Agricultural Drought Indices - US Department of Agriculture

Accumulated Drought Index (ADI): This drought index has rainfall (P) and potential
evapotranspiration (ETP) as inputs. Its determination is based on the relationship between these
two variables (Table 3), and the drought classification follows the categories of the accumulated
index (Table 4). ADI is calculated as
ADI = ∑ DI / (3 n N) (1)
where DI is determined for each period by the P and ETP relationship presented in Table 3, n is
the number of periods considered, and N is the number of periods without rain above 10 mm
(NDWR>10). For N = 0, ADI is calculated by
ADI = ∑ DI / (3 n) (2)
Table 3. P and ETP relationship for determination of the drought index (DI). Source:
www.infoseca.sp.gov.br.
P & ETP relationship Classification DI
P ≥ 2 ETP Wet 5
ETP ≤ P < 2 ETP Lightly wet 4
½ ETP ≤ P < ETP Normal 3
0 < P ≤ ½ ETP Lightly dry 2
P = 0 Dry 1
Table 4. ADI and drought classification. Source: www.infoseca.sp.gov.br.
ADI Classes
ADI ≥ 1.50
0.80 ≤ ADI < 1.50 Very wet
0.40 ≤ ADI < 0.80 Wet
0.20 ≤ ADI < 0.40 Normal
Classification
Extremely wet
0.04 ≤ ADI < 0.2 Dry
ADI < 0.04
Very dry
Accumulated Water Deficit index (AWD): Water deficit is an output of the climatological water
balance, determined by Thornthwaite and Mather’s WB model. The water deficit (WD) is the
difference between potential (ETP) and actual (ETa) evapotranspiration. The WD magnitude for a
given condition will depend on the soil water holding capacity (SWHC) adopted for the WB. When
accumulated during the growing season, this index will have a good correlation with crop yield
losses. Figure 5 presents two examples of the AWD maps for Brazil during the dry and wet seasons.
Negative values represent the AWD, whereas positive values represent the accumulated water
surplus (AWS).
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