Understanding the effects of pasture type and stocking rate on the hydrology of the Southern Great Plains

Authors: NIRAULA, REWATI - Tarleton State University; SALEH, ALI - Tarleton State University; BHATTARAI, N. - University Of Michigan; KANNAN, NARAYANAN - Tarleton State University; OSEI, EDWARD - Tarleton State University; Gowda, Prasanna; Neel, James; XIAO, XIAOMAO - University Of Oklahoma; BASARA, JEFF - University Of Oklahoma

Submitted to: Science of the Total Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/5/2019
Publication Date: 11/17/2019
Citation: Niraula, R., Saleh, A., Bhattarai, N., Kannan, N., Osei, E., Gowda, P.H., Neel, J.P., Xiao, X., Basara, J. 2019. Understanding the effects of pasture type and stocking rate on the hydrology of the Southern Great Plains. Science of the Total Environment. 708:134873. https://doi.org/10.1016/j.scitotenv.2019.134873.
DOI: https://doi.org/10.1016/j.scitotenv.2019.134873

Interpretive Summary: Grasslands are among the major ecosystems in the world. Most US grasslands are concentrated in the Great Plains, where native prairies and introduced pastures serve as a major forage source for beef cattle production, a major economic activity in the region. Understanding the spatiotemporal dynamics of the water cycle in these grassland ecosystems under different managements has a direct implication on both climatic and economic processes. In this study, a biophysical model “APEX (Agricultural Policy Environmental eXtender) was used to assess the effects of pasture type and stocking rate on the hydrology of the Southern Great Plains. Two pasture types (native and introduced) were evaluated at different stocking rates. Long term simulations indicated that introduced pastures produce higher water yield (runoff and recharge) while reducing the evapotranspiration rates. Results also suggested that increasing the stocking rate will further enhance this trend. Further, simulations results showed that introduced pastures are relatively more water efficient than native pastures in the Southern Great Plains.

Technical Abstract: Grassland is one of the major biomes in the United States (US) and the world. In the US, the majority of grasslands are concentrated in the Great Plains and has undergone significant interventions or management changes over the last few decades. A key economy-driven intervention in the Southern Great Plains (SGP) include the introduction of new forage species and conversion of native grassland to introduced pasture to increase productivity and its nutritive value for improved cattle production. Since water is one of the fundamental resources needed to sustain grassland productivity, it is important to understand how such pasture conversion and prevailing cattle grazing practices affect water balance and biomass production in a given pasture system. In this study, the Nutrient Tracking Tool (NTT) with its core APEX (Agricultural Policy Environmental eXtender) model was used to assess the hydrological impacts of the pasture introduction, i.e., native pasture (little bluestem, Schizachyrium halapense) vs. introduced pasture (old world bluestem, Bothriochloa caucasica), and the stocking rate in the SGP. Monthly evapotranspiration (ET) and biomass estimates from NTT compared well with observed data at two USDA-ARS experimental pastures (native and introduced) near El Reno, Oklahoma, for the years 2015 and 2016. Simulated long-term average annual hydrologic fluxes (i.e. ET, runoff, and groundwater recharge) from the introduced pasture were slightly lower than the observed data but not significantly different than those from the native pasture under the current management conditions. NTT predicted higher water yield (runoff and recharge) and significantly lower ET for the introduced pasture than the native pasture. Results suggest that grazing has the potential to alter the hydrological balance in the SGP. For example, the increase in stocking rate within the carrying capacity of the farm decreases ET and increases runoff and groundwater recharge for both pastures. Comparison of estimated biomass production between native and introduced pastures indicated that introduced pastures are more efficient in using the available water and thus produce a higher forage biomass per unit of water in the SGP. This study highlighted the potential significance of considering hydrological and other biophysical impacts of new forage introduction and stocking rate changes for the sustainable management of grazing and pasture systems in the SGP.