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Time Series Analysis of Field-Measured Water Content of a Sandy Soil

Dep. of Soil and Environ. Sci., Univ. of California, Riverside, CA 92521

R. R. Allmaras

USDA-ARS, Dep. of Soil, Water, and Climate, Univ. of Minnesota, St. Paul, MN 55108

*Corresponding author (laowu{at}mail.ucr.edu).

ABSTRACT

Prediction of the field soil water regime is important for assessing agrochemical transport and scheduling irrigation. This research was conducted to evaluate the potential for employing the time series analysis technique to predict average water content (W_AVG) of a soil profile and water content at depths of interest from measurements made at a single depth. Volumetric water content of a Zimmerman fine sand (mixed, frigid Argic Udipsamment) in Princeton, MN, was measured in situ by time domain reflectometry at six depths during the early 1993 growing season. The time series made up of hourly measurements of soil water content was first-order differenced to obtain stationarity. The differenced data were used to conduct analyses in the frequency domain to evaluate the coherence and cross-amplitude between two soil water content time series and were subsequently fitted to autoregressive moving average models to obtain coefficients for the transfer function models in the time domain. The transfer function models were then used to predict water contents at depths of interest (50-, 75-, and 100-cm depths) and W_AVG in the top 100-cm profile from measured water content at the 25-cm depth. Overall, the predictions were reasonable, with accuracy increasing as the separation distance from the 25-cm depth decreased.

Received for publication May 13, 1996.

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