 Explanation of the weekly Palmer drought and crop moisture data
 products.
 
      The Palmer Drought Severity Index (PDSI) and Crop Moisture Index
 (CMI) are indices of the relative dryness or wetness effecting water
 sensitive economies.  The PDSI indicates the prolonged and abnormal
 moisture deficiency or excess.  The CMI gives the short-term or current
 status of purely agricultural drought or moisture surplus and can
 change rapidly from week to week.  Both indicies indicate general
 conditions and not local variations caused by isolated rain.
 Calculation of the PDSI and CMI are made for 350 climatic divisions in
 the United States and Puerto Rico.  Input to the calculations include
 the weekly precipitation total and average temperature, division
 constants (water capacity of the soil, etc.) and previous history of
 the indices.
 
      The PDSI is an important climatoligical tool for evaluating the
 scope, severity, and frequency of prolonged periods of abnormally dry
 or wet weather.  It can be used to help delineate disaster areas and
 indicate the availability of irrigation water supplies, reservior
 levels, range conditions, amount of stock water, and potential
 intensity of forest fires.  The CMI can be used to measure the status
 of dryness or wetness affecting warm season crops and field activities.
 
      The equation for the index was empirically derived from the monthly
 temperature and precipitation scenarios of 13 instances of extreme
 drought in western Kansas and central Iowa and by assigning an index
 value of -4 for these cases.  Conversely, a +4 represents extremely wet
 conditions.  From these values, 11 categories of wet and dry conditions are
 defined (Table 1).  The index is a sum of the current moisture anomaly
 and a portion of the previous index to include the effect of the
 duration of the drought or wet spell.  The moisture anomaly is the
 product of a climate weighting factor and the moisture departure.  The
 weighting factor allows the index to have a reasonably comparable local
 significance in space and time.  A value for a division in Florida
 would have the same local implication as a similiar value in a more
 arid division in western Kansas.  The moisture departure is the
 difference of water supply and demand.  Supply is precipitation and
 stored soil moisture and demand is the potential evapotranspiration,
 the amount needed to recharge the soil, and runoff needed to keep the
 rivers, lakes, and resevoirs at a normal level.
 
      The duration of the drought (or wet spell) is determined by
 calculating indices for different weather spells (incipient and
 established wet and dry spells).  A week of normal or better rainfall
 is welcome in an area that has experienced a long drought but may be
 only a brief respite and not the end of the drought.  Once a weather
 spell is established (by computing a 100% "probability" that an
 opposite weather spell has ended), the final value is assigned.  In
 order for the program to have a real-time significance, a value is
 assigned based on a greater than 50% "probability" that the opposite
 weather spell has ended.  This is not entirely satisfactoty, but it
 does allow the index to have a value when there is a doubt as to
 whether it should be positive or negative.  A "F" is placed after the
 PDSI when a weather spell is established and a "P" when a weather spell
 is not established.
 
      The CMI was developed from some of the moisture accounting
 procedures used in computing the PDSI.  This index 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.  If the potential moisture demand exceeds available
 moisture supplies, the CMI is negative.  However, if moisture meets or
 exceeds demand the index is positive.  It is necessary to use two
 separate legends because the resulting effects are
 different when the moisture supply is improving than when it is
 deteriorating (Table 2).  The stage of crop development and soil type
 should be considered when using this index.  In irrigated regions, only
 departures from ordinary irrigation requirements are reflected.
 
      A parameter obtained from the calculations is the monthly moisture
 anomaly (Z) index which is the product of the moisture departure of the
 most recent 4 weeks and a climate weighting factor.  This index can be
 used as an indicator of forest fire ignition.  The classes of dry and
 wet periods for the different index values are given in table 3.
 
      Another parameter derived from the calculations is the
 additional precipitation in inches needed to bring the PDSI to near
 zero.  This parameter is computed for all values of the current week's
 PDSI less than -.5 and left blank for all values greater than or equal to
 -.5.  The precipitation values are the theoretical, additional amounts
 required to end the drought in each climatic division.  In using
 this parameter to make projections, it must be realized that these
 values are instantaneous, valid only for the current week.  To end the
 drought in a given climatic division for the oncoming period, the amount
 listed plus near-normal rainfall must occur.
 
      The following is a listing of the parameters in the files and
 their meaning.  Temperature and precipitation are data received from
 the field and the other parameter are results of the Palmer drought and
 crop moisture data calculations.  The week number in the heading is the
 week of the growing season where week one is the week with the first
 Wednesday in March. The computations are reinitated each year for
 week one using the output of the February Palmer data run.  All initial
 data are replaced with the historical data received from the National
 Climatic Data Center in Asheville when available and the calculations
 rerun.
 
                    Columns of the Weekly Palmer Drought
                        and Crop Moisture Data Files
 
     ST   -  State (states are grouped in each file by NWS region).
 
     CD   -  Climate division (CD) number in the state.
 
     TMP  -   Average weekly temperature (F) in the CD.
 
     PRCP -  Total weekly precipitation (inches) in the CD.
 
     SOIL MOIST UPPR LAYR  -  Soil moisture in the upper layer at the
                              end of the week (water capacity is one inch).
 
     SOIL MOIST LOWR LAYR  -  Soil moisture in the lower layer at the
                              end of the week (water capacity in
                              inches is a function of the average
                              soil type in the CD).
 
     PCT FLD CPC END WEEK  -  The percent of field capacity of
                              moisture in the soil at the end of the
                              week. This value is the ratio of the
                              soil moisture in the upper and lower
                              layers to the available water
                              capacity expressed in percent.
 
     POT EVAP  -  Potential evapotranspiration using Thornwaites method
                  (based on temperature, solar declination angle, and
                  division constants such as mean latitude).
 
     RUN OFF  -   Run off in inches at the end of the week.
 
     CROP MOIST INDEX -  Crop moisture index (CMI).  Values indicate
                         dry or wet conditions in the short term.
 
     CHNG FROM PREV WEEK - The difference of the previourmal
      -1.99 to -1.25                       Mild to moderate drought
      -2.74 to -2.00                       Severe drought
      -2.75 and below                      extreme drought
 
