Global snow and ice cover are significant factors in the seasonal and long-term water balance equations on worldwide and regional scales. Snow and ice also have a strong influence on climatic conditions, or more properly, they reflect climatic change. Each year, between a fifth and a quarter of the earth's surface is covered by snow, and nearly 80% of the earth's fresh water supply is in temporary storage in the snow and ice masses that cover land surface alone. Seasonal snow cover represents a much smaller portion of the fresh water supply, but locally it is critical. For example, the Colorado River derives 90% of its flow from snowmelt.