SPATIAL-ANALYSIS OF SATURATED HYDRAULIC CONDUCTIVITY IN A SOIL WITH MACROPORES

Saturated hydraulic conductivity (K-S) is an important soil hydraulic parameter for it establishes a limit on the rate of water and solute t ransmission through soil. However, its determination in the laboratory has been shown to be much influenced by column size. We evaluated the spatial variability of laboratory K-S measurements using three differ ent column sizes: firstly, sixty 5.1 cm long columns of 5 cm diameter were used (type-I), next, thirty 20 cm long and 20 cm diameter columns were considered (type-II), and finally, thirty columns 100 cm long an d of 30 cm diameter (type-III) were studied. All columns were collecte d along a transect in a sandy loam soil with macropores. Estimates of macroporosity at three depths (2.5, 12.5, and 16.5 cm) for twenty-four of the type-II columns were calculated from stained dye patterns obta ined during ponded infiltration. The geometric mean of K-S decreased w ith increasing column size, i.e., from 2.24, 1.68 to 0.56 cm/h for typ e-I, -II, and -III columns, respectively. The coefficient of variation (CV) based on a log-normal distribution showed a similar trend: 619% for type-I, 217% for type-II, and 105% for type-III. Type-II and type- III columns were large enough to encompass a representative elementary volume (REV). The percentage of dye-staining (macropore cross-section al area) decreased from 3% at 2.5 cm to 1.7% and 1.6% at 12.5 and 16.5 cm, respectively. Percentage of depth-averaged macropore area was mod erately variable with CV = 51%. A geostatistical analysis revealed tha t a weak spatial structure existed for type-I K-S measurements whereas type-II and type-III columns displayed better spatial correlation wit h a range of approximately 14 m and 11 m, respectively. Spatial correl ation was also observed for depth-averaged macropore area with a range of 12 m. The cross-semivariogram calculated between type-II K-S value s and depth-averaged macropore area obtained from the same columns ind icated positive spatial cross-correlation for all lags.