一般干旱分类通常分三种：气象干旱、水文干旱、农业干旱。我认为，土壤干旱应居独立的一席之地。

  有些研究（见附1）拉扯上社会经济干旱，将干旱分为四种类型：气象干旱、水文干旱、农业干旱、社会经济干旱。

   在自然地理要素中，“气水土生”往往联袂登台。在四者之后各加“干旱”，可以组成“气象干旱”、“水文干旱”、“土壤干旱”、“生物干旱”。农业干旱中的作物干旱可视作“生物干旱”的一部分。

   以“土壤干旱”而言，它与“气象干旱、水文干旱、农业干旱”既有联系，又有区别。“土壤干旱”除受“气象干旱、水文干旱“的宏观控制外，与地质构造、矿物结构、地形特征、土壤质地等关联密切。

   目前的土壤干旱往往作为农业干旱的附庸出现，比如“墒情”反映的其实就是土壤含水量状况。墒情告急的话，就意味着土壤干旱比较严重。  

   以往，由于土壤含水量很难观测、资料难以获取。不过，随着遥感监测干旱技术、水文气象定量模拟技术及土壤水分地面观测技术的逐渐发展，土壤干旱数据获取的难度正在降低。因此，“农业干旱”可以拆分成“作物干旱”与“土壤干旱”，“土壤干旱”已开始拥有与“气象干旱、水文干旱、作物干旱”平起平坐的资格。

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附1：

http://www.cma.gov.cn/2011xzt/2012zhuant/20120302/2012030205/201203020502/201203/t20120306_163756.html

                                                        干旱及气象干旱

　　干旱的定义

　　干旱通常包含两种含义：一是干旱气候，指某地多年无降水或降水很少的一种气候现象，世界气象组织将干燥度（年可能蒸散量与年降水量之比）大于10的地区定为严重干旱区或沙漠区，又称常年干旱区；二是干旱灾害，指某地在某一时段内的降水量比其多年平均降水量显著偏少，导致经济活动（尤其是农业生产）和人类生活受到较大危害的现象。

　　干旱的类型及其相互关系

　　干旱是指因水分的收与支或供与求不平衡而形成的持续的水分短缺现象。这种水分的短缺可以表现为降水量的不足、土壤水分的缺乏或江河湖泊水位偏低等。从类型上，干旱可以分为气象干旱、农业干旱、水文干旱和社会经济干旱。

附2：http://www.weather.com.cn/drought/ghzs/04/362370.shtml

                                                      干旱的标准与类型
                                                       2010-04-09 08:35:30   来源： 中国天气网

   干旱指数是表征干旱程度的标准, 也是旱情描述的数值表达 。

   美国气象学会在总结各种干旱定义的基础上将干旱分为4种类型: 气象干旱、农业干旱、水文干旱和社会经济干旱。

   （1）气象干旱也称大气干旱, 根据中华人民共和国国家标准, 气象干旱是指某时段时, 由于蒸发量和降水量的收支不平衡, 水分支出大于收入而造成的水分短缺现象。常见单要素有降水量指数、降水标准差指数、降水Z指数、标准化降水指数等。常见多要素指数有干燥度、湿润度、德马顿干旱指数、降水温度均一化指数、帕默尔干旱指数等。

   （2）农业干旱是指作物生长过程中因水分不足而阻碍作物正常生长而发生的水量供需不平衡现象。可分为土壤干旱和作物干旱。常用指标有降水量、土壤含水量、作物旱情指数和综合性旱情指数4种。

   （3）水文干旱是指由降水量和地表水或地下水收支不平衡造成的异常水分短缺现象。利用年（月）径流量、河流日流量、水位等要素作为指标。常用有水文干湿指数、最大供需比指数、水资源总量短缺指数等作指标。

   （4）社会经济干旱是指自然系统与人类经济系统中, 水资源供需不平衡而造成的水资源短缺现象。通常拟用损失系数法、水分供需平衡模式等来作指标。

附3：http://drought.unl.edu/DroughtBasics/TypesofDrought.aspx

                                                          Types of Drought

    Research in the early 1980s uncovered more than 150 published definitions of drought. The definitions reflect differences in regions, needs, and disciplinary approaches.

    Wilhite and Glantz1 categorized the definitions in terms of four basic approaches to measuring drought: meteorological, hydrological, agricultural, and socioeconomic. 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 socioeconomic systems.

                                                                Meteorological Drought

Meteorological drought is defined usually on the basis of the degree of dryness (in comparison to some “normal” or average amount) and the duration of the dry period. Definitions of meteorological drought must be considered as region specific since the atmospheric conditions that result in deficiencies of precipitation are highly variable from region to region.

For example, some definitions of meteorological drought identify periods of drought on the basis of the number of days with precipitation less than some specified threshold. This measure is only appropriate for regions characterized by a year-round precipitation regime such as a tropical rainforest, humid subtropical climate, or humid mid-latitude climate. Locations such as Manaus, Brazil; New Orleans, Louisiana (U.S.A.); and London, England, are examples. Other climatic regimes are characterized by a seasonal rainfall pattern, such as the central United States, northeast Brazil, West Africa, and northern Australia. Extended periods without rainfall are common in Omaha, Nebraska (U.S.A.); Fortaleza, Ceará (Brazil); and Darwin, Northwest Territory (Australia), and a definition based on the number of days with precipitation less than some specified threshold is unrealistic in these cases. Other definitions may relate actual precipitation departures to average amounts on monthly, seasonal, or annual time scales.

                                                                 Agricultural Drought

Agricultural drought links various characteristics of meteorological (or hydrological) drought to agricultural impacts, focusing on precipitation shortages, differences between actual and potential evapotranspiration, soil water deficits, reduced groundwater or reservoir levels, and so forth. Plant water demand depends on prevailing weather conditions, biological characteristics of the specific plant, its stage of growth, and the physical and biological properties of the soil. A good definition of agricultural drought should be able to account for the variable susceptibility of crops during different stages of crop development, from emergence to maturity. Deficient topsoil moisture at planting may hinder germination, leading to low plant populations per hectare and a reduction of final yield. However, if topsoil moisture is sufficient for early growth requirements, deficiencies in subsoil moisture at this early stage may not affect final yield if subsoil moisture is replenished as the growing season progresses or if rainfall meets plant water needs.

                                                              Hydrological Drought

Hydrological drought is associated with the effects of periods of precipitation (including snowfall) shortfalls on surface or subsurface water supply (i.e., streamflow, reservoir and lake levels, groundwater). The frequency and severity of hydrological drought is often defined on a watershed or river basin scale. Although all droughts originate with a deficiency of precipitation, hydrologists are more concerned with how this deficiency plays out through the hydrologic system. Hydrological droughts are usually out of phase with or lag the occurrence of meteorological and agricultural droughts. It takes longer for precipitation deficiencies to show up in components of the hydrological system such as soil moisture, streamflow, and groundwater and reservoir levels. As a result, these impacts are out of phase with impacts in other economic sectors. For example, a precipitation deficiency may result in a rapid depletion of soil moisture that is almost immediately discernible to agriculturalists, but the impact of this deficiency on reservoir levels may not affect hydroelectric power production or recreational uses for many months. Also, water in hydrologic storage systems (e.g., reservoirs, rivers) is often used for multiple and competing purposes (e.g., flood control, irrigation, recreation, navigation, hydropower, wildlife habitat), further complicating the sequence and quantification of impacts. Competition for water in these storage systems escalates during drought and conflicts between water users increase significantly.

                                                                    Socioeconomic Drought

Socioeconomic definitions of drought associate the supply and demand of some economic good with elements of meteorological, hydrological, and agricultural drought. It differs from the aforementioned types of drought because its occurrence depends on the time and space processes of supply and demand to identify or classify droughts. The supply of many economic goods, such as water, forage, food grains, fish, and hydroelectric power, depends on weather. Because of the natural variability of climate, water supply is ample in some years but unable to meet human and environmental needs in other years. Socioeconomic drought occurs when the demand for an economic good exceeds supply as a result of a weather-related shortfall in water supply. For example, in Uruguay in 1988–89, drought resulted in significantly reduced hydroelectric power production because power plants were dependent on streamflow rather than storage for power generation. Reducing hydroelectric power production required the government to convert to more expensive (imported) petroleum and implement stringent energy conservation measures to meet the nation’s power needs.

In most instances, the demand for economic goods is increasing as a result of increasing population and per capita consumption. Supply may also increase because of improved production efficiency, technology, or the construction of reservoirs that increase surface water storage capacity. If both supply and demand are increasing, the critical factor is the relative rate of change. Is demand increasing more rapidly than supply? If so, vulnerability and the incidence of drought may increase in the future as supply and demand trends converge.

1 Wilhite, D.A.; and M.H. Glantz. 1985. Understanding the Drought Phenomenon: The Role of Definitions. Water International 10(3):111–120.