Drought

A Guide To Climate Change Impacts, On Scotland’s Historic Environment, published by Historic Environment Scotland in October 2019, defines a drought as: ‘An extended period of unusually low rainfall that can lead to water shortages.’

AR5 Climate Change 2014: Impacts, Adaptation, and Vulnerability, Glossary, published by the Intergovernmental Panel on Climate Change (IPCC) defines drought as: ‘A period of abnormally dry weather long enough to cause a serious hydrological imbalance. Drought is a relative term; therefore any discussion in terms of precipitation deficit must refer to the particular precipitation-related activity that is under discussion. For example, shortage of precipitation during the growing season impinges on crop production or ecosystem function in general (due to soil moisture drought, also termed agricultural drought), and during the runoff and percolation season primarily affects water supplies (hydrological drought). Storage changes in soil moisture and groundwater are also affected by increases in actual evapotranspiration in addition to reductions in precipitation. A period with an abnormal precipitation deficit is defined as a meteorological drought. A megadrought is a very lengthy and pervasive drought, lasting much longer than normal, usually a decade or more.’

Tree species selection for green infrastructure, A guide for specifiers, published by the Trees and Design Action Group Trust in 2019, states: ‘Strictly speaking we should refer to ‘drought’ as ‘water deficits’ because drought has a precise meteorological definition. Water deficits may arise as a result of impermeable surfaces, small rooting volumes or root damage but without the lack of precipitation needed to define a climatic drought.’

Climate Change 2021 – The Physical Science Basis, Annex VII: Glossary, written by the Intergovernmental Panel on Climate Change (IPCC) and published by Cambridge University Press in 2023, defines a drought as: ‘An exceptional period of water shortage for existing ecosystems and the human population (due to low rainfall, high temperature, and/or wind).’

It defines an agricultural and ecological drought as: ‘Depending on the affected biome: a period with abnormal soil moisture deficit, which results from combined shortage of precipitation and excess evapotranspiration, and during the growing season impinges on crop production or ecosystem function in general.’

It defines a hydrological drought as: ‘A period with large runoff and water deficits in rivers, lakes and reservoirs.’

It defines a meteorological drought as: ‘A period with an abnormal precipitation deficit.’

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