Livestock impacts on hydrological connectivity

• Main Author: Zhao, Yiwen
• Published: University of Leeds 2008
• Subjects: 631.4
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.485753

In response to sustainable land management requirements and catchment-scale hydrological modelling needs, scientific interest in hydrological'connec~ivity'hasgrown. However, little is ~ . known about how hydrological connectivity is impacted by management. While there has been much research into livestock grazing and soil erosion in the UK; relatively little research has been conducted to characterize the hydrological patterns of livestock impacts, especially in organic soils which might be more vulnerable to environmental degradation. This thesis examines livestock impacts, as a land use case study, to conceptualize the 'dynamic nature of hydrological connectivity and its potential changes on hillslope and catchment hydrology. Different grazing strategies on organic soils (blanket p~ats and peaty gleys) in the uplands of Upper Wharfedale and Teesdale and organo-mineral soils (stagnogleys) in lowlands at Kirkby Overblow, northern England are investigated. Field measurements and monitoriIig are combined with laboratory experiments and proces~-based distributed modelling. Results suggest that compaction effects from sheep differ between soil types and topographic context but that they were generally confined to the upper 20 cm of the soil profile but with greater impacts in the upper 5 cm. Light-medium grazing (-9 a ha-1 ) significantly raised mean bulk density compared to adjacent soils with no grazing in the upper 10 cm of the soil profile of'stagnogleys. The relatively heavy grazing associated with frequently trampled areas under sheep tracks on stagnogleys was associated with soils with a mean bulk density 10 % greater (in the upper 10 cm of the soil) than those in the surrounding lightly grazed soil for stagnogleys, 2 % in 0-5 cm arid 9 % in 5-10 cm layers of peaty gleys, and at least 50 % in the near surface of peats. The effects of sheep tracks on soil properties and hydrological function were found to extend by at least 2 m further either side ofthe track rather than just being a feature ofthe visible track alone. Removing sheep led to a substantial fall in compaction effects in peats (10.4-21.7 % in bulk ~ensity) and stagnogley soils (0.5 % in bulk density) within a short time period (4-6 years). There were also significant differences in jnfiltration rates between short-term exclosure sites ' and grazed sites. For peat soils differences between lightly graZed areas and the long-term ' exclosures (35-45 years) were not much greater than those between the lightly grazed areas and the short-term exclosures suggesting that soil response to removal of sheep can be rapid. This thesis demonstrated that both the value and actual spatial distribution of livestock may .significantly decrease vertical connectivity and increase lateral connectivity, and hence increase both the spatial and temporal frequency of overland flow. In the organic soils of the humid-temperate zone studied this is through alteration of saturation-excess processes rather than infiltration-excess flow generation; antecedenfwater stores or soil moisture content are therefore of great, importance in hydrological connectivity responses to livestock impacts. Sheep tracks can be crucial in producing more rapid and connected overland flow on peats and stagnogleys, and effects extend several metres ether side of the track. Blanket peats are more vulnerable to livestock grazing than stagnogleys. The modelling approach used on stagnogleys from Kirkby Overblow suggests that increasing sheep density in very wet environments would lead to enhanced flood risks in terms of overall runoff amount and more rapid response due to e$anced coupling of hillslope arid river corridors, especially in extreme rainfall conditions.