WATER-QUALITY CHARACTERISTICS ASSOCIATED WITH SOUTHERN PLAINS GRASSLANDS

Water quality information regarding grasslands in the Southern Plains of Oklahoma and Texas is sparse. The objective of this study was to determine the extent to which the area's surface and ground-water quality is influenced by native and introduced grass management practices. Concentrations and amounts of sediment, N, and P in surface runoff water were determined for 14, 1- to 6-ha watersheds in the Reddish Prairie and Rolling Red Plain land resource areas for periods of 3 to 13 yr. In general, surface water quality was influenced slightly during the study periods by dominant grass species, area grazing practices, or recommended fertilizer rates, but sediment losses were curtailed greatly by gully amelioration. Mean annual discharge ranged from 30 to 71 000 kg ha-1 for sediment, 0. 1 to 11 kg ha-1 for total N, and 0.02 to 4 kg ha-1 for total P. Highest losses were from watersheds with established, active gullies. Annual soluble nutrient losses in surface runoff tended to be small, often <1 kg ha-1 N or P. Successful prediction of soluble P, particulate P, and particulate N in surface runoff was achieved using appropriate kinetic desorption and enrichment ratio procedures. Soluble N in surface runoff posed few water quality problems, but soluble and total P concentrations often exceeded proposed 0.01 and 0.02 mg L-1 respective critical limits for eutrophication, even on the more pristine watersheds. In the case of groundwaters, occasional, elevated NO3-N (i.e., 18 mg L-1 maximum) concentrations were observed beneath certain N fertilized, introduced and native grasses on shallow water table (3-20 m depth), medium-textured soils.