The Assessment of Drought Intensity By A New Index

T H E A SSESSM EN T O F D R O U G H T IN T EN S IT Y BY A N EW IN D EX J ames A. Shear and Donald Steila* D ro u g h t is a p h en o m en o n of w id esp read sig nifican ce. T h e use of th e term in b o th p ro fessio n al an d n o n -p ro f essio n al literatu re, h o w ev er, is extrem ely v aried . T o th e f o rester, d ro u g h t is th o u g h t o f p rim arily in term s of th e su scep tib ility of su rf ace co nd itio n s to th e th re at of fire. T o th e ag ricu ltu rist, th e co n ce rn of d ro u g h t cen ters ab o u t his cro p ’s w ater-n eed s an d soil m o istu re co nd itio ns. T o th e h y d ro lo g ist, d ro u g h t is in d icated b y lo w ered w ate r levels in w ells, stream s, an d reserv o irs. D ep artu res of p recip itatio n d u rin g an y p erio d of tim e (f o r a g iv en area) f ro m th e n o rm al am o u n t f o r th at p erio d are e xp e cte d o ccu rre n ce s. R arely d o av erag e clim atic d ata exp ress p ersisten t con d itio ns o v er exten siv e tim e p erio d s. If a p articu lar m o istu re d e p artu re d oes n o t significantly d ev iate f ro m m ean m o istu re co n d itio n s, th e en v iro n m en t m ay w ell acco m m o d ate th at d ep artu re w ith o u t d eleterio u s effects (1), b u t d ev ia­ tions th at re ach th e stress lim its of th e p h y sical en v iro n m en t are of m ajo r co n seq u en ce to m an. B asic to m o st p h y sical definitions of d ro u g h t is a n eg ativ e d ep artu re from th e e xp e cte d m o istu re av ailab ility of an area. S ev eral statistical tech n iq u es h av e b een em p lo y ed in an eff ort to d eriv e a q u an titativ e m easu re of d ro u g h t inten sity . T h e m o st w id ely ap p lied an d co m p re ­ h en sive o f th ese, to d ate, is Palm er’s In d ex. (2) Palm er atte m p te d to d iv o rce h im self f ro m th e su b jectiv ity and n arrow ness o f sco p e asso ciated w ith p rev io u s d ro u g h t m e asu re (e.g ., ag ricu ltu ral an d h y d ro lo g ical) an d ch aracte riz e d d ro u g h t solely in term s o f m eteo ro lo g ical p h en o m en a. H e d efined a d ro u g h t p erio d as: A n in terv al o f...