WATER FLOW PATH CHARACTERIZATION IN SHALLOW VADOSE ZONE USING TENSIOMETERS. A TECHNICAL REPORT AND THE PRELIMINARY RESULTS OF A PILOT PROJECT

We have measured the subsurface water flow paths in sandy clay loam soil following infiltration experiment using Tensiometers. The head gradient (matric potential) and hydraulic conductivity measurements show that subsequent infiltration, water movement in the vadose zone is vertical, but it can have a large lateral component under steady condition. This shows that water moves generally from high water content to lower water content region. Average Pressure head for the percolation test conduction locations EB and HB was -30 and -80 cm respectively. Hysteresis produces another interesting situation when the soil is drained. We found that the wetter portion of the soil in the vadose zone could be at a lower potential (head) than the drier portions, resulting in lateral driving force for a preferential flow of water from the drier to the wet soil. The infiltration rate for the 5cm ponded water was calculated at 5.45 cm/hr. The infiltration rate curve shows, that the rate of infiltration decreases with the time. When infiltration first starts, the wetting front is steep and very close to the surface. Due to the pressure head gradient, large value for infiltration is recorded. Under these conditions, we believe that the gradient in pressure head is responsible for the rapid movement of water into the dry soil. The effect of gravity is less on water during the initial stages of infiltration, however, it is more effective for preferential flow pattern. In the latter infiltration event, the wetting front has moved deeper into the soil. As a result, the pressure head gradient at the surface is much smaller and consequently has little effect. When pressure head is equal zero, infiltration rate approaches almost to the lowest level. We also observed that even a minor change in soil-water pressure due to slope could change both direction and magnitude of water flux.