Simulated water quality effects of alternate grazing management practices at the ranch and watershed scales

Abstract

Inappropriate grazing management with high stocking rates can result in significantly higher levels of runoff, sediment and nutrient losses to surface water resources. An assessment of water quality effects of various grazing management practices enables the selection of appropriate management practices. The overall objective of this study was to assess the impacts of alternate grazing management practices including the heavy continuous (HC), light continuous (LC) and adaptive multi-paddock (MP) grazing, and no grazing (EX; exclosure) on water quality at the ranch and watershed scales in the rangeland-dominated (71% rangeland) Clear Creek Watershed (CCW) in north central Texas using the Soil and Water Assessment Tool (SWAT). The SWAT model was calibrated and validated for water quality predictions using the measured data on county-level crop yield (1980–2013), and monthly sediment (1994–2009), total nitrogen (TN) and total phosphorus (TP) loads (1986–2009) at the watershed outlet. The ranch-scale assessment results at two study ranches indicated that when the grazing management was changed from the baseline MP grazing to HC grazing, the simulated average (1980–2013) annual surface runoff, sediment, TN and TP losses increased within the ranges of 106%–117%, 6.0–8.1tonha−1, 8.3–11.5kgha−1, and 1.6–2.6kgha−1, respectively. At the watershed-scale, shifting grazing management from the baseline HC grazing to the improved MP grazing decreased surface runoff, sediment, TN and TP loads by 47.0%, 39.7%, 35.1% and 34.1%, respectively. Thus, adaptive MP grazing was found to be the best grazing management practice for the CCW in terms of water quality protection and improvement in ecosystem functions such as reduced soil erosion and increased nutrient retention at both ranch and watershed scales. However, the magnitudes of water quality benefits due to adoption of MP grazing vary according to the extent of grazing lands in a watershed.