Pj. Vaughan et al., WATER-CONTENT EFFECT ON SOIL-SALINITY PREDICTION - A GEOSTATISTICAL STUDY USING COKRIGING, Soil Science Society of America journal, 59(4), 1995, pp. 1146-1156
A geostatistical analysis of soil salinity in an agricultural area in
the San Joaquin Valley included measurements of electrical conductivit
y of soil paste extract (EC(e)) and water content of soil samples supp
lemented by surface measurements of apparent electrical conductivity (
EM(H)). Prediction of soil salinity at unsampled points by cokriging l
og(e)(EC(e)) and EM(H) is worthwhile because EM(H) measurements are qu
icker than soil sampling. This work studies how patterns of log(e)(EC(
e)) predicted by cokriging with EM(H) are influenced by variation in g
ravimetric water content (W). The data are mean EM(H) = 1.00 +/- 0.13
dS m(-1) for 2378 locations, mean log(e)(EC(e)) = 1.40 +/- 0.29 dS m(-
1), and mean gravimetric W = 0.260 +/- 0.003, both averaged for four s
amples from 0.3-m intervals to 1.2-m depth for 315 locations. The coef
ficient of determination (R(2)) for EM(H) vs. log(e)(EC(e)) increased
with depth from 0.05 to 0.54 whereas the R(2) for EM(H) vs. W decrease
d from 0.48 to 0.28. A gray-scale EM(H) map contained nine out of 56 q
uarter-section boundaries coinciding with step variations in EM(H). Th
e t-statistics for differences in mean W were six of nine significant
at 0.001 and nine of nine at 0.05, but mean log(e)(EC(e)) had only two
of nine at 0.05, implying that W caused EM(H) steps. Water-affected E
M(H) impaired prediction of EC(e) at depth by cokriging, because near-
surface variations in W masked EC(e). Two subareas were defined, one w
here management factors, such as irrigation, controlled EM(H), causing
steps, and one where near-surface W varied less, making cokriging pre
dictions more reliable.