Water floods over land that is normally dry when it rains heavily or the water level in rivers, streams, channels or drainage ditches rise. It can also be caused by other natural disasters such as a tsunami in coastal areas or a tropical cyclone with a storm surge.
The shape of land, vegetation, and soil determine whether it is prone to flooding. Land that has been cleared for farming and urban development, paved with asphalt or concrete, or burned by wildfire may be less capable of absorbing excess rainfall. Other factors can increase the risk of floods, including the timing and duration of rain, the intensity of the rainfall, and the amount of rainfall in an area at one time.
Some floods occur very quickly and are known as flash floods, enveloping communities in just a few hours. Other floods occur slower and last longer, harming homes, farms, and businesses as water slowly moves in over land that is not designed to hold it.
Scientists studying floods use data from NASA’s Global Flood Monitoring System, “GFMS”. It is based on rainfall data from the satellites and a computer model of the land surface. The model combines data on topography, vegetation, soil type, and climate to predict how much water will soak into the ground and how much will flow into streamflow. GFMS provides information to help communities prepare for and respond to floods. It also informs policy makers and the general public.