Evapotranspiration in a prairie ecosystem: Effects of grazing by cattle

Grazing by ungulates is common in grasslands and may influence evapotranspi ration (ET). The Bowen ratio energy balance method (BREB) was used to measu re ET from grazed (GR) and ungrazed (UGR) tallgrass prairie sites in northe astern Kansas, USA. Yearling steers mere stocked on the GR site from day of year (DOY) 128 to 202 in 1999, and ET data were collected from DOY 141 to 295. Grazing reduced ET by 28% between DOY 179 and 207; mean ET values were 3.6 (GR) and 5.0 mm d(-1) (UGR). During that period, leaf area index (LAI) was an average of 78% lower on the GR site, and below-normal precipitation kept soil dry near the surface; hence, transpiration and evaporation of wa ter from soil decreased. Lower ET during that period, conserved soil water in the 0- to 0.30-m profile on the GR site. Before that (e.g., DOY 152-179) , ET was similar between treatments, despite an average 70% lower LAI on th e GR site compared with the UGR site. Above-normal precipitation during tha t period probably maintained high evaporation of water from soil, thereby c ompensating for reductions in transpiration (via LAI removal) on the GR sit e. Cumulative ET values during the 155-d study were estimated at 526 and 49 4 mm on the UGR and GR sif es, respectively. Thus, grazing reduced seasonal ET by; 6.1%. Late in the study, ET nas higher on the GR site, despite a lo wer LAI compared vith the UGR site. Younger leaves in regrowth after grazin g resulted in delayed senescence, causing higher ET on the GR site.