Water Flow in Unsaturated Soil Below Turfgrass Observations and LEACHM (within EXPRES) Predictions

In cropped soils, water sustains the plants, affects the transport of nutrients within the root zone, and controls the leaching of nutrients and chemicals to ground water. The objectives of this study were (i) to investigate the effects of turfgrass on water flow in sandy loam soil during the growing season using field lysimeters, and (ii) to test the abilities of the models EXPRES and LEACHN with free‐drainage and lysimeter bottom‐boundary conditions, respectively, to simulate water movement in the lysimeters. Twelve field lysimeters were packed with a three‐horizon profile, topped with Kentucky bluegrass ( Poa pratensis L.) sod, and monitored for 2 yr. Saturated hydraulic conductivity, measured on cores, was much greater and more variable for turf than soil. The moisture‐retention curve for turf also had a much steeper drop in water content at low applied negative head than soil. The lysimeters became very dry during the summer, and only drained during the spring and autumn. The model EXPRES generally predicted water flow well, but had some difficulty with water redistribution during the drying periods (gravity drainage and evapotranspiration). In general, with the free‐drainage bottom‐boundary condition, EXPRES predicted more drainage and less drying during the summer than was observed. Under conditions of little to no irrigation, the free‐drainage condition over‐predicted and the lysimeter condition under‐predicted the total amount of measured drainage. Model predictions of drainage under heavily irrigated conditions were similar for both bottom‐boundary conditions.