Agricultural Drought Indices - US Department of Agriculture

to compare moisture conditions at different locations (Hayes 2000). Thompson and Wehmanen
(1979) utilized remote-sensing data (Green Number Index) for growth stages for wheat with
segments classified as drought or non-drought. A high degree of agreement was found between
the 18-day remote sensing data and the ground-based Crop Moisture Index. Similar close results
were obtained in parts of the (then) USSR and Australia.
Indices Based on Precipitation, Temperature, Soil Moisture/Soil Characteristics
As compared to the precipitation- and temperature-based drought indices, the precipitation,
temperature, and soil moisture based indices are generally those calculated and derived using
information provided by water balance (WB) models. However, in this case, the WB information
can be provided by different methodologies. These can range from the very simplistic models,
such as those addressing the precipitation (P) and potential evapotranspiration (PET) relationship,
to the very complex models, such as those proposed by Ritchie (1998) and Gevaerd and Freitas
(2006). These varied approaches are critical both conceptually and operationally in tracking
agricultural drought, which is dependent on what is occurring in terms of moisture in the plant/tree
rooting zones throughout the growing season. Attempting to depict and understand this water
balance approach through various indices will allow one to better understand how crops or the
environment may be reacting or stressing during times of drought and at various stages of
plant/crop development. This, in turn, can help us better determine how growth or yield may be
influenced by drought during the growing season and throughout critical off-season recharge
periods. Some examples are shown below.
Relative Soil Moisture (RSM)
Soil moisture (SM) or soil water storage (SWS) is an output of the water balance. This variable
can be obtained by different WB methods, such as those proposed by Thornthwaite and Mather
(1955), Molinas and Andrade (1993), Allen et al. (1998), Ritchie (1998), and Gevaerd and Freitas
(2006). Each kind of WB method will require specific inputs, but basically they require climate
(rainfall and potential evapotranspiration), soil (physical properties), and plant (crop type, leaf area,
crop sensitivity to water stress, crop water requirement for each phenological phase, and
management practices) information, depending on their complexity. The RSM index is the
relationship between SWS and soil water holding capacity (SWHC). The results are given in
percentage.
Relative Water Deficit (RWD)
This index is obtained by calculating the relative difference between actual (ETa) and potential
(ETP) evapotranspiration:
RWDI = (1 – AET / PET) 100
where AET is the actual evapotranspiration, an output of Thornthwaite and Mather’s climatological
water balance, considering the SWHC of the respective soil type for the region, ranging from 50 to
150 mm for a 1-m soil profile, and PET is the potential evapotranspiration, calculated by physical
or empirical methods. This index is non-accumulative and is calculated by the total AET and PET
for the period considered.
Accumulated Water Deficiency (AWD)
Water deficiency is an output of the climatological water balance, as determined by Thornthwaite
and Mather’s WB model. The water deficit (WD) is the difference between potential (PET) and
actual (AET) evapotranspiration. The WD magnitude for a given condition will depend on the soil
water holding capacity adopted for the WB. When water deficiency is accumulated during the
growing season, this index will have a definite correlation with crop yield losses.
Accumulated Drought Index (ADI)
This drought index has as inputs rainfall (P) and potential evapotranspiration (PET). Its calculation
is based on the relationship between these two variables (Table 4), and the drought classification
follows the categories of the accumulated index (Table 5). ADI is calculated as:
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