Hydrological drought refers to shortages of water resources, when for example; groundwater, reservoir, or stream levels are significantly reduced. This is considered the drinking water type of drought. Conditions for hydrologic drought are built over extended periods of time.
Drought is often grouped into four basic types: 1) meteorological or climatological, 2) agricultural, 3) hydrological, and 4) socioeconomic. Meteorological and climatological drought is defined in terms of the magnitude of a precipitation shortfall and the duration of this shortfall event.
Socioeconomic drought occurs when the demand for an economic goods exceeds supply as a result of a weather-related shortfall in water supply. The supply of many economic goods, such as water, forage, food grains, fish, and hydroelectric power, depends on weather.
Generally, however, soil moisture is the water that is held in the spaces between soil particles. Surface soil moisture is the water that is in the upper 10 cm of soil, whereas root zone soil moisture is the water that is available to plants, which is generally considered to be in the upper 200 cm of soil.