The research presented in this thesis posed and answered a number of fundamental questions regarding the controls on runoff generation in a tropical dry forest; yet a number of questions remain to be answered with respect to annual variations in runoff response and the specific source area contributions to runoff in this catchment. The following suggestions will continue to improve our understanding of the hydrology of tropical dry forests:
1. Conduct similar investigations over a multi-year time period. Chapters three and four examined the threshold response and water flow pathways in a year in which precipitation was 24% below the annual average. This leads to the questions regarding the impact of interannual variability on the runoff response, specifically in a year with above average rainfall. Characterising the threshold runoff response over a greater range of rainfall
frequencies, intensities and magnitudes will improve the threshold response derived from the piecewise regression analysis, thereby improving our ability to assess how climatic shifts will affect the runoff response in this catchment (Ali et al., 2013).
2. Explore the effectiveness of the soil water storage – rainfall threshold approach at other tropical dry forest site. The technique used in chapter three provides a simple and easily repeatable approach. However, unless it is tested at other dry forest sites, the approach cannot be recommended for catchment wide adoption.
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3. Extend the soil moisture measurement locations for threshold analysis to include near-stream areas. Although most work in steep catchments with incised stream channels and narrow riparian zones suggest that contributions from near-stream areas are very low. However, without direct measurement, the possibility of contributions to streamflow activation cannot be excluded.
4. Map the occurrence and extent of hillslope groundwater across the catchment, using network of wells and pressure transducers. Multi-level well should be installed to map vertical flow gradients to provide direct evidence of flow pathways during storm events.
5. Explore the direct contributions of subsurface water at depths below one metre to stormflow using isotopic and geochemical tracers. Such exploration would confirm if the high pre-event water contributions and strong baseflow geochemical signature recorded during stormflow (chapter four) originated from deep subsurface soil layers or transient groundwater. This would further our process based understanding of runoff generation processes in tropical dry forests.
6. Application of more appropriate isotopic tracers to improve the estimate of the mean stream water residence time. The use of stable isotope tracers (δ2H and δ18
O) while suitable have been shown to underestimate the age of baseflow in residence time analyses. Tritium based characterisation of the baseflow reveals larger proportions of older water and would like improve the estimation of residence time in this catchment (Stewart et al., 2010; 2012). 7. Examine the partitioning of water among the major tree species and dominant forest types
across the catchment. Canopy interception, transpiration and the rooting depth have strong impact on the partitioning of incoming rainfall and the pool of stored soil water. While these processes have been shown to strongly impact the amount of water available for runoff in wet tropical forests (Bonell and Bruijnzeel, 2005) other work has suggested that two pools of water exists, a mobile pool for runoff and a less mobile one for transpiration (Goldsmith et al., 2011). Investigating these processes in tropical dry forests will improve our
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5.4 References
Ali, G, Oswald, CJ, Spence, C, Cammeraat, ELH, McGuire, KJ, Meixner, T, Reaney, SM. 2013. Towards a unified threshold-based hydrological theory: necessary components and recurring challenges. Hydrological Processes 27: 313-318, doi: 10.1002/hyp.9560. Bonell, M, Bruijnzeel, LA. 2005. Forests, water and people in the humid tropics: past, present
and future hydrological research for integrated land and water management. Cambridge University Press.
Dunne, T. 1983. Relation of field studies and modeling in the prediction of storm runoff. Journal of Hydrology 65 (1-3): 25-48.
Goldsmith, GR, Muñoz-Villers, LE, Holwerda, F, McDonnell, JJ, Asbjornsen, H, Dawson, TE. 2011. Stable isotopes reveal linkages among ecohydrological processes in a seasonally dry tropical montane cloud forest. Ecohydrology 5: 779-790. doi: 10.1002/eco.268. Karmalkar, AV, Bradley, RS, Diaz, HF. 2011. Climate change in Central America and Mexico:
regional climate model validation and climate change projections. Climate Dynamics 37: 605-629. doi: 10.1007/s00382-011-1099-9.
Stewart, MK, Morgenstern, U, McDonnell, JJ. 2010. Truncation of stream residence time: how the use of stable isotopes has skewed our concept of streamwater age and origin. Hydrological Process 24: 1646-1659. doi: 10.1002/hyp.7576.
Stewart, MK, Morgenstern, U, McDonnell JJ, Pfister, L. 2012. The „hidden streamflow‟ challenge in catchment hydrology: a call to action for stream water transit time analysis. Hydrological Processes 26: 2061-2066. doi: 10.1002/hyp.9262.
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Appendices
Appendix 1. Change in the soil surface hydraulic conductivity (mm/h) at variable pore pressures at four locations at a) the deciduous forest and b) pine forest.
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Appendix 2. Example of the graphical hydrograph separation using the local minimum method. The hydrographs are separated by the dashed line, connecting the local minima. The line separates the quickflow (black area) from the baseflow (grey area).
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Appendix 3. Copyright release agreement for Chapter 1 “Left high and dry: a call to action for increased hydrological research in tropical dry forests”.
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Appendix 4. Copyright release agreement for Chapter 2 “Infiltration and soil water dynamics in a tropical dry forest: it may be dry but definitely not arid”
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Appendix 5. Copyright release agreement for Chapter 3 “Soil water storage, rainfall and runoff relationships in a tropical dry forest catchment”.
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Curriculum Vitae
Name: Kegan Farrick
Post-secondary University of the West Indies
Education and St. Augustine, Trinidad and Tobago
Degrees: 2002-2005 B.Sc.
The University of Waterloo Waterloo, Ontario, Canada 2006-2008 M.Sc.
The University of Toronto Mississauga, Ontario, Canada. 2009-2010 Ph.D.
The University of Western Ontario London, Ontario, Canada
2010-2014 Ph.D.
Honours and Don Gray Award: Best Student Paper in Hydrology
Awards: Canadian Geophysical Union, 2014
CGU Travel Award
Canadian Geophysical Union, 2013
Second runner up, Best talk in sciences Western Research Forum, 2012
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Arcangelo Rea Family Foundation Award in Environmental Research Department of Earth Sciences, 2011
Don Gray Scholarship in Canadian Hydrology Canadian Geophysical Union, 2010
Related Work Teaching Assistant
Experience The University of Western Ontario
2010-2014
Research Assistant University of Toronto 2009-2010
Research Professional
University of the West Indies 2009
Environmental trainee
Ministry of Planning and Environment 2005
Publications:
Farrick, Kegan K. and Branfireu, Brian A. (2014) Soil water storage, rainfall and runoff relationships in a tropical dry forest catchment. Water Resources Research, 50, doi: 10.1002/2014WR016045.
Farrick, Kegan K. and Branfireun, Brian A. (2014). Infiltration and soil water dynamics in a tropical dry forest: it may be dry but definitely not arid. Hydrological Processes, 28, 4377-4387 Farrick, Kegan K. and Branfireun, Brian A. (2013). Left high and dry: a call to action for increased hydrological research in tropical dry forests. Hydrological Processes, 27, 3254-3262
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Farrick, Kegan K. and Price, Jonathan S. (2009). Ericaceous shrubs on abandoned block-cut peatlands: Implications for soil water availability and Sphagnum restoration. Ecohydrology, 2, 530-540.