The NLDAS experimental drought monitor is derived from near real-time soil moisture output from both the NASA MOSAIC and NCEP Noah land surface models. The anomalies and percentiles are based on a 28 year climatology (1980 - 2007). Two separate climatology files are used; one for the calculation of anomalies, and one for the calculation of percentiles. The anomaly climatology file contains 1 soil moisture value per day (daily average over 28 years) for each gridbox. The percentile climatology file contains 140 soil moisture values per day (5 for each year) for each gridbox.
Anomalies are calculated by comparing the current soil moisture values to mean values for the same time of year over each gridbox. For weekly anomalies, the past week's worth of soil moisture values are first averaged together to form a single value. The same procedure is applied to the corresponding week of the anomaly climatology file. The two values are then differenced to derive an anomaly soil moisture value. A similar procedure is followed to compute monthly (30 day) anomalies.
Percentiles are based upon an 5 day moving window of soil moisture values. This acts to smooth out the soil moisture record. Weekly analyses for each gridbox are computed by comparing the past 7 days to corresponding period in the percentile climatology. Taking day 1 of the week as an example, hourly soil moisture values from this day are averaged together to form a single value. This value is then ranked against the soil moisture values from each day of the 5 day window surrounding day 1 of the corresponding week in the percentile climatology. This same process is then repeated for days 2-7 of the week, with each day of the week contributing equally to the overall ranking value. Monthly (30 day) percentile analyses are computed in a similar fashion.