In general it can be said that water flows faster in large than in smaller pores and in thicker water films than thin. This is because there is less friction and attraction to surfaces and water flow is less interrupted in wet soil. Therefore, the wetter the soil the more freely it can move.
It should also be noted that water does not flow appreciably from small water filled pores into large air filled pores. Water flow from a fine-textured layer (Clay Loam) into a course textured one (Sand) is slow except at water potentials near saturation (free flowing water).
Indicative values of saturated hydraulic conductivity (Ksat) for various soil texture classes are shown in the table below.
Soil Texture Class | Ksat (mm/hr) | Ksat class |
Coarse Sand | 360 | Very Rapid |
Sand | 208 | Rapid |
Loamy Sand | 61 | Rapid |
Loam Fine Sandy | 36 | Moderately Rapid |
Sandy Loam | 26 | Moderately Rapid |
Fine Sandy Loam | 19 | Moderately Rapid |
Loam | 13 | Moderate |
Silt Loam | 7 | Moderate |
Silt | 7 | Moderate |
Sandy Clay Loam | 4 | Moderately Slow |
Clay Loam | 2 | Moderately Slow |
Silty Clay Loam | 1.5 | Moderately Slow |
Sandy Clay | 1.2 | Slow |
Silty Clay | 0.9 | Slow |
Clay | 0.6 | Very Slow |
Generally Ksat can be divided into three main classes.
Rapid - includes horizons which are largely gravel, sand or both and have very little silt and clay. Soil in this class has large connecting pores that transmit water rapidly. Loamy sands and some sandy loams and loams that have strong granular structure may also have high hydraulic conductivity.
Slow - includes horizons with clayey textures (Sandy Clay, Clay, Silty Clay) which have weak, massive, platy, or no structure. Fragipans and horizons with slickensides are also included in the slow class.
Moderate - includes texture and structure combinations not included in the other two classes.