Abstract: A float valve assembly, integral with a water supply system, responds to changes in a predetermined water level inside a test bore to regulate water flow through the float valve into the bore to maintain this water level. The float valve assembly can be positioned at different depths below ground level by suspension at the lower end of a premarked flexible hose (16) hanging freely inside the test bore. The float valve housing (1) is open at its lower end (3), so that water around it in the test bore can raise the float (8) within to throttle the water flowing down through a reducer (13) at the end of the hose and directly above the float. After an initial transient stage, the water in the test bore percolates away from the bore through the soil around it at a steady rate, measured to obtain the steady state percolation rate at that site at the selected depth.