Crop Moisture Index (CMI)

Crop Moisture Index (CMI)


Index name: Crop Moisture Index (CMI).

Ease of use: Yellow.

Origins: As part of original work done by Palmer in the early 1960s, CMI is usually calculated weekly along with the Palmer Drought Severity Index (PDSI) output as the short-term drought component in which the impact on agriculture is considered.

Characteristics: As some of the drawbacks associated with PDSI became apparent, Palmer responded to them with the development of CMI. It is intended to be a drought index especially suited to drought impacts on agriculture, in that it responds quickly to rapidly changing conditions. It is calculated by subtracting the difference between potential evapotranspiration and moisture, to determine any deficit.

Input parameters: Weekly precipitation, weekly mean temperature, and the previous week’s CMI value.

Applications: Used to monitor droughts in which agricultural impacts are a primary concern.

Strengths: The output is weighted, so it is possible to compare different climate regimes. Responds quickly to rapidly changing conditions.

Weaknesses: As it was developed specifically for the grain-producing regions in the United States, CMI may show a false sense of recovery from long-term drought events, as improvements in the short term may be insufficient to offset long-term issues.

Resource: US Drought Portal.

Reference: Palmer, W.C., 1968: Keeping track of crop moisture conditions, nationwide: the new Crop Moisture Index. Weatherwise, 21: 156–161. DOI: 10.1080/00431672.1968.9932814. (For more information on this paper, please contact the IDMP HelpDesk).

Currently used by: Brazil, USA.

17 Responses

  1. cmi = pet – soil moisture ( monthly tensiometer value)
    pet estimated through Thornthwaite’s equation is Thornthwaite equation (1948)

    P E T = 16 ( L /12 ) ( N/ 30 ) ( 10 T d /I ) power α


    P E T is the estimated potential evapotranspiration (mm/month)

    T d is the average daily temperature (degrees Celsius; if this is negative,of the month being calculated

    N is the number of days in the month being calculated

    L is the average day length (hours) of the month being calculated as for example

    α = ( 6.75 × 10 − 7 ) I 3 − ( 7.71 × 10 − 5 ) I 2 + ( 1.792 × 10 − 2 ) I + 0.49239

    I = ∑ i = 1 12 ( T m i 5 ) 1.514

    1. Dear Chanderpal Singh Maurya

      The formula for CMI that you have posted contains the main variables (PET and soil moisture), however, it does not account for the fact that the CMI is calculated as a function of current and previous weeks’ PET and soil moisture values and the fact, that the CMI is calculated on a weekly basis.

      Please see our response to Zhq where you can find more information about CMI and its calculation.

      Best regards
      IDMP Team

  2. i want to calculate the CMI in China,but i can’t find the equations of it on line,can you please share the equations Thanks.

    1. Dear Zhq

      Thank you for your comment.

      The CMI was developed based on the Palmer Drought Severity Index (PDSI), and it gives the short-term or current status of purely agricultural drought or moisture surplus. This value can change rapidly from week to week, hence the index is based on weekly data. Please note that both PDSI and CMI have been developed empirically using drought period data in the USA. These empirical relationships are based for example on soil types, which might strongly vary between geographical regions.

      For further information, you might find the following website helpful: Excerpt: “[The CMI] is the sum of the evapotranspiration anomaly (which is generally negative or slightly positive) and the moisture excess (either zero or positive). Both terms are a function of the previous week and a measure of the current week. The evapotranspiration anomaly is weighted to make it comparable in space and time. […] The stage of crop development and soil type should be considered when using this index.”
      An equation for CMI calculation can be found here:

      Best regards
      IDMP Team

    1. Dear Tobias,

      I am responding to both of your questions. I try to find software documentation on the CMI.

      I would also look the main Palmer paper: Palmer, W.C., 1965: Meteorological Drought. Research Paper No. 45, US Weather Bureau, Washington, DC. The link is here:

      Be definition, the CMI is a weekly index. Please also note, that the CMI was developed for the climate and soils of the Midwest of the USA. It may not be applicable is other geographic areas.

      Best regards,

  3. I need information regarding CMI is it possible to use in Pakistan in rainfed region to see impact on production. What time series length will be ideal

    1. Dear Saira,

      It is best to use the longest time series record that you have available. But you should use a time series length of at least 30 years.

      Best regards,
      IDMP Team

  4. How can we get the crop moisture index by division weekly maps and drought severity index by division weekly maps for 2018? We can only get the current maps each week as they are sent. The previous week maps are not available. Can you help us. -Chris-

    1. To calculate the CMI, you will need Weekly precipitation, weekly mean temperature, and the previous week’s CMI value.

  5. Hi, Can you please share the equations or the reference study in this webpage. I am trying to run CMI over some data I have. Thanks in advance, Munish

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