Procedure to standardize Illinois soil moisture data

Because the soil moisture data from the different stations in Illinois were not always taken regularly, or at the same time at different stations, it was necessary to fit the data into a regular interval. The timing chosen, on the 8th , 18th, and 28th of each month, is the same as the technique used for the Russian, Mongolian and Chinese data. For each observation time, a bin was created that extended four days in either direction, so, for example, the May 8th bin was from May 4th to May 12th. The criteria for filling the bin with data were:

If there was no observation during the bin then the value was set to undefined.
If there was only one observation taken during the bin time then the bin value was set to that observational value.
If there was an observation taken on the bin center (e.g., May 8th) then the bin took on that value, regardless of other observations taken during that time.
If more than one observation was taken during the bin time, but all the observations were either before or after the bin center, then the value taken on the day closest to the bin center was the value used for the bin. (So if there were observations taken on the 4th, 5th, and 7th, the value from the 7th was used.)
If there was more than one observation taken, with at least one observation before and one after the bin center, then the two values closest to the bin center were chosen to be averaged. The averaging was done with a weight proportional to time from the bin center. For example:
If there were observations on the 4th, 6th, and 11th, then the values from the 6th and 11th were chosen. If the value on the 6th was 15 cm and the 11th was 18 cm, then the weighting equation was:
```
     |8-11| * 15 cm      |8-6| * 18 cm
     --------------  +  --------------  = Value for bin
     |8-11| + |8-6|     |8-11| + |8-6|
```
The first value has a weight equal to difference between the bin center and the other observation, in this case |8-11| or 3. The second weight is equal to the difference between the bin center and the first observation date, or |8-6 | or 2. The total weight is 5. So the resultant value would be:
```
      3          2
(15 * -) + (18 * -) = (9 + 7.2) = 16.2 for the bin
      5          5
```

For additional questions, you may contact Jared Entin.

Prepared by Alan Robock( robock@envsci.rutgers.edu) and Govindarajalu Srinivasan (srini@envsci.rutgers.edu)
Last updated on January 28, 1999