Water flow paths during the rainy season in an acid sulphate soil

Management of acid sulphate soils is one of main problems in the search for sustainable development of the Mekong Delta. This study was carried out within the Management of Acid Sulfate Soil Project (MASSP) at Tan Thanh experimental farm situated in middle of the Plain of Reeds of the Mekong delta, Vietnam, from 1990 until now. The present study concentrated on water flow patterns through the soil profile especially in the rainy season, a time when the chemical and physical processes in acid sulphate soils are very important. With measurements in the field under the two controlling boundary conditions: local weather (upper boundary) and canal water levels (dynamic lower boundary), SOIL model was used as a tool to simulate soil water contents, matric potentials and the distribution of water flow in soils. The results showed an agreement between model simulations and measurements especially in ground water level and soil temperatures. However, simulation of the ground water level became complicated since it was affected by the both high quantity of infiltration and the fluctuation of water flow from canals. The major part of the rain infiltrated and rarely formed soil surface runoff because of high storage capacity and high infiltration capacity. Partitioning between bypass flow and total water flow in the unsaturated part of the profile was estimated at about 50 %, 35 % and 20 % in 10, 20, 30 cm levels respectively. The water exchange between canal water and groundwater was established by the hydraulic gradient of flows (canal levels - ground levels). The ratios of inflow to outflow were about 4.5 from observed data and from simulated data. The reasonable value of the saturated hydraulic conductivity (ks) used in the simulations was in the range of 1.4 - 1.8 cm min-1 for distances to canals in the range of 1 to 5 m