Considerations under climate change

Ockenden et al. (2017) found that under climate change (high emissions scenario, 2050s) winter P loads in the Newby Beck will increase up to 30%. To mitigate such

129 increases they report that major agricultural changes would be required (20-80% reduction in P inputs). Such predictions will critically impact the retention of P in this small headwater catchment. Higher flows in winter will mean more overland flow from grasslands; it is likely that SRP concentrations will remain high throughout the year giving way to increased likelihoods of eutrophication pressures. It is perhaps unlikely that point sources will remain as significant in contributing more frequently to load in Newby Beck. As high loads in winter driven by diffuse source contribution will likely “outwash” (i.e. whereby point sources decrease in significance due to the combined effect of the dilution and removal of sediments exposed to point sources) point sources at Morland village. Such high loads will give way to more P being retained within the catchment, as more is mobilised, therefore overbank flooding (which was observed in this study) may become increasingly frequent and important for retaining P in the headwaters.

Ockenden et al. (2017) also show that lower flows and a decrease in P export (-20%) during summer is likely under climate change. This poses significant risk to eutrophication issues during the summer, as mass balance calculations suggest that bed sediments are likely to desorb P contributing to the P load. On the other hand, during increased precipitation and loads in winter months, increased loads of sediment are likely to enter the stream reach. Particles will be washed out the system or deposited as new, this flushing effect is likely to “re-set” the system, changing whether sediments absorb or desorb P which will have implications for stream SRP concentrations in summer.

Therefore, it might be hypothesised that climate change will yield extremes between nutrient quality in summer (high concentrations, low flows) and winter (high loads, high flows). Yet, Ockenden et al. (2017) found that mitigation measures farmers are

130 likely to implement under climate change scenarios (e.g. increased livestock housing, covered slurry stores) are unlikely to be effective in reducing P loads. The likely disparities between summer and winter stream water quality, alongside the likely inefficiencies of mitigation measures calls for policy and regulation to reflect the urgency of the potential impacts of climate change on the riparian health in rural headwater communities.

5.6 Conclusion

There is evidence to suggest that this headwater system has been impacted by the activities occurring in the catchment such as agriculture. Flow and P data suggest that diffuse sources are critical sources upstream of Morland village, but downstream of this point sources increase in importance. Load apportionment modelling implies that diffuse sources dominate (>65%) TP load, but point sources downstream of Morland village contribute up to 64% of the time therefore potentially diminishing the importance of diffuse sources beyond this point. Mass balance calculations also suggest that bed sediments could play a critical role in the desorption and remobilisation of P to the river reach during summer contributing the P load downstream, whilst in winter results suggest the opposite is the case, that P is retained, most likely by P retention in floodplains. Sediment EPC0 values suggest that there is possibly a limited or non-existent retention capacity for SRP within the headwaters, but that the influence of point sources downstream of Morland village alter this buffering capacity as sediments show the ability to absorb and desorb SRP. The hypothesis that anthropogenic point sources of P can alter the degree of retention of P even at the headwater scale where agricultural diffuse sources dominate can be accepted. In this case, this headwater catchment can potentially act as a source of P to downstream reaches, most likely due to anthropogenic inputs reduce the capacity of

131 sediments to retain P. This remobilisation potential of P is especially critical during low flows when river ecological degradation is a more prevalent risk. Such low flow periods are likely to increase under climate change therefore, this headwater catchment may be at risk of further degradation. However, climate change will increase flows in Newby Beck by up to 30% in winter months. Such increases in flow may have the potential to “outwash” point sources at Morland village, and “re-set” instream sediment buffering capacity. It might be hypothesised therefore that climate change will yield extremes between nutrient quality in summer (high concentrations, low flows) and winter (high loads, high flows).

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136

Chapter 6 Conclusions

In this chapter I firstly outline the specific conclusions reached from the work in this thesis. Secondly, I summarise and discuss the work completed and then outline future work which would be beneficial to enhance and progress my findings further.

6.1 Conclusions

My overall aim was to gain a better understanding of the possible changes which will be caused by climate change, including both individual processes (e.g. mobilisation) and catchment scale processes which encompass the P transfer continuum, in a rural headwater catchment (Newby Beck, Eden DTC).

I therefore highlight the possible risks of climate change exacerbating, rather than changing, the processes described in the P transfer continuum for the Newby Beck catchment (Figure 6.1 and Figure 6.2). This study has therefore encompassed a range of scales, from fine-scaled laboratory based experimentation; to small catchment scale monitoring; and multi-catchment coarse predictions; in order to bring together aspects of measuring and modelling to further our understanding of within catchment complexity (Neal, 2013). Objectives were achieved by analysis of current literature, laboratory experimentation, high and low- resolution river quality (P, chloride), flow, and sediment P data, and the modelling of process-based model predictions for P and flow for future climatic conditions. The work was underpinned by existing work being carried out on the national DTC framework, and research under the NUTCAT project.

137

Figure 6.1 A conceptual diagram of the main conclusions of the thesis based on the phosphorus (P) transfer continuum diagram outlined in Chapter 2. The recycling arrows represent buffering properties on land (green) and within the river reach (blue). Writing in orange depicts changes under climate change.

Bold arrows show linkages that influence the next process/component, dashed arrows show result links between chapters.

138 The following conclusions, relevant to the research objectives, have been drawn from this thesis and are highlighted in Figure 6.1 and Figure 6.2:

Objective 1: To appraise the likely changes to the P transfer continuum under climate