Near-saturated soil hydraulic properties as influenced by land use management systems in Koohrang region of central Zagros, Iran

Soil management and land use via the effects on soil characteristics can indirectly change soil hydraulic properties. This study was conducted to investigate the impacts of different management and land uses on the near-saturated soil hydraulic properties in Koohrang region of central Zagros, Chaharmahal-va-Bakhtiari province, Iran. Major land uses in the area were pasture, dryland farming, irrigated farming and fallow. Unsaturated water infiltration was measured at the consecutive inlet matric suctions (h) of 2, 5, 10 and 15 cm using a tension infiltrometer at 100 locations (40 in pasture, 33 in dryland farming, 15 in irrigated farming and 12 in fallow). The infiltration data was modeled using Wooding's analytical method and best-fit values of Gardner's parameters of saturated hydraulic conductivity (Ks) and macroscopic capillary length (λc) were calculated. A completely random design was used in which soil texture and land use system were analyzed separately. The averages were compared by the least significant difference test at 5% of probability. The λc and unsaturated/saturated hydraulic conductivity [K(h)] values were not significantly affected by soil textural classes. However, the land use systems significantly affected soil hydraulic parameters (K(h), steady-state flux, q(h), and sorptivity, S(h)), and the differences became greater with decreasing h (towards saturation). The averaged K(h), q(h) and S(h) values were lower in pasture soils when compared with the cultivated lands which were associated with lower organic matter and higher degree of compactness of pasture soils due to overgrazing. The λc was significantly greater in the fallow and pasture land uses than in dryland farming, and intermediate value belonged to the irrigated farming. For all of the land use systems, minimum values of S(h) were observed at h = 10 cm. The S(h) decrease with h decrease in the range 15-10 cm might be partially associated with swelling of smectite clays, reducing the size of soil pores. The dryland farming increased water infiltration and unsaturated hydraulic conductivity when compared to the other land uses. In this region, averages of soil hydraulic properties are mainly influenced by soil structure and management practices rather than by intrinsic soil properties like texture. A small change in degree of compactness in the swelling soils would significantly influence water infiltration and hydraulic properties, indicating structural susceptibility of the soils to management practices. Therefore, the degree of compactness (e.g. relative bulk density) could be considered as an important index of land use management.