TIME-SERIES ANALYSIS OF FIELD-MEASURED WATER-CONTENT OF A SANDY SOIL

Prediction of the field soil water regime is important for assessing a grochemical transport and scheduling irrigation, This research was con ducted to evaluate the potential for employing the time series analysi s technique to predict average water content (W-AVG) of a soil profile and water content at depths of interest from measurements made at a s ingle depth, Volumetric water content of a Zimmerman fine sand (mixed, frigid Argic Udipsamment) in Princeton, MN, was measured in situ by t ime domain reflectometry at six depths during the early 1993 growing s eason. The time series made up of hourly measurements of soil water co ntent was first-order differenced to obtain stationarity, The differen ced data were used to conduct analyses in the frequency domain to eval uate the coherence and cross-amplitude between two soil water content time series and were subsequently fitted to autoregressive moving aver age models to obtain coefficients for the transfer function models in the time domain. The transfer function models were then used to predic t water contents at depths of interest (50-, 75-, and 100-cm depths) a nd Wave in the top 100-cm profile from measured water content at the 2 5-cm depth, Overall, the predictions were reasonable, with accuracy in creasing as the separation distance from the 25-cm depth decreased.