Oklahoma DroughtWatch

Science Center Objects

The U.S. Geological Survey Oklahoma Water Science Center continuously monitors the status of surface water in the State's principal rivers, reservoirs, and aquifers. Selected index stations with long-term continuous record spanning many drought periods are used to compare existing streamflows, reservoir storage, and ground-water levels with normal and extreme recorded values. These comparisons primarily are conducted in published USGS reports and webpages.

OK drought watch example
Oklahoma DroughtWatch example. (Public domain.)

Maps from USGS Drought Watch show the 7-day, 14-day, and 28-day average streamflow conditions for hydrologic units and streamflow stations. Thus, the maps show conditions adjusted for this time of the year. The colors represent average streamflow percentiles for the day of the year. Only USGS sites having at least 30 years of record are used. The data used to produce this map are provisional and have not been reviewed or edited. They may be subject to significant change.

Streamflow and Ground-Water Conditions

About Drought

Droughts are among the most costly hydrologic hazards in the United States. They are generally slow in developing, frequently occur over a long period of time, and can affect large areas and populations. Droughts occur with some severity almost every year in parts of the Central United States. The regional differences in drought characteristics across the United States are caused by various physical processes that affect precipitation.

 

Definitions of Drought

Droughts do not have the same meaning or significance to all people. No generally accepted definition is adequate, nor is one practical, because drought is the result of many different factors. In general, however, "drought is a condition of moisture deficit sufficient to have an adverse effect on vegetation, animals, and man over a sizeable area" (Warrick, 1975). Three common definitions are:

  1. Meteorological drought: "A period of abnormally dry weather sufficiently prolonged for the lack of water to cause serious hydrologic imbalance in the affected area" (Huschke, 1959).
  2. Agricultural drought: "A climatic excursion involving a shortage of precipitation sufficient to adversely affect crop production or range production" (Rosenberg, 1979).
  3. Hydrologic drought: "A period of below average water content in streams, reservoirs, aquifers, lakes, and soils" (Yevjevich and others, 1977).

About Hydrologic Drought

Drought begins with precipitation deficiency, high temperatures and winds, and low humidity, collectively resulting in "meteorological" drought. As soil moisture is reduced and plants are stressed, "agricultural" drought occurs. If the agricultural drought continues for several seasons, "hydrologic" drought results. In general, when the water content in streams, reservoirs, aquifers, or soils falls below the long-term average, a pending or potential hydrologic drought may exist. The severity of a hydrologic drought is not always obvious until these water supplies are seriously depleted. Even though a hydrologic drought may be widespread, many people may not be aware of its severity or extent. Some people may not be affected either because they live in an arid region where they have adapted to limited water supplies or because they live in areas where stored water supplies remain adequate to meet their needs during a drought. Water in rivers and reservoirs commonly has multiple and competing purposes including flood control, irrigation, drinking-water supply, recreation, navigation, hydropower generation, and wildlife habitat. Likewise, water in aquifers supplies agricultural, municipal, and domestic uses. Competition for surface water and groundwater escalates during hydrologic droughts, resulting in conflicts between water users.

Drought Indexes

Most commonly used drought indexes have significant limitations for adequately describing the status of an existing or pending hydrologic drought, one of the major limitations being that they do not always consider the status of available surface-water and groundwater supplies. For example, these techniques commonly measure only the deficiency in shallow soil moisture and antecedent precipitation of recent weeks and do not reflect availability of the deeper groundwater supplies that result from the previous months or years of precipitation.

Assessing Hydrologic Drought Severity with Index Stations

The U.S. Geological Survey Oklahoma Water Science Center continuously monitors the status of surface water in the State's principal rivers, reservoirs, and aquifers. Selected index stations with long-term continuous record spanning many drought periods are used to compare existing streamflows, reservoir storage, and ground-water levels with normal and extreme recorded values. These comparisons primarily are conducted in published USGS reports and webpages. The USGS WaterWatch and DroughtWatch websites, for example, develop maps and graphs of current streamflow and groundwater-levels in comparison to long-term normal (median) or extreme conditions. These products provide a quick visual overview of local and regional hydrologic drought conditions.

References

Huschke, R.E., ed., 1959, Glossary of meteorology: Boston, American Meteorological Society, 638 p.

Rosenberg, N.J., ed., 1979, Drought in the Great Plains- Research on impacts and strategies: Proceedings of the Workshop on Research in Great Plains Drought Management Strategies, University of Nebraska, Lincoln, March 26-28: Littleton, Colorado, Water Resources Publications, 225 p.

Warrick, R.A., 1975, Drought hazard in the United States: A research assessment: Boulder, Colorado, University of Colorado, Institute of Behavorial Science, Monograph no. NSF/RA/E-75/004, 199 p.

Yevjevich, Vujica, Hall, W.A., and Salas, J.D., eds., 1977, Drought research needs, in Proceedings of the Conference on Drought Research Needs, December 12-15, 1977: Colorado State University, Fort Collins, Colorado, 276 p.

Text modified from Hanson, R. L., 1987, Base flow as an indication of drought occurrence. In: S, Subitzky (ed.), Selected Papers in the Hydrologic Sciences: U.S. Geological Survey Water Supply Paper 2330. pp. 115-129.