Drought is defined, biogeophysically, as water deficits that are carried over in a series of chain mechanisms starting from atmospheric moisture to precipitation, soil moisture and eventually to plant water.
National Drought Mitigation Center: What is Drought?
Shinoda, M.: Land: Proactive Management of Drought and Its Derived Disasters. In R. Shaw and T. Phong eds.: Environment Disaster Linkages. Community, Environment and Disaster Risk Management, Vol. 9, Emerald Publishers, Bingley UK, 61-78, 2012.