Resource book: Earth observation and climate data analysis for agricultural drought monitoring in South Asia

Abstract

Droughts are manifestations of climatic fluctuations associated with large-scale anomalies in the planetary circulation of the atmosphere. They imply the absence of, or weak precipitation, for an extended period over large areas. It is very difficult to identify and to clearly determine the onset as well as the termination point of a drought as it is a usually a slow-developing phenomenon. The effects of drought tend to accumulate slowly and persist over long periods of time. Making matters worse, droughts are typically not confined by topography that affects weather conditions, and may extend over large areas. Droughts are defined as the temporary reduction in quantity of rainfall, runoff, and soil moisture compared to the climatology of a region. Dry climates are prone to drought effects due to the soil moisture deficiency and high variability in occurrence of rainfall events and quantity. Because drought does not have a precise and universally accepted definition, it is difficult to assess its occurrence or measure its degree of severity. This ambiguity is further aggravated as drought implies different meanings and implications for different people. For example, a meteorologist, an agriculturalist, a hydroelectric power plant operator, and a wildlife biologist will have differing definitions of drought. In addition, drought can be defined conceptually as well as operationally (Wilhite & Glantz, 1985). Conceptual definitions are stated in relative terms (e.g., a drought is a long, dry period), whereas operational definitions attempt to identify the onset, severity, and termination of drought periods. Generally, operationally defined droughts can be used to analyse drought frequency, severity, and duration for a given return period. Among others, the main natural causes of drought are climate effects. Nonetheless, mismanagement of water resources, acid rain pollution, overexploitation, and many other man-made effects also contribute to the extent, appearance, continuity, and severity of droughts. Severity of droughtsis a factor of high temperatures, winds, and low relative humidity. In many parts of the world, drought is also related to the timing of precipitation including principal season of occurrence, delays in the start of the rainy season, the occurrence of rains in relation to crop growth stages and the effectiveness of the precipitation ( i.e. rainfall intensity, its areal extent, and the frequency of occurrence). All these variables indicate that each drought event is unique in its climatic characteristics, spatial extent, and impacts. In many cases, drought severity is hard to define. Indeed, drought severity is determined by its duration, intensity and geographical extent, coupled with water demand from human activities and vegetation in the given region. Drought characteristics and extensive impacts render it more difficult to identify and quantify the drought effects on the society, economy and environment. To fully appreciate the significance of drought, the societal context in which it is defined also plays an important role. Despite a drought episode occurring during one or several consecutive seasons, its impacts on society can persist for many years thereafter. Additionally, drought impacts depend on the vulnerability of a particular society or population to drought at a particular time. As such, subsequent drought episodes of similar intensity, duration and extent, will likely result in different effects in different areas. The common theme in all drought episodes is the deficiency of precipitation, resulting in moisture stress for socio-ecological processes (e.g. agriculture). While a number of natural and human factors can have important impacts, impact on water availability, water shortage and stress resulting from drought must however be understood as a relative condition and temporary condition, and not an absolute one.