.