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Groundwater Drought Indicators

Groundwater is an important factor, or indicator, for drought. Like soil moisture, which is the amount of moisture in the top layers of the ground where plants and crops grow, groundwater is the amount of moisture in the ground but at much deeper levels. Groundwater is recharged from the surface, from sources such as rainfall and streamflow. If there is a substantial amount of groundwater, an aquifer is formed which can be tapped for agricultural, municipal, or industrial use via extraction wells. The depth at which the ground is completely saturated with water is called the water table.

Groundwater is measured via a network of wells, which monitor the depth of the water table. For drought-monitoring purposes, this well data needs to be compared to the historical record of the well, usually in the form of a percentile. The following factors affect the usefulness of well data:

  • the length of record of each well varies,
  • some wells report on a near-real time operational basis, while others report on a delayed basis and only periodically,
  • the water table level is affected by water drawn out for irrigation, municipal, industrial, and other non-drought purposes, so well data may not reflect true drought conditions, and
  • there is limited spatial coverage of wells across the country.

As a result, other groundwater monitoring methods are being developed. One is satellite observations. Water has a different density than land. Saturated ground has a different weight, or "mass", than dry ground. Mass affects gravity. Gravity affects satellites. By measuring the effect of varying gravity fields on a pair of satellites as they orbit the earth, estimates of groundwater can be calculated from the GRACE (Gravity Recovery and Climate Experiment) satellite project.