A Closer Look at Drought: Defining Drought

“Research in the early 1980s uncovered more than 150 published definitions of drought. The definitions reflect differences in regions, needs and disciplinary approaches”           The National Drought Mitigation Center describing (Wilhite and Glantz, 1985)

As news agencies across the country have begun reporting the record heat and drought that plagued much of the U.S. throughout 2012 (see two articles from NYT, one on drought and one on heat) I thought it would be worthwhile to take a closer look at the issue by focusing on how we measure and report drought. The statistics used to underscore just how extensive the drought has been often belie the intricacies that go into measuring such events. Although this may seem semantic, how we report drought (and indeed what we report) has an immediate impact on farmers, water resource managers and tax payers. In the first part of this two-part post I’ll focus on what we report, and how drought affects different stakeholders.

To give an idea of how extensive the recent drought has become, and how different sources are thinking about and reporting the event, it is worthwhile to take a look at some of the statistics used to describe the drought. Being that 2012 recently came to a close, one metric that has been widely circulating is the year’s average temperature. The average temperature across the continental U.S. in 2012 (55.3 F) exceeded the previous record by a full degree Fahrenheit. According to an article published in EOS (Karl et al., 2012): “As of September [2012], every month since June 2011 had above normal average temperatures” (meaning top 1/3 according to data collected since 1895) “a record that is unprecedented” . As of January 1st 2013, more than 61% of the country was still experiencing Moderate-Exceptional drought according to the U.S. Drought Monitor. Although snow storms have swept across the Midwest recently, according to the Drought Monitor such precipitation is “enough to arrest further deterioration but insufficient to improve the drought depiction.  Precipitation in Oklahoma had little impact on reservoir and lake levels, and agricultural reports indicated that soil moisture remained depleted”.The USDA described the drought as “seriously affecting U.S. agriculture, with impacts on the crop and livestock sectors and with the potential to affect food prices at the retail level.”

Although statistics such as those above may be taken together to describe a single event, they in fact allude to all four major categories of drought – meteorological, agricultural, hydrological and socioeconomic – described by Wilhite and Glantz in 1985.

Meteorological Drought is often defined as the relative dryness of a region compared to the expected seasonal precipitation. This measure requires the definition of a normal or baseline period and as such is region specific.

Agricultural Drought links agriculture to the impacts that meteorological or hydrological drought. This definition is often dependent on the susceptibility of crops during the growing season (i.e. topsoil is relevant for planting but subsoil moisture is more important for maturing plants)

Hydrological Drought focuses on the effects that deficits in precipitation have on the hydrological system. These droughts are often out of phase with meteorological or agricultural drought because a below normal precipitation cycle will take time to work its way through the hydrologic cycle.

Socioeconomic Drought ties the impacts of hydrological, agricultural and meteorological drought to economic impacts. Socioeonomic drought occurs when demand outstrips supply as a result of drought. This definition incorporates the spatial and temporal distribution of supply and demand into the classification of drought.

[For a more complete description of these four main categories of drought, see here]

While news sources will often report some form of meteorological drought because it is immediately intuitive, it is important to consider the cases in which different definitions of drought will provide different perspectives on current conditions. Reservoir managers and hydroelectric operators, for example, will be more interested in tracking hydrological drought than meteorological or agricultural drought. A drought that is relatively short-lived but ill-timed for planting or growing crops may have serious agricultural and socioeconomic implications without posing much of an issue for those concerned with long-term hydrological conditions. Similarly, multi-year droughts can pose serious hydrological concerns (i.e. reservoir depletion) that will not necessarily be reflected in the meteorological or agricultural measures of drought. As seasonal rainfall returns to the expected quantity following a long drought, meteorological and agricultural drought will be alleviated long before hydrological conditions are restored.

To a certain extent, all of these modes of drought are dependent on defining a climatologically “normal” baseline. Although that may seem straight forward, it is not always as intuitive as one might think. Part of the problem is that low-frequency variability often operates on time scales longer than recorded histories of hydrologic measurements which are in turn longer than the baselines established by individual experience alone. The allocation of water rights to the Colorado River is an iconic example of incorrectly estimating a hydrologic baseline. Flow from the river was measured and a baseline established during an anomalously wet period, thus over-allocating the river (and although the scientific overestimation of the streamflow was substantially compounded by political jockeying from interested stakeholders, the compact still provides an example of how easily a hydrological baseline can be misjudged). Compounding the definition of a scientific baseline is the establishment of an internal baseline. It is difficult to establish an internal perception of “normal” that runs counter to our own personal experiences. Although a common perception of “normal” conditions may play a fairly insignificant role in terms of scientific consensus, they play an enormous role in creating public pressure for political action.

Changes in the hydroclimate (either gradual or abrupt changes) further confound efforts to define drought by making the definition of “normal” a bit of a moving target. The changes occurring in such systems may be characterized in two ways: (1) the variability in the system is changing (a non-stationary system), or (2) the mean state in the system is changing (system with a mean trend). The issue of a shifting normal state is particularly relevant for the American West as climatologists study the causes of past drought in order to forecast future conditions. In July 2011 Richard Seager, a climate scientist at Columbia University and long a prominent voice on drought, was asked about the possibility of a perpetual drought. He had this to say: “You can’t really call it a drought because that implies a temporary change. The models show a progressive aridification. You don’t say, ‘The Sahara is in drought.’ It’s a desert. If the models are right, then the Southwest will face a permanent drying out”.

As arid conditions persist across much of the Continental US, we need to take the time to evaluate how we define this ongoing dry-spell in the context of a changing climate. The way in which we refer to these events has implications beyond semantics. Our language reflects what we expect the consequences of such events to be and the frequency with which we expect them to occur in the future.