Evapotranspiration from Northern Semiarid Grasslands

Proper grazing management reduces ET from grasslands by reducing transpiration losses and grazingManagement of forage production for livestock grazing on semiarid induced surface energy relationships that reduce evapograsslands depends on water availability. Evapotranspiration (ET) ration (Svejcar and Christiansen, 1987; Wraith et al., was measured using the Bowen ratio energy balance method on three 1987; Bremer et al., 2001). Cattle (Bos taurus) springgrasslands at Mandan, ND: a nongrazed mixed-grass prairie (prairie), grazing a tallgrass prairie site reduced season-long ET a grazed mixed-grass prairie (grazed prairie), and a grazed western by 6.1% compared with an ungrazed site (Bremer et wheatgrass [Pascopyrum smithii (Rybd) Löve] site (western wheatgrass). Measurements were made from 24 April to 17 October al., 2001). Heavy grazing of a northern mixed prairie (the growing period) in 1996, 1997, and 1998. Peak ET rates generally by prairie dogs (Cynomys ludovicianus) increased ET coincided with periods of peak biomass production and occurred over light grazing (Day and Detling, 1994). Stewart and between early July and early August. Peak biomass averaged 1097 Verma (1992), however, reported no differences in ET kg ha 1 for the prairie, 1227 kg ha 1 for grazed prairie, and 1725 kg from grazing in a tallgrass prairie even when leaf area ha 1 for western wheatgrass, and peak leaf area index averaged 0.38 index (LAI) differences were large and soil water was for the prairie, 0.44 for grazed prairie, and 0.59 for western wheatgrass. nonlimiting. Soil water extraction by grazed and unGrowing period (175 d) ET averaged 489 mm for the prairie, 455 mm grazed crested wheatgrass [Agropyron desertorum for the grazed prairie, and 497 mm for the western wheatgrass while (Fisch. Ex. Link) Shultes.] was shown to vary during growing period precipitation averaged 320 mm. Evapotranspiration the season, but cumulative soil water use was not differof grazed prairie was 7% less than nongrazed prairie and 8% less ent to 193-cm depth (Wraith et al., 1987). Generally than western wheatgrass. Evapotranspiration of the nongrazed prairie grazed or clipped vs. ungrazed grasslands have greater and the grazed western wheatgrass were similar. The ratio of the soil water content available for regrowth later in the latent heat of ET to net radiation averaged 0.25 for grazed prairie season or for use during dry periods (Owensby et al., and 0.28 for prairie, suggesting that grazing changed the canopy struc1970; Bremer et al., 2001; Wraith et al., 1987). ture and energy budget components that affected ET. These results suggest that in a semiarid environment, proper grazing of prairie Water loss from grasslands is affected by precipitagrasslands conserves soil water. tion, soil water content, air temperature, vegetation diversity and density, energy availability, and canopy structure. The canopy structure in native prairie grasslands of the Northern Great Plains is typically sparse B production on semiarid grassland is limited and generally not the dominating factor in determining primarily by lack of soil water (Rogler and Haas, ET rates. Stewart and Verma (1992) confirmed the inde1947; Coughenour, 1985). The native grass species that pendence of latent heat flux (LE) and LAI by showing occur on the mixed-grass prairie of the Northern Great no effect on ET for LAI ranging from 0.5 to 1.7. They Plains evolved under limited water conditions (Axelrod, showed further that ET was reduced because of an in1985). These grasses often have leaf characteristics that crease in aerodynamic resistance to canopy water vapor favor water conservation, such as reduced stomatal size flux. Ham and Knapp (1998) found that energy fluxes and distribution, surface ridging, leaf rolling, pubesduring senescence caused a shift from a LEto a sencence, epicuticular waxes, and early senescence (Frank sible heat (H)–driven system. Overall, grazing-induced et al., 1996). Hanks et al. (1968) found that when soil changes in energy partitioning and canopy structure water was not limiting, ET for native grasses was nearly generally increase evaporative processes and decrease equal to pan evaporation whereas the ratio of ET to plant transpiration processes in grasslands. pan evaporation was 2 for oat (Avena sativa L.) and 1.5 This study reports on the ET rates for three Northern for winter wheat (Triticum aestivum L.). Precipitation Great Plains grasslands. The objective was to determine events also affect the rate of water loss from grasslands. ET rates for grazed and ungrazed native prairie in the Parton et al. (1981) showed that the greatest water loss Northern Great Plains grasslands and for a grazed site from a shortgrass steppe occurred within 2 to 14 d after that had been seeded to western wheatgrass. Informaa large precipitation event. tion has been previously lacking on rates and seasonal dynamics of ET for the predominately cool-season grazed grasslands of the Northern Great Plains. USDA-ARS, Box 459, Hwy. 6 S., Mandan, ND 58554. USDA-ARS, Northern Plains Area, is an equal opportunity/affirmative action emMATERIALS AND METHODS ployer, and all agency services are available without discrimination. The mention of commercial products in this paper is solely to provide The three sites were located at the Northern Great Plains specific information for the reader. It does not constitute endorsement Research Laboratory, Mandan, ND (46 46 N, 100 55 W; by the USDA-ARS over other products. Received 5 Dec. 2002. *Corresponding author ([email protected]). Abbreviations: BREB, Bowen ratio energy balance; ET, evapotranspiration; H, sensible heat flux; LAI, leaf area index; LE, latent heat Published in Agron. J. 95:1504–1509 (2003).  American Society of Agronomy flux; LEt/Rn, ratio of latent heat of evapotranspiration to net radiation. 677 S. Segoe Rd., Madison, WI 53711 USA