Assessing the Effects of Native and Introduced Pastures on the Hydrology of Southern Great Plains Using Nutrient Tracking Tool.

Grassland ecosystems which occupy approximately 40% of the world's land surface comprise one of the largest land cover types in the United States. Native grassland are managed by introducing new forage species with the aim of enhancing pasture productivity to feed millions of cattle in the Great Plains of the United States . Water is one of the fundamental resources that drives the grassland ecosystem in this region. Thus it is important to understand how the conversion of native to introduced pasture affect the water balance and biomass to ensure both the forage production and water demand in the region. We used Nutrient Tracking Tool (NTT) which is based on APEX (Agricultural Policy Environmental eXtender) model to assess the effect of native pasture (Little bluestem) and introduced pasture (Old world bluestem) on the hydrology of the Southern Great Plains. NTT simulated monthly evapotranspiration (ET) and biomass for the year 2015 and 2016 was validated with the measured data at two USDA-ARS experimental plots near El Reno, Oklahoma. Results showed that the long term average annual hydrologic fluxes from the introduced pasture were not significantly different with that from the native pasture even though the fluxes between the plots vary significantly for some years. The average annual yield was however higher for the introduced pasture compared to the native pasture.