The definitions reflect differences in regions, needs and disciplinary approaches. The first three approaches deal with ways to measure drought as a physical phenomenon. The last deals with drought in terms of supply and demand, tracking the effects of water shortfall as it ripples through socio-economic systems. Meteorological drought is usually an expression of precipitation's departure from normal over some period of time. These definitions are usually region-specific, and presumably based on a thorough understanding of regional climatology.
Meteorological measurements are the first indicators of drought.
Agricultural drought occurs when there isn't enough soil moisture to meet the needs of a particular crop at a particular time. Agricultural drought happens after meteorological drought but before hydrological drought. Agriculture is usually the first economic sector to be affected by drought. Hydrological drought refers to deficiences in surface and subsurface water supplies. It is measured as streamflow, and as lake, reservoir and ground water levels.