between k and the mean values of clay content. Instead of using a pure

cokriging you could use a kriging with external drift in which the mean

value of k is related to the map with clay content. Goovaert'book

Geostatistic for Natural Resources Evaluation chapter 6 could give you an

hand. Gstat code permit you easily to perform kriging with extrernal drift.

Sincerely

Sebastiano Trevisani

At 04:35 PM 11/27/2003 +0100, Sigrun kvarno wrote:>Dear AI-GEOSTATS members,

[Non-text portions of this message have been removed]

>

>I tried co-kriging for the first time yesterday, and now I have some

>questions:

>

>I have a dataset with 42 measurements of saturated hydraulic conductivity

>(Kfs) within a grid where I have measured soil texture in 256 points with

>10 m spacing. Kfs measurements are irregularly spaced. Kfs is

>approximately log-normal, and there is a significant correlation between

>log(Kfs) and log(clay content), R2 = 0.61.

>

>I know that there is a significant spatial trend in clay content. Earlier,

>when using geostatistics on the clay content alone, I computed the

>variogram and kriged on the residuals, and then added back the trend

>function to the kriged estimates.

>

>Considering co-kriging, I need a variogram for both the primary variate

>(Kfs) and the covariate (log(clay content)), but due to the trend in clay

>content I have to compute the variogram on the residuals (this is

>important - a variogram on the original data gives a range of about 600 m

>(when using the GS+ program), whilst a variogram on the residuals reduces

>the range to about 55 m). But the correlation between residuals and logKfs

>is not so good. It is significant, and R2 = 0.20, which is not too good

>compared to 0.61 for the original data. What is the correct way to proceed

>in this case? Are there any rules of thumb for such problems?

>

>I'd also like to know which search radius should be used - the range of

>the logKfs data is about 40 m, the effective range (exponential model) of

>logclay is 600 m for original data (55 if residuals can be used), and the

>range of the cross-variogram is 53 m. I've learnt that the search radius

>should be approximately the same as the (effective) range when performing

>kriging. Is it the range of the cross-variogram I should use in this case?

>

>I hope there are some simple solutions out there!

>

>Kind regards,

>Sigrun

>

>

>

>

>

>******************************************************************************

>Miss Sigrun H. Kværnø

>Arealressursavd./Dept. of Land Resources

>Jordforsk - Norwegian Centre for Soil and Environmental Research

>Frederik A. Dahls vei 20

>N-1432 Ås

>NORWAY

>phone: +47 64948159

>fax: +47 64948110

>******************************************************************************