Ƙididdigar bushewa

Alamar busasshiyar (AI) alama ce ta lambobi game da matakin bushewa yanayi a wani wuri. Ƙungiyar Meteorological ta Amurka ta bayyana shi a cikin yanayin yanayi da yanayin yanayi, a matsayin "matakin da yanayi ba shi da tasiri, mai inganta rayuwa". Rashin ruwa ya bambanta da fari saboda bushewa na dindindin ne yayin da fari na wucin gadi ne. An gabatar da alamun busasshiyar ƙasa da yawa (duba ƙasa); waɗannan alamun suna aiki don ganowa, ganowa ko ƙayyade yankunan da ke fama da karancin ruwa, yanayin da zai iya shafar tasirin amfani da ƙasar don irin waɗannan ayyukan kamar noma ko noma.

## Tarihin tarihi da alamomi

### Köppen
A farkon karni na 20, Wladimir Köppen da Rudolf Geiger sun haɓaka manufar rarraba yanayi inda aka bayyana yankuna masu bushe a matsayin wuraren da tarin ruwan sama na shekara-shekara (a cikin centimeters) bai kai ba R/2. Inda 

* R = 2 × T  idan ruwan sama ya faru galibi a lokacin sanyi,
* R = 2 × T + 14 idan ruwan sama ya rarraba daidai a duk shekara, kuma
* R = 2 × T + 28  idan ruwan sama ya faru galibi a lokacin zafi.

inda T  shine matsakaicin zafin jiki na shekara-shekara a cikin Celsius.

Wannan yana daya daga cikin yunkurin farko na bayyana alamar busasshiyar ƙasa, wanda ke nuna tasirin tsarin zafi da adadin da rarraba hazo wajen tantance tsire-tsire na asali wanda zai yiwu a wani yanki. Ya fahimci muhimmancin zafin jiki wajen ba da izinin wurare masu sanyi kamar arewacin Kanada don a gani a matsayin mai laushi tare da matakin hazo iri ɗaya kamar wasu hamada na wurare masu zafi saboda ƙananan matakan yiwuwar evapotranspiration a wurare masu sanyi. A cikin subtropics, izinin rarraba ruwan sama tsakanin lokutan zafi da sanyi ya fahimci cewa ruwan sama na hunturu ya fi tasiri ga ci gaban shuke-shuke wanda zai iya bunƙasa a cikin hunturu kuma ya yi barci a lokacin rani fiye da adadin ruwan sama na rani a lokacin zafi zuwa zafi. Don haka wani wuri kamar Athens, Girka wanda ke samun mafi yawan ruwan sama a cikin hunturu ana iya la'akari da shi yana da yanayi mai laushi (kamar yadda aka tabbatar a cikin ganye masu laushi) tare da kusan adadin ruwan sama wanda ke sanya yanayin hamada a Midland, Texas, inda ruwan sama ya fi faruwa a lokacin rani.

### Thornthwaite
 
A cikin 1948, C. W. Thornthwaite ya ba da shawarar AI da aka bayyana kamar haka:

AI_T = 100\times\frac{d}{n}

inda karancin ruwa ana lissafa shi azaman jimlar bambance-bambance na kowane wata tsakanin hazo da yiwuwar evapotranspiration ga waɗancan watanni lokacin da ruwan sama na yau da kullun ya fi ƙasa da evapotranspiracy na yau da kullum; kuma inda yana tsaye ne don jimlar ƙimar kowane wata na yiwuwar evapotranspiration don watanni marasa kyau (bayan Huschke, 1959). Wannan AI daga baya Meigs (1961) ya yi amfani da shi don tsara yankunan busassun duniya a cikin mahallin shirin Binciken Yankin Arid na UNESCO.

### Shirin Muhalli na Majalisar Dinkin Duniya
A cikin shirye-shiryen da suka kai ga taron Majalisar Dinkin Duniya na 1977 kan hamada (UNCOD), Shirin Muhalli na Majalisar Dinkinobho (UNEP) ya ba da taswirar bushewa bisa ga wani nau'i daban-daban na busasshiyar ƙasa, wanda Mikhail Ivanovich Budyko (1958) ya gabatar da shi da farko  kuma ya bayyana kamar haka: 

AI_B = 100\times\frac{R}{LP}

inda R shine matsakaicin radiation na shekara-shekara (wanda aka fi sani da ma'aunin radiation), P shine matsakaicin ruwan sama na shekara-shekara, kuma L shine zafi mai zurfi na tururi don ruwa. Lura cewa wannan alamar ba ta da girma kuma masu canji R L da kuma P ana iya bayyana shi a cikin kowane tsarin raka'a wanda ya dace da kansa.

## Manazarta