Grazing can have a profound impact on infiltration and thus runoff and
erosion. The objectives of this study were to quantify the effects of
select grazing systems on rainfall and snowmelt induced runoff and
sediment yield from sloped areas of the foothills fescues grasslands of
Alberta, Canada.
Previous studies have indicated that grazing has hydrologic impacts on the Canadian foothills through alteration of plant species composition, density, ground cover, and litter. Moreover, it has been demonstrated that livestock trample vegetation and may compact the soil, reduce infiltration, increase runoff and alter soil water regime. Runoff and sediment yield from rainfall simulations have been found to increase with increased grazing intensities and reduced ground cover. However, runoff and sediment yield have not been quantified for these rangelands under snowmelt or natural rainfall.
In 1988, research sites were established in the 40-year-old continuous grazing treatments and four-year-old short duration grazing treatments. Treatments contrast between the historical grazing regime (continuous throughout the growing season) and short-duration, high-intensity grazing. Continuous treatments were grazed from May through October; short-duration treatments were grazed for one week in Mid-June. Heavy and very heavy stocking densities at 2.4 and 4.8 animal unit month (AUM) /ha, respectively, in both the short duration and continuous treatments, represent common grazing intensities for the area. Runoff and sediment yield were monitored following snowmelt and after rainfall events using three 1-m2 runoff frames per treatment.
Average snowmelt runoff among treatments ranged from 3.3e to 105.7. Runoff was generally three to four times higher from mid-slope than from the upper slope, likely due to a slightly steeper slope in the mid-slope position and more snow trapped there in the woody vegetation. The investigators found that snowmelt runoff decreased with increasing grazing intensity from heavy to very heavy. The trend is likely due to higher litter and vegetation accumulations in the heavily grazed treatments which trapped more snow, resulting in increased runoff. Thus, any grazing regimes that reduces vegetation markedly will also reduce snowmelt runoff in this area.
Sediment concentrations under snowmelt were generally low and similar across grazing treatment, likely due to frozen soil conditions. Most concentrations were between 0.07 and 0.18 g/L.
Runoff was measured from 23 rainfall events during the four-year monitoring period (1988-1991). Runoff increased as grazing intensity increased (heavy to very heavy). The authors argued that vigorous plant growth was a factor controlling runoff through its reduction of soil moisture and thus the hazard of runoff. Moreover, antecedent soil moisture conditions appear to play a significant role in determining whether summer runoff occur.
Sediment concentrations increased under grazing compared to the control and were 4 to 10 times higher under rainfall than snow melt. Soil particle detachment occurs more easily under rainfall than snowmelt, resulting in the higher ratios under rainfall. Also detachment increases as protective vegetative cover is removed by grazing.
Many of the world's rangelands have topographic characteristics that predispose them to runoff and erosion. To obtain optimum sustained yields from such rangelands, range management strategies must incorporates soil and water conservation. A practical and economical means of achieving such conservation may be through management of animal use with selected grazing regimes.
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