Unsaturated Hydraulic Conductivity

AGVISE Laboratories now offers “Unsaturated Hydraulic Conductivity”. Unsaturated Hydraulic Conductivity is one of the soil parameters recommended on ground water studies by government agencies.

AGVISE Laboratories has offered “saturated hydraulic conductivity” (SHC) soil analysis for several years. The principles involved in saturated hydraulic conductivity are relatively simple. Soil is placed into a plastic tube or column. The bottom of the tube is covered with a mesh that lets water flow through but not soil particles. Water is then applied to the column of soil until it is saturated. Once the soil is saturated, additional water is pumped on to the top of the column and held several inches  above the soil in the column. While the water level above the soil column is held constant by a small pump, water begins to flow out the bottom of the column through the mesh. The volume of water that is flowing out the bottom of the column is collected and measured over time. The water flow through the column is recorded for several consecutive hours. Once the water flow is consistent hour to hour, the saturated flow rate has been achieved. A “Saturated Hydraulic Conductivity” can then be calculated for that soil sample. The value calculated for “Saturated Hydraulic Conductivity” is a measure of how fast water flows through a soil profile, if the soil is saturated at the start. This information is very useful in determining the movement of nutrients and pesticides through the soil in a saturated condition.

The method for “unsaturated hydraulic conductivity” (UHC) is a somewhat different and more complex measurement. Unsaturated hydraulic conductivity is a measure of how water flows through a soil profile when the soil is not saturated with water. The unsaturated hydraulic conductivity of a soil is important when evaluating the movement of pesticides and nutrients through the soil at different water contents. In this method we start out with soil in a column and saturate it with water just as in the saturated hydraulic conductivity method. We then reduce the water content of the soil to a specific water content (i.e. 15 bar or wilting point). Water is then applied to the bottom of the column of soil at a slow but constant rate. A computer records changes in a tensiometer mounted at the top of the soil column to record the exact time the water reaches the top of the column of soil. The amount of time required for water to fill the column is then used to calculate the unsaturated hydraulic conductivity of that soil. This procedure is then repeated several times, with the soil starting at different initial water contents.