Soil moisture refers to how much water is being held in the ground at any given time. It is important in determining whether plants are able to grow, flower, and survive. It also plays a role in how much water reaches streams and rivers during rain storms or spring snow melt.
For the Forest Health Index, we use soil moisture data from GRACE, a NASA satellite that uses changes in gravity to measure water on Earth’s surface and in the ground. It operates at a broad scale, grouping together moisture from 16 mile (or more areas). It can provide a good general sense of whether a summer is wetter or drier than the year before and reveal when soil is drying from month to month.
Near average: Soil moisture near average last year. Forest ecosystems are well adapted to this amount of soil moisture. Other factors, however, are also important factors in determining forest health relative to water needs, such as air temperature, seasonal timing of precipitation, and conditions in the previous year.
Because most plants draw the majority of their water from the soils, soil moisture is necessary to plant survival, and unusually prolonged periods of dry conditions can stress plants, preventing them from flowering or seeding or even leading to death. Soil moisture also plays an important role in our water cycle. Soils act as the middle-man between precipitation that falls from the sky, water taken up by plants, water lost back to the atmosphere during evaporation, and water that is stored in the ground or runs off to join streams and rivers.
Rainfall, snowmelt, air temperature, and prior-year soil moisture conditions all drive annual average soil moisture values, and soil moisture can experience large swings from year to year. Since our record of soil moisture is relatively short (dating back to 2002), no clear trends have yet emerged, but as air temperature in Colorado continues to rise due to climate change, we may see more soil drying unless changes in rainfall patterns also take place.