About the dataset

Model:

Soil moisture is estimated by a one-layer hydrological model (Huang et al., 1996, van den Dool et al., 2003). The model takes observed precipitation and temperature and calculates soil moisture, evaporation and runoff. The potential evaporation is estimated from observed temperature.

Model parameters are constant spatially (tuned based on Oklahoma observed runoff data).

Tuning the model to runoff of serveal small river basins in eastern Oklahoma resulted in a maximum holding capacity of 760mm of water. Along with a common porosity of 0.47 this implies a soil column of 1.6 meter.

Procedure:

The observed precipitation and temperature are monthly data over 344 Climate Divisions from NCDC (1931-present). Because the monthly data is usually available on the 5th of next month, the daily soil moisture in the current month is calculated with real time daily temperature from CPC in-house product (so called 1000-day archive) and real time precipitation from daily station data with 1 day lag. The climatology is calculated based on climate division data.The soil moisture in the current month will be re-calculated with climate division monthly data on the 5th of next month.

Warning:

Please be aware of the problems caused by using real time station data in calculating soil moisture for the current month: Daily station preciptation tends to be heavier compared to climate division data. 

Please be aware that when using Grads to plot station data, we implicitly use Cressman analysis. It may create some unrealistic values, especially in areas with big gradients or in the boundary area.