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FrankS
Average Member
USA
15 Posts |
 Posted - 05/29/2008 : 18:44:45
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I am interested in the distribution of VG parameters for sandy loam agricultural mineral soils, and how different VG parameter estimation methods influence modeling uncertainty. I noticed Rosetta (SSCrho - theta33 - theta1500)-based estimates of theta-r for several sandy loam soils in the supplied UNSODA database are in the range of 0.15 - 0.30 (!!!). In addition, the measured (fitted?) theta1500 values reported in that UNSODA database are also in this range. Does anybody know whether such high theta-r/theta1500kPa are possible for sandy loam soils? or are these outliers, errors or just very very unusual soils?
I'm aware of the empirical nature of the VG parameters, where alpha and n are best viewed as fitting parameters, but theta-r and theta-s do constrain possible water contents in HYDRUS and so are more than just empirical fitting parameters. I also understand the asymptotic nature of theta-R will necessarily yield uncertain estimates, but values > 0.15 seem incredible. Comments anyone? |
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bjohnson
Junior Member
USA
4 Posts |
Posted - 03/02/2010 : 23:00:13
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Since you subsequently did a nice analysis of this issue and are too modest to post it, I am going to post it here. A quote from this analysis addresses the issue that you brought up in May 2008:
“Estimates of #952;r derived from fits to limited ranges of #952;(h) data are sometimes unrealistic (Figure 4). The fitted parameters for the two soils in Figure 4 were among those used to calculate the textural class parameter means comprising ROSETTA PTF#1. Across all 321 Sand–Loam UNSODA soils, estimated #952;r from such limited #952;(h) data are significantly greater than when estimates were derived from a broader range of #952;(h) data, regardless of soil textural class (analysis of variance, p<0.001; Appendix 2). Moreover, in nearly all cases, when high values of #952; r are observed, the #952;(h) data from which those estimate were obtained only include #952;(h) measurements in the “wetter” range (e.g. where minimum h > -1000 cm) (Appendix 2).”
For those interested, the full work is at
http://www.cdpr.ca.gov/docs/emon/pubs/ehapreps/analysis_memos/2066_rosetta.pdf
and details an analysis of the impact of various pedotransfer function estimation procedures on cumulative flux for a volatile compound.
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bjohnson
Junior Member
USA
4 Posts |
Posted - 03/02/2010 : 23:03:13
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| In my preceding post, please mentally insert the Greek theta symbol for #952. |
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residual theta - coarse soils - Rosetta/UNS
