Soil Moisture Memory

Assesses the ability of soil to retain its moisture state, or more specifically the time it takes for soil moisture to decay to a condition beyond 1/e (i.e. the e-folding time).

Quantifies the time it takes for soil moisture to lose information of its initial state, meaning the time it takes for the lagged autocorrelation of soil moisture to fall below its e-folding time. This method can be calculated from most in-situ and remotely sensed observations to be compared against model data in an effort to show regions where models might be incorrectly retaining or losing soil moisture information. May also shed light on potential soil moisture retention biases related to various land cover type.

Needs a sufficiently long time series of daily soil moisture. Sufficiently is generally regarded as being greater than or equal to 90 days. Can be either model output, in-situ observations, or remotely sensed. Be mindful of the relative errors associated with the inputs when evaluating soil moisture memory from various data products.
Be cautious when comparing soil moisture memory across different sites when using in-situ data because measurements may have been taken at various depths or with different sensors. These differences result in variance errors. Therefore some measure of error should be recorded in conjunction with soil moisture memory values. The current code does not provide this error estimate.

Prototype Subroutine Call

subroutine soilm_memory ( dim2, ntim, soilm, smemory, missing )

Required Input

Output

*** Note: Memory is defined as time when the lagged autocorrelation falls below the e-folding time. The day at which this occurs is smemory so the unit of time depends on the timestep used for the lagged correlation. For example if daily soil moisture data are used, then the smemory would be returned as the day when the lagged autocorrelation is less than 1/e.

How to Calculate

1
Simply calculate lagged autocorrelation of soil moisture and find the instance where the correlation falls below 1/e.

Relevant Citations

Method Description 

Thomas L. Delworth and Syukuro Manabe, 1988: The Influence of Potential Evaporation on the Variabilities of Simulated Soil Wetness and Climate. J. Climate, 1, 523–547.

  Detailed Evaluation 

Thomas Delworth and Syukuro Manabe, 1989: The Influence of Soil Wetness on Near-Surface Atmospheric Variability. J. Climate, 2, 1447–1462.