What is Drought?
The word drought conjures up images of various kinds of water deficits, and means different things to different people. For instance, a farmer looks at lack of soil moisture for plants as a water deficit, low ground ground water levels may imply lack of water for irrigation or for drinking water to a water engineer, a hydrologist may be concerned with low water levels in a reservoir for hydropower generation or recreational purposes, environmental engineers and ecologists would be worried about low streamflows and its influence on water quality and aquatic life, climatologists and atmospheric scientists are worried about precipitation deficits and associated impacts on climate dynamics, economists are interested in what water deficits mean to crop prices, livestock, and food industry, while social scientists grapple with the problem of how communities deal with water deficits and what kinds of risks is society willing to accept.
Drought maps and information on the intensity, spatial extent, and duration of droughts are crucial to a range of socio-economic sectors, such as agriculture, hydrology, and water resource distribution. Many scientific and economic interests depend on accurate drought information.What are the various kinds of droughts?
When a drought event occurs, moisture deficits are identified from many hydrologic variables such as precipitation, streamflow, soil moisture, snow pack, ground water levels, and reservoir storage. Because of the various ways droughts are understood by people, and the myriad ways in which it impacts them, a commonly-accepted definition of drought does not exist. Rather, droughts are defined based on whatever impacts a local community in the most direct manner. For scientific purposes, three types of drought are in common use (Dracup et al., 1980):
- Meteorological droughts are usually based on deficits in precipitation amounts as when compared to the long-term average of a region. However, how large the region should be to calculate the long-term average (precipitation varies in space as well), or how much of a departure from this long-term average constitutes a drought is not defined or standardized.
- Hydrological droughts are defined in terms of deficiencies in surface and subsurface water resources. Commonly used variables include stream flows at gauging stations, water levels in lakes and reservoirs, and ground water levels.
- Agricultural droughts reflect deficits in moisture that is available to specific crops occurs and assume an important role during crop growth stages when moisture is essential.
Though these different types of deficits tend to be positively correlated and are likely responding to the same trigger, they exhibit diverse temporal and spatial scales. An overall drought indicator, that encompasses multiple types of deficits along with their corresponding temporal scales, is therefore difficult to construct because of the complicated dependencies in the variables that are used to characterize droughts.How are droughts characterized? Droughts are assessed by various indices that have derived from hydrologic variables and have been motivated by varying needs. A partial list includes:
- Munger's Index (Munger, 1916)
- Blumenstock's Index (Blumenstock, 1942)
- Antecedent Precipitation Index (McQuigg, 1954)
- Palmer Drought Severity Index, PDSI (Palmer, 1965)
- Crop Moisture Index (CMI; Palmer, 1968)
- Surface Water Supply Index (SWSI; Shafer and Dezman, 1982)
- Vegetation Condition Index (VCI; Kogan, 1995)
- Climate Prediction Center (CPC) Soil Moisture Index (Huang et al., 1996)
- Standardized Precipitation Index (SPI; McKee et al. (1993)
There is no "best" approach or index for characterizing droughts. The drought status that is assessed from one indicator often does not correspond well with that obtained from a different indicator because of the complicated physical mechanics between infiltration, evapotranspiration, groundwater motion, base flow and direct runoff. In addition, droughts result from cumulative effects of water shortages over different periods of time. While drought status around the globe can be assessed from different sources, a commonly used source is the US Drought Monitor where the severity of a drought (D0 ~ D4) is determined based on various indicators (PDSI, CPC Soil Moisture, USGS weekly, Percent of normal, SPI, and VCI) as described by Svoboda et al. (2002). Through an Objective Blend of Drought Indicators (OBDI, a linear weighted average of several indicators), an overall measure of severity is obtained.