Study on soil water retention and infiltration rate: a case study in eastern Iran

Knowing the water retention properties of soil can help with predicting water infiltration into the soil, the potential for runoff generation and sediment yield, and ultimately, better management of natural areas. The purpose of this study was to investigate the characteristics of the soil water retention curve (SWRC) and its effects on infiltration rate and runoff generation. For this purpose, an arid area in eastern Iran for which rainfall and runoff data were available was selected. Two variables that are assumed to affect soil water retention and runoff generation were used in this study, namely geological formation and topographic wetness index (TWI). The geological formations layer for the study area was extracted from 1:100,000 geological maps, and TWI was derived from the combination of slope and the area of upstream basins. Initial infiltration rate, stable infiltration rate, and time to stable infiltration rate were determined using the double ring infiltration apparatus and were used to create the soil infiltration potential curve (SIPC). In addition, saturation soil water content, field capacity, permanent wilting point, plant available water, and area under the curve were determined using pressure plates, and were used to create SWRC. The results showed that the characteristics of SWRC, SIPC, and soil water conditions after rainfall are affected by geological formation and TWI. All characteristics of SWRC were higher in the shale formation than the sandstone formation, and areas with higher TWI had lower soil water retention. Also, soils in the sandstone formation had higher initial and stable infiltration rates and required a shorter time to reach stable infiltration rates. The results showed that conversion of the soil water curve from the soil water-suction mode to the suction-time mode allows for predicting post-rainfall soil water conditions. Finally, areas in the shale formation with maximum TWI had the highest water retention potential, lowest infiltration rates, and maximum runoff generation capacity.