Soil Temperature Under A Dormant Bermudagrass Mulch: Simulation And Measurement

The ENergy and WATer BALance (ENWATBAL) model is a mechanistic, numerical model that simulates soil water and temperature profiles, evaporation from soil, and transpiration from crops, but it does not simulate the effects of a mulch layer. Surface vegetative mulches are becoming more common, especially in reduced-tillage systems, limiting the model’s applicability. Our objective was to modify ENWATBAL to enable physically based simulation of the effects of a dense mulch. As a preliminary evaluation of the model, soil temperatures simulated with the modified model were compared with those measured at Watkinsville, Georgia, in Cecil sandy loam (clayey, kaolinitic, thermic, Typic Kanhapludult) under a dense, thatchy layer of dormant bermudagrass (Cynodon dactylon, [L.] Pers.) that acted as a mulch during the simulation period. Measured daily soil temperature amplitudes at 0.04 m depth were about 2.5 C during an 8-day period in December 1995. Simulated amplitudes were 12 C with the original ENWATBAL model (configured for a bare soil) and 3.5 C with the mulch-enhanced model. The root mean square error between hourly measured and simulated soil temperatures was 4.1 C using the original ENWATBAL model and 1.1 C using the mulch-enhanced model. Measured soil temperatures lagged behind those simulated, indicating that conduction may be an important process of heat transfer through the mulch. Two solution methods were tested: an iterative solution for mulch and soil surface temperatures implicit in the energy balance equations, and a linearized explicit solution of the energy balances. The latter method was 50 times faster than the iterative method without compromising accuracy; the largest linearization error was only 0.01 C. The capability to simulate mulch effects increases the scope of problems where ENWATBAL is applicable.