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ESTIMATION OF DURATION INDICES FOR REPEATED TENSIOMETER READINGS

David W. Meek* and Jeremy W. Singer

USDA-ARS, National Soil Tilth Lab., 2150 Pammel Dr., Ames, IA 50011



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  		Fig. 1. The time response curve and scattergram is shown for Plot 2423's wetness index, y(t). For both estimation methods, the wetness duration index is 36.5. The model has R2 = 0.974 for 31 observations (P < 0.0001). The selected regression model is two splined line segments with a common join point; formally, it can be written as follows:


The gray band is the 95% confidence interval, the dashed line represents the model estimates, and the black circles are the wetness index values estimated by scaling the soil matric potential observations.

 


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  		Fig. 2. Berg (1992) equivalence plot for comparing {theta}reg to {theta}trap. Here n = 15. For {theta}reg, the mean is 27.946 ± 2.416 (P < 0.0001), the maximum value is 36.499, the minimum value is 10.602, and the median value is 31.959 (see vertical box plot). For the {theta}trap, the mean is 27.927 ± 2.423 (P < 0.0001), the maximum value is 36.504, the minimum value is 10.612, and the median value is 32.189 (see horizontal box plot). Based on the {theta}trap – {theta}reg, the mean bias is 0.0192 ± 0.0484 (P < 0.6513), the maximum value is 0.415, the minimum value is –0.300, and the median value is 0.008 (see difference box plot). The Kerrich (1966) regression slope is 1.001 with a 95% confidence interval ranging from 0.997 to 1.005 with r = 0.9998 (P < 0.0001).

 




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