From: Lindsay Hutley (L_HUTLEY) To: sowacs@aqua.ccwr.ac.za Date: Friday, June 21, 1996 1:52 pm Subject: sowacs: TDR calibration > Date sent: Thu, 20 Jun 96 09:09:17 PST > From: "Andrew Gray" > To: sowacs@aqua.ccwr.ac.za > Subject: Re: TDR calibration Andrew Gray wrote; > I agree that a two-point calibration is probably notappropriate, because of > the non-linear (1/V) response of TDR at low soil-moistures(especially > oven-dry), but if your soils in the field never gets too dry(e.g. 12% or > less), maybe a low and high calibration point would be enough? > > I am extremely skeptical of any calibration involving mixingand packing of > soils, particularly after sieving the soils to 2 mm. Thatdestroys much of > the soil structure, removing aggregates and air pockets (thelatter which > may greatly influence the TDR response), so my feeling is thatyou're > calibrating something unrelated to the field soil,particularly in the > high-organic content forest soils I work in. > > I conducted my calibration similar to the method in Herkelrathet al > 1991 (Water Resources Research 27: 857-864) and collected soilsamples by > pounding 10 cm diameter PVC pipe into the ground andextracting the core > (with minimal compaction), letting the core dry in the lab,then sealing the > bottom and adding water to the top and taking measurements. Ididn't wait > more than a few minutes before sampling, because TDR is robustto vertical > heterogeneity in H2O content along the rods. Even thoughhorizontal > heterogeneity might have been a factor, the confidenceintervals on my > regressions were still around 2.5% H2O, which isn't much worsethan Topp et > al's. > > Some folks I know have been doing point sample calibrations,by taking TDR > readings in the field at different times of the year, thenextracting a soil > core at the sample point. My concern there is that a standardsoil sample > tool is only about 3 cm in diameter, and I doubt you can getan accurate > volume sample over a depth of 30 cm or more (soil compacts anddoesn't enter > the bottom of the sampler). > > I'd really appreciate any feedback on these methods. AlthoughI published > my calibrations (Can. J. Forest Res. 25: 376-385), I haven'theard a peep! > > Andrew Gray > graya@fsl.orst.edu I agree with Andrew's point re calibrations done in the lab (2point or otherwise). I used TDR in rainforest soils of Southern Quensland. The soils had high clay content and destructive volumetric sampling was very difficult as the soils were highly compactable, especially when wet. The high density of roots near the surface also made accurate volumetric sampling difficult. Thus we felt lab calibration was not going to tell us much given the large alteration in structure that would result from destructive sampling. We still sampled in the field to make comparisons with the TDR, which were only within 10%. However our interest wasmainly relative storage changes for ET calculations so we felt that was acceptiable. Lindsay Hutley CRC and the School of Biological Sciences Northern Territory University Darwin 0909 Ph (08) 8946 7103 Fax (08) 8946 7107