NOAA Drought Index (NDI)
Index name: NOAA Drought Index (NDI).
Ease of use: Yellow.
Origins: Developed in the early 1980s at the Joint Agricultural Weather Facility as part of the United States Department of Agriculture’s attempt to use weather and climate data for crop production estimates around the world.
Characteristics: A precipitation-based index in which the actual precipitation measured is compared with normal values during the growing season. Mean precipitation for each week is calculated and a running eight-week average of measured average precipitation is summed and compared. If the actual precipitation is greater than 60% of the normal precipitation for the eight-week period, then the current week is assumed to have little or no water stress. If stress is detected, it remains until the actual precipitation is at 60% or more of normal.
Input parameters: Monthly precipitation converted to weekly precipitation values.
Applications: Used as an indicator of drought conditions affecting agriculture.
Strengths: The only input is precipitation, in a monthly time step. The calculations and explanation of use are simple.
Weaknesses: At least 30 years’ worth of data are required to compute normalized monthly values that are used in the computation of the weekly values. It has very specific applications related to agriculture and crop progression and development.
Reference: Strommen, N.D. and R.P. Motha, 1987: An operational early warning agricultural weather system. In: Planning For Drought: Toward a Reduction of Societal Vulnerability (D.A. Wilhite, W.E. Easterling and D.A. Wood, eds.). Boulder, CO, Westview Press.