Effects of sheep grazing episodes on sediment and nutrient loss in overland flow

Abstract

The effect of sheep grazing on the loss of sediment and nutrients in overland flow was investigated on a hill-country farm in the Waikato, New Zealand. The losses were measured in runoff produced artificially with small (0.5 m2) and large (1050 m2) rainfall simulators. Immediately after intensive winter grazing, rainfall applied at high intensity increased concentrations by a factor of 13–16 for sediment and particulate nutrients, 33–76 for dissolved reactive phosphorus and ammonium-nitrogen, and 5–7 for dissolved organic nitrogen and phosphorus. During summer, when there was less removal of vegetative cover, there was a smaller effect of grazing. The concentrations of sediment and particulate nutrients in overland flow were strongly correlated with the percentage of bare ground. The concentrations returned to background levels within 6 weeks after grazing, and the infiltration rate and ground cover also recovered from grazing in this time. The small rainfall simulator experiments showed that the infiltration rate decreases with grazing, which results in greater runoff after grazing. The greater runoff combines with the increased concentrations to give higher loads after grazing. In late winter, the infiltration rates were approximately half the summer values and the soil erodibility was approximately double, so the risk of high sediment and nutrient loads is greatest in winter, especially considering the higher rainfall and lower grass growth. The management implications are that exposure of bare ground associated with intensive grazing should be avoided, especially in winter.

Using a rainfall simulator and a physically based hydrological model to investigate runoff processes in a hillslope

“…The main disadvantage of small simulator experiments is that only small-scale erosion processes, principally raindrop detachment (inter-rill erosion), take place in the plot area, and larger processes, such as flow-induced shear detachment (rill erosion), are not normally observed. Use of a large-scale simulator (Elliott and Carlson, 2004) overcomes this shortcoming because it allows flow convergence and the development of rill and sheet erosion. The large rainfall simulator was constructed at Pukemanga in November 2002, initially for investigating the effects of sheep grazing at a larger scale (Elliott and Carlson, 2004).…”

“…Use of a large-scale simulator (Elliott and Carlson, 2004) overcomes this shortcoming because it allows flow convergence and the development of rill and sheet erosion. The large rainfall simulator was constructed at Pukemanga in November 2002, initially for investigating the effects of sheep grazing at a larger scale (Elliott and Carlson, 2004). Use of a large rainfall simulator allows measurement of surface flow rates, collection of samples for the analysis of sediment, pathogens and nutrients, and the deployment of instruments for measuring soil moisture such as time-domain reflectometry (TDR) probes.…”

Using 137Cs and 210Pbex measurements to estimate soil redistribution rates on semi-arid grassland in Mongolia

“…In general, soil in semi-arid areas has a low organic content and large percentage of silt, which results in high soil susceptibility to surface sealing and crusting (Abu-Awwad and Shatanawi, 1997; Mills and Fey, 2004). Thus, the loss of surface vegetation cover due to herbivory by domestic grazing animals decreases infiltration rate, and hence markedly increases overland flow generation and surface erosion (Gutierrez and Hernandez, 1996;Elliott and Carlson, 2004). The increase of sediment load in surface runoff accelerates nutrient loss from surface soil (Schlesinger et al, 1999;Brazier et al, 2007), which is considered one of the primary causes of rangeland desertification.…”

Analysis of runoff generation and soil erosion processes by using environmental radionuclides in semiarid areas of Mongolia

“…Therefore, the surface vegetation cover is considered to be the primary reason for soil erosion (Cammeraat, 2004). A considerable number of studies have been conducted on the impact of grazing pressure on soil erosion (Evans, 1977;Elliott and Carlson, 2004). Soil erosion in semiarid grasslands may also trigger desertification (Faraggitaki, 1985;Zhao et al, 2005), which is one of the key issues concerning the global environment.…”