Hydrology and the River Environment PDF

# Hydrology and the River Environment ## 5. Artificial Influences on Runoff Volume and Timing ### 5.1 History of Concepts and Experimentation - Chapter 1 briefly examines the history of hydrology as a science while Section 2.3 adds the use of catchment experiments as a device for accruing data o...

# Hydrology and the River Environment ## 5. Artificial Influences on Runoff Volume and Timing ### 5.1 History of Concepts and Experimentation - Chapter 1 briefly examines the history of hydrology as a science while Section 2.3 adds the use of catchment experiments as a device for accruing data on components of the hydrological cycle. - Hydrologists have recognized the human impacts on the hydrological cycle processes and patterns of rainfall, runoff, and storage in the hydrological cycle. - Early influences on water resources are evident from the use of riparian rights and by-laws stipulating good practice which date in the UK to the medieval period. - There has been a challenge in understanding the relationship between land use and surface water and also between surface land-use activities and the quantity and quality of groundwater. - Table 2.3 lists some of the important catchment studies which have developed our concepts of the hydrological cycle, and also revealed the influence of land use (notably forest cover) on runoff. - Table 5.1 details the progress of policies on the management of the relationship between land use and runoff. - An early phase in the perception of these relationships was the use of policies on afforestation to address environmental extremes such as floods and droughts, together with river instability. - Later attention turned to water resources and in the last two decades to water pollution from both point sources and diffuse sources. - Lastly, considerations have extended to freshwater habitats in downstream locations in relation to upstream land use and management. ### 5.2 A Return to Processes: Points and Extent of Intervention - The hydrological effects of direct intervention in surface water systems are easy to imagine, though research has revealed many complexities and sensitivities which have influenced our perception of the spatial and temporal extent of impacts. - We therefore give greater highlight here to the often hidden processes of runoff generation considered for natural slopes and channels in Chapter 3. - A useful way to catalogue the potential for human impact is to superimpose our knowledge of land-use and management effects on a basic-process diagram from Chapter 3 (see Fig. 5.1). - Table 5.2 fills out the range of potential impacts and the reader is referred to the many compilation volumes of research results from individual land-use and land-management studies. ### 5.3 Fundamental Effects: Volume and Timing - The influence of mankind's use of land and exploitation of other resources on runoff is still debated both between research hydrologists and, where technical consensus is reached, between scientists and politicians. - Fig. 5.1 is a simple “tank” model of the runoff cascade extended to allow for a dynamic contributing area and labelled by relationship of components to artificial modifications (land-use and water development). ### 5.4 Forests and Water - It was the perception that loss of natural forest cover had a profound and deleterious effect on rivers which paved the way for an expansion of scientific hydrology. - The need to retain forest covers to modulate flows, moderate extremes, and reduce erosion is now a common perception. - The impact of forestry on the hydrological cycle is not restricted to natural forest covers and their loss during settlement/development. - Research hydrologists have directed attention to the replacement forest plantations of the long-settled humid (especially mountain) zones and their management cycle. ### 5.4.1 Elements of Forest Land Use - Plantation forests are often the first managed crop for a long period on land hitherto labelled as surplus to development. - The activities shown in Table 5.4 can clearly be adapted to minimize environmental impacts; to this end codes of practice have been published in several parts of the world - Table 5.4. includes: - Ground Preparation - Planting to canopy closure - Mature Crop - Harvesting - These activities encourage foresters to consider the inevitable off-site impacts of land-surface modifications because simple educational approaches are appropriate since the “message” needs to cross professional disciplinary boundaries and be interpreted in the driver’s cab of a tractor as well as in the board room. - Fig. 5.4 (a) and (b) summarizes some of the management guidance available to UK and Canadian foresters respectively. ### 5.4.2 Regional Effects - Newson and Calder (1989) label forest hydrology a “regional science.” - One cannot extrapolate the water use by a conifer forest in continental tundra to aid prediction of the effects of groves of eucalyptus in semi-arid India. - Table 5.5 draws process-controlled contrasts between regions in the following aspects: - Climate - Trees - Soil/Geology - Management - The spatial scale of forests has an effect on their hydrological impact (edge effects often magnify the impact of evaporative processes). - Before this regional variability was formally recognized by forest hydrologists and incorporated in research design and techniques of extrapolation it had been considered impossible to reach scientific consensus, particularly on the impacts on loss rates by interception and transpiration. - Controversy still rages over the effects of forests on flood and drought extremes, sediment yield, pollution, and habitat viability. - Even at canopy level it is vital to separate transpiration and interception as processes, together with their controls. ### 5.4.3 Water Use by Forests - Whilst water use applies exclusively to transpiration this process was confused and conflated with interception for so long that we now often apply it to total water losses from a given vegetation canopy. - Newson and Calder (1989) separate, within the tropical zone, the humid and seasonally dry climates of, respectively, Java and southern India. - In the humid tropics it has been thought possible to estimate that all net radiation is converted to latent heat (as water vapour). - In southern India and, from analysis of the literature, in similar climates with seasonal moisture stress in the soil, some species of eucalyptus control their water use according to soil conditions and others according to atmospheric conditions; the latter can effectively act as “water pumps” where the roots are continuously supplied. ### 5.4.4 Riparian/Phreatophyte Effects - Riparian/phreatophyte forest effects have been studied closely in western USA where “phreatophyte vegetation ‘stands with its roots in water’ for most of its life and can therefore almost ‘pump’ water by transpiration. - Graf (1985) tables phreatophyte transpiration rates. ### 5.4.5 Predicting Forest Influences on Runoff Volume - We have a scientific paradox in land-use hydrology: the need for prediction, but the regional uniqueness of much of the measurement and modelling activity in the results. - The simplest process to measure (see Chapter 2) but the resulting predictions such as the Calder-Newson equation below are limited in their application to those conditions. - Calder and Newson (1979) put forward the simple prediction for total loss from a forested area. - Bosch and Hewlett (1982) make an ambitious compilation of the results of many catchment experiments worldwide to plot the forest effect, largely from felling operations which increase streamflow yields. - Fig. 5.5 (b) shows how runoff increases slightly more for conifers and eucalypts. ### 5.5 Agriculture and Hydrology - Plantation forestry may be considered as trees grown by agricultural principles and whilst, strictly speaking, agriculture deals with shorter crops. - Agriculture and forestry are often allocated to different climatic or soil zones; - Agriculture has a shorter crop cycle with a relatively minor impact in establishment and harvesting, compared with those of forestry. - Agriculture (and especially horticulture) makes considerable use of irrigation. ### 5.5.1 Cultivation Effects - Fig. 5.2 suggests cultivation increases roughness but also increases depression storage and infiltration capacity. - The complicating factors are so numerous that controversy has long surrounded the agricultural (and forestry) drainage impact, especially where the impact of increased or reduced floods (or droughts!) is predicted for sites far downstream from the drained area. ### 5.5.2 Drainage Effects - It is possible that drainage studies rival forest hydrology in qualifying as a “regional science.” - Fortunately Robinson (1990) has recently gathered many of these apparently conflicting studies between two covers and Table 5.7 shows the impact of pipe underdrainage schemes on the parameters of the unit hydrograph at the field scale. ### 5.5.3 Canopy Effects - Agricultural-crop canopies are shorter and often less dense than those of either natural or plantation forests. - Processes on the canopy do not differ; interception and transpiration both contribute to crop water use-typical values of crop-interception coefficients are shown in Table 5.8. - under present conditions of relatively abundant water supplies (either natural or enhanced by reservoir supply schemes) crop water use is dominated by transpiration under irrigation-water deployment but, in future, canopy processes in crops need to be understood in order for spray irrigation to be applied with less waste by interception. ### 5.6 Urbanization - One of the salient phases in the development process in the modern world is the wholesale movement of people from rural homes and livelihoods to urban centers. - The corollaries of urban growth were, and continue to be: Hazardous conditions in respect of water supply for both health and fire within the densely settled parts of cities (especially the poorer parts) and problems of waste disposal and a surplus of drainage water gathering from impervious surfaces. - Often the waste and drainage problems are cured first with an obvious disposal route in most sites being a joint one (with the drainage water propelling the waste through buried pipes) to the nearest watercourse. - The completed urban development can amount to a very extensive and a very profound modification to natural climatological and hydrological processes. ### 5.6.1 Elements of Urban Surfaces - The section through a compressed slice of urban and suburban surfaces shown in Fig. 5.8 (a) indicates that there is not one surface but many. - Fig. 5.10 b and c illustrates changes in the unit hydrograph and in flow frequencies in the Canon’s Brook, Essex, UK during three decades of urbanization. - The black box techniques for predicting the effects of urbanization on runoff, especially on flood peaks, tended to use a simple rational formula linking the flood discharge to catchment area. ### 5.6.2 Urban Drainage - There are two functions of urban drainage: storm drainage which removes surplus rainfall (otherwise constituting a flood hazard and dangerous transport conditions) and sewerage, the provision of routes for waterborne wastes created by washing and toilet facilities. - Fig. 5.9 shows commercially available techniques of drainage to serve new urban developments in the UK. ### 5.6.3 Experimental Evidence at Various Scales - Urban hydrology is not an easy or pleasant occupation: vandalism besets instrument systems, there are dangers from traffic and property owners, and the quality of water one measures is often equally, if not more dangerous. - Fig. 5.8 shows commercially available techniques of drainage to serve new urban developments in the UK. ### 5.6.4 Effects on Urban River Channels - In many urban areas the former natural-surface channels have been culverted underground over long distances or so completely constrained between concrete banks and beds that they are effectively open drains. - Those open non-rigid channels which remain in or close below urban areas are in receipt of a changed inflow regime in terms of flow, sediments, and chemistry. ### 5.7 Land-Use Hydrology: What Next? - We have learned so far that the investigation of land-use effects on the hydrological cycle is a major stimulus to research. - Is there sufficient consensus and perception for river-management institutions, both national and international, to seek control over land-use change and land-management practice? ### 5.7.1 International Practice - We have already referred to the complex path by which scientific knowledge (especially uncertain knowledge) becomes incorporated in institutional practice. - The authors go on: Thereby the hydrologic cycle and associated ecosystems were disturbed in the search for agricultural and industrial production and urban amenities. Determining the full nature of those transformations is handicapped by lack of statistics as to the human activities and waterflows, and also by unprecise understanding of how natural systems have been affected. - The environmentally sound management of water resources projects should be implemented by an active interdisciplinary and intersectoral learning process. The interaction between water projects and their host river basins should be studied in depth. ### 5.7.2 River Basin Land-use Data - Given the hindsight of 10,000 years or more in their review of Holocene river dynamics in the UK, Macklin and Lewin (forthcoming) conclude that changes wrought in the valley/channel system over such time-scales are “climatically-driven but culturally blurred,” in other words the effects of land use take second place to those of climate. - Given, therefore, that international practice is increasingly validating the relationship between catchment activity and streamflow/ecosystem response it is essential to consider data-collection systems which allow relevant extension of hydrological information into land-use information. - Such extensions can be made by using: - GIS (geographical information systems) to store and manipulate the two types of information pooled from, existing sources. - Satellite surveys to update land-use change in real time, including seasonal changes of, say, cultivation. - The development of interventionist agricultural policies including production quotas and buffer zones will provide information akin to that held by planning authorities on urban land-use patterns. ### 5.8 Effects Deriving from Water Management - Land-use effects on the basin hydrological cycle are indirect on the channel component, by comparison, that is, with the profound manipulations of flow brought about by impoundment_and_transfers, channelization and regulation. ### 5.8.1 Water Abstraction and Use - We have already outlined the relatively small proportion of global surface, fresh water which is utilized—approximately 10 per cent or 3,600 km³ per year. - Locally, however, and in the context of some large and internationally important rivers, water abstraction and consumptive use threatens both downstream supply and the river ecosystem. - Fig. 5.11 (a) illustrates the situation in the Colorado River, a system with other management problems (see Sections 4.5.4 and 5.4.4). ### 5.8.2 Irrigation - Section 4.5.4 charted the spread of irrigated agriculture and some of its hydrological pitfalls both within the irrigated area and downstream. - Irrigation makes very heavy demands on water resources; - Perhaps the most widely publicized hydrological impact of irrigation development in recent years has been in central southern Asia where Soviet plans to irrigate up to 9 million ha. have virtually destroyed the Aral Sea, whose level fell 8 m in twenty years (Fig. 5.11 (b)). ### 5.8.3 Land Drainage/Flood Protection and “Channelization” - The natural river channel is the outcome of water flows and sediment loads of a certain magnitude and frequency (see Section 3.6.2). - Options include: - Widening - Deepening - Straightening - Embanking - Bank stabilization - Clearing and snagging - Dredging of silt - Clearing trash from urban areas ### 5.8.4 Reservoir Operation: Direct Regulation of River Flow - Whole books are now available on this subject; the number of dams across world rivers continues to grow despite popular protest and scientific concern over impacts downstream. - A large number of research projects have been carried out on the effects of river impoundment on flows, channels, pollution, and biota; the experimental context is attractive, with good data available, a measurable change from natural conditions, and the rare element of flows controlled at the touch of a switch (at least in the reaches nearest to dams). - The review by Petts (1984) is very comprehensive, but more recently the challenge has been taken up to agglomerate small-scale studies to national impact assessments (Petts, 1989) and to extend into the influences of impoundments on the world’s larger rivers (e.g. Van de Ven et al., 1991). - Petts provides basic conceptual guidance on the impacts of impoundments, presented here as Fig. 5.14. - Reservoirs have a limited impact on flows in the Mekong; nevertheless, with a catchment area of 32675 km² feeding existing tributary dams the effect on mean annual runoff and mean annual minimum flows can be discerned at a catchment area of 545000 km². - In Europe the river receiving the most attention on river regulation (as a result of political differences between the twelve nations included in its vast catchment) is the Danube. - Box 5.1 indicates the complications which these manipulations introduce to the use of gauging-station flow records. ### 5.9 References - Acreman, M. C. (1985) “Changes in water runoff in UK uplands” In: The hydrology of upland catchments. (Ed. P. G. Whitehead). pp. 613-637. Chichester (UK): John Wiley & Sons. - Alvares, E. F. (1988). “Social constraints to environmental management” In: The Role of Water Resources in Development. (Ed. D. W. Chapman). pp. 143-147. Proceedings of the Conference held at the University of New England, Armidale, Australia. - Arnell, N. W. (1989) “The influence of land use on runoff” In: Land use and water quality. (Ed. M. J. Perrow & A. J. B. Zehnder). pp. 1-55. Chichester (UK): John Wiley & Sons. - Atkinson, B. W. (1979) “Precipitation modification in the urban environment” In: Urban hydrology. (Ed. I. D. C. Crapper). pp. 3-18. Chichester (UK): John Wiley & Sons. - Billorey, K. A. (1990) “Hydropower in India” In: Major watershed management projects of the world: An international evaluation of their performance. (Ed. J. D. Kalma, J. G. Arnold, D. A. Gilmour & W. J. Young). pp. 575-586. Proceedings of the International Conference held in 1989 in Canberra, Australia. - Boardman, J. (1988) “Soil erosion” In: Water quality in the UK. (Ed. P. G. Whitehead). pp. 135-160. Chichester (UK): John Wiley & Sons. - Bosch, J. M. & Hewlett, J. D. (1982) “A review of catchment studies related to the effect of forest clearing on water yield” In: Forest Hydrology and Watershed Management. (Ed. T. G. Burt & R. P. Hooper). pp. 203-208. Proceedings of the International Symposium held in 1982 at the Oregon State University, Corvallis, Oregon, USA. - Boucher, D. (1990). “The Danube: River of empires.” Geographical Magazine, 62 (10), 20-29. London: Geographical Magazine - Brookes, A. (1988) “The regulation of stream flow.” In: Water quality in the UK. (Ed. P. G. Whitehead). pp. 161-180. Chichester (UK): John Wiley & Sons. - Bruenig, E. F. (1986) “The effects of forest management on the water cycle” In: Effects of forest management on hydrological processes. (Ed. E. F. Bruenig). pp. 1-10. Proceedings of the International Symposium held in 1986 at the University of Hamburg, Hamburg, Germany. - Calder, I. R. (1990) “Interception loss of rainfall” In: Modelling the water balance of forest ecosystems. (Ed. J. D. Kalma & M. J. Stern). pp. 1-13. Proceedings of the Workshop held in 1989 at the Monash University, Clayton, Victoria, Australia. - Calder, I. R. & Newson, M. D. (1979) “The effects of Scots pine and Sitka spruce plantations on the rainfall interception of a small upland catchment.” Journal of Hydrology, 40, 299-313. - Calder, I. R., Newson, M. D. & Gash, J. H. C. (1986) “A study of the influence of tropical rain forest on rainfall interception and stemflow.” Journal of Hydrology, 87, 223-241 - Courtney, R. (1978) “Interception by Sitka spruce-a new method for calculating interception loss.” Journal of Hydrology, 37, 231-238. - Cummings, W. (1990) “The environmental impact of hydropower development in Brazil” In: Major watershed management projects of the world: An international evaluation of their performance. (Ed. J. D. Kalma, J. G. Arnold, D. A. Gilmour & W. J. Young). pp. 569-574. Proceedings of the International Conference held in 1989 in Canberra, Australia. - DOE (1982) The Register of Gauging Stations, Volumes 1, 2 and 3. London: Department of the Environment, UK - Ellis, J. B. (1979) “Urban runoff” In: Urban Hydrology (Ed. I. D. C. Crapper). pp. 19-30. Chichester (UK): John Wiley & Sons. - Essery, R. L. H & Wilcock, P. (1990) “Hydrological effects of drainage system modifications” In: Channel change. (Ed. A. D. Abrahams). pp. 55-85. Chichester (UK): John Wiley & Sons. - Flader, S. L. (1987) The Great Lakes Forest: Aldo Leopold and the ecological conscience. Madison, Wisconsin: University of Wisconsin Press. - Ford, E. D & Deans, J. (1978) “The interception of rainfall by Scots pine and Sitka spruce in upland Britain.” Journal of Hydrology, 40, 1-16. - Gash, J. H. C., Wright, I. R. & Lloyd, C. R. (1980) “A comparison of interception loss from Sitka spruce stands in upland Britain.” Journal of Hydrology, 46, 145-163. - Gash, J. H. C., Stewart, J. B. & Calder, I. R. (1982) “Interception loss of rainfall by Scots pine in upland Britain.” Journal of Hydrology, 57, 377-390. - George, R. J. (1990) “Deep root characteristics and forest hydrology.” In: Modelling the water balance of forest ecosystems. (Ed. J. D. Kalma & M. J. Stern). pp. 14-24. Proceedings of the Workshop held in 1989 at the Monash University, Clayton, Victoria, Australia. - Gore, J. A. (1985) “Channel restoration” In: The Role of Water Resources in Development. (Ed. D. W. Chapman). pp. 219-224. Proceedings of the Conference held at the University of New England, Armidale, Australia. - Graf, W. L. (1985) “The effects of phreatophytes on groundwater resources.” In: Proceedings of the Conference on Western Water: Alternatives for the Future. pp. 1339-1356. Tucson, Arizona: University of Arizona. - Greenwood, R. G., George, R. J. & Turner, J. (1985) “Water use by eucalypts in a Mediterranean climate.” Australian Journal of Botany, 33, 243-252. - Gregoire, J. M. (1991) “Remote sensing and GIS: Tools to plan forest management and the sustainable use of forest resources.” In: Towards sustainable forest management. (Eds. P. J. Kuusela, D. A. Dymond & S. A. A. Kellomäki). pp. 101-108. Proceedings of a symposium organized by the IUFRO Working Party S.5.02 “Remote sensing and GIS in Relation to Forest Management” held in 1990 at the University of Helsinki, Helsinki, Finland. - Higgs, G. (1987) “The effects of land drainage schemes on flood regimes.” Journal of Hydrology, 94, 349-364. - Higgins, G., El-Sawah, S. & O’Donnell, T. (1988) “A study of the impact of the Aswan High Dam and irrigation project on the Egyptian water balance with forecasts of water availability under changing demands.” Journal of Hydrology, 105, 229-250. - Hollis, G. E. (1974) “The effect of urban development on flood discharge.” Journal of Hydrology, 21, 65-81. - Hollis, G. E. (1975) “Urban storm water management.” In: Urban Hydrology. (Ed. I. D. C. Crapper). pp. 167-194. Chichester (UK): John Wiley & Sons. - Hollis, G. E. (1979) “Urban hydrology.” Hydrological Sciences Bulletin, 24, 361-381. - Hollis, G. E. & Ovenden, L. (1988a) “Infiltration rates under urban pavement.” Hydrological Sciences Bulletin, 33, 285-290. - Hollis, G. E. & Ovenden, L. (1988b) “Infiltration rates under various urban covers.” Hydrological Sciences Bulletin, 33, 419-424. - Kirby, C. S., Whitehead, P. G. & Lewis, R. G. (1991) “A study of interception on a coniferous plantation in North Wales.” Journal of Hydrology, 123, 1-23. - Knight, D. W. (1979) “Stream morphology and urban development.” In: Urban Hydrology. (Ed. I. D. C. Crapper). pp. 159-166. Chichester (UK): John Wiley & Sons. - Landsberg, H. E. (1970) The Urban Climate. New York: Academic Press. - Law, F. M. (1956) “The interception of rainfall by a forest canopy.” Forestry, 29, 113-120. - Lenton, R. L. (1985) “Water Resources Development.” In: Encyclopedia of the Earth’s Surface. (Eds. D. S. Gorsline & R. J. Huggett). pp. 1002-1010. Chichester (UK): John Wiley & Sons. - Leopold, L. B. (1991). “A view of the river landscape.” In: River conservation and management. (Eds. P. Calow & G. E. Petts). pp. 3-10. London: Blackwell Scientific Publications. - L’vovitch, S. & White, G. F. (1991) “The global water balance.” In: River conservation and management. (Eds. P. Calow & G. E. Petts). pp. 225-239. London: Blackwell Scientific Publications. - Macklin, M. G. & Lewin, J. (forthcoming) “Late Holocene alluvial dynamics in the UK, and the contribution of cultural activity.” In: The Physical Geography of Britain and Ireland. (Ed. J. A. Gordon). London: Edward Arnold. - Maitland, P. S., Hildrew, A. G., & Ormerod, S. J. (1990) “The effects of forestry on the freshwater environment.” In: Ecology and management of freshwater systems. (Ed. P S. Maitland). Oxford: Blackwell Scientific Publications. - Marsh, G. P. (1864) Man and Nature. New York: Charles Scribner & Co. - Moffat, A. J. (1989) “How do forests influence streamflow?” In: The role of forests in watershed management. (Eds. T. B. Mowrer & J. M. Hammond). Proceedings of the Conference held in 1989 at the Oregon State University, Corvallis, Oregon, USA. - Newson, M. D. (1992) “Freshwater science and river basin management.” Geographical Journal, 158, 1-10 - Newson, M. D. & Calder, I. R. (1989) Forest hydrology: processes and management. London: Chapman and Hall. - Newson, M. D. & Robinson, J. (1983) “Rainfall and runoff in two upland catchments with different drainage densities: the effect of drainage on streamflow response.” Journal of Hydrology, 63, 19-34. - Nunnally, N. R. & Keller, E. A. (1979) “Stream channel adjustments to urbanization.” In: Urban Hydrology. (Ed. I. D. C. Crapper). pp. 147-158. Chichester (UK): John Wiley & Sons. - OECD (1989) Environmental policies for integrated water management. Paris: OECD. - Oyebande, L. (1988) “Estimating actual evapotranspiration from various agricultural crops.” Agricultural Water Management, 15, 129-142. - Packman, J. C. (1979) “Estimation of flood frequency for small urban catchments” In: Urban Hydrology. (Ed. I. D. C. Crapper). pp. 127-138. Chichester (UK): John Wiley & Sons. - Park, C. C. (1977) “The impact of urbanization on stream channels.” In: Sedimentation in River Systems. (Eds. R. D. Hey & J. C. Bathurst). pp. 157-170. Proceedings of the International Conference held at the University College of Swansea, Swansea, UK. - Penman, H. L. (1963) Vegetation and Hydrology. Commonwealth Bureau of Soils Technical Communication no. 53. Harpenden. UK. - Petts, G. E. (1984) Impounded Rivers: Perspectives for ecological management. Chichester (UK): John Wiley & Sons. - Petts, G. E. (1989) “The effects of water resource development on the physical characteristics of rivers.” In: River conservation and management. (Eds. P. Calow & G. E. Petts). pp. 241-258. London: Blackwell Scientific Publications. - Piper, D. J. W., Heathwaite, A. L. & Burt, T. P. (1991) “River flow regulation: the influence of headwater impoundments on the downstream hydrological regime.” Earth Surface Processes and Landforms, 16, 113-138. - Ponce, V. M. (1986) “Forest management effects” In: Effects of forest management on hydrological processes. (Ed. E. F. Bruenig). pp. 31-39. Proceedings of the International Symposium held in 1986 at the University of Hamburg, Hamburg, Germany. - Reid, I. (1979) “The effect of agricultural practices on hydrological processes.” Hydrological Sciences Bulletin, 24, 383-397. - Reid, I. & Parkinson, J. A. (1984) “A field investigation of the effects of agricultural drains on water flow and sediment flux.” Journal of Hydrology, 67, 171-89. - Reynolds, R. (1990) “The role of agriculture in the hydrological cycle of developing countries” In: River conservation and management. (Eds. P. Calow & G. E. Petts). pp. 283-299. London: Blackwell Scientific Publications. - Roberts, J. (1983) “The influence of forest plantations on the hydrological cycle in Britain” In: Hydrology of British upland catchments. (Ed. P. G. Whitehead) pp. 95-133. Chichester (UK): John Wiley & Sons. - Roberts, J. (1989) “River channel changes and their implications for the management of urban drainage” In: The hydrology of urban areas. (Ed. P. G. Whitehead) pp. 193-208. Chichester (UK): John Wiley & Sons. - Robinson, J. (1986) “The influence of drainage on the hydrological regime of a small upland catchment.” Journal of Hydrology, 83, 25-43. - Robinson, J. (1990) “The hydrological impact of agricultural drainage.” In: River conservation and management. (Eds. P. Calow & G. E. Petts). pp. 261-282. London: Blackwell Scientific Publications. - Robinson, J. & Beven, K. J. (1983) “The hydrological effects of field drainage measured at different spatial scales.” In: Hydrological processes. (Ed. T. O’Donnell) pp. 117-136. Chichester (UK): John Wiley & Sons. - Robinson, J. & Newson, M. D. (1986) “A comparison of the hydrological behaviour of six upland catchments with differing drainage characteristics.” Journal of Hydrology, 86, 305-327. - Ross, N. L. (1990). “The effect of afforestation on the yield and sediment transport.” In: The Hydrology of Upland Catchments. (Ed. P. G. Whitehead). pp. 591-611. Chichester (UK): John Wiley & Sons. - Schwarz, G., Kates, R. W. & Goldewijk, K. (1991) “The Nile River: a shared resource, different perspectives.” In River conservation and management. (Eds. P. Calow & G. E. Petts). pp. 341-360. London: Blackwell Scientific Publications. - Shuttleworth, W. J. (1991) “Evaporation.” In: River conservation and management. (Eds. P. Calow & G. E. Petts). pp. 169-186. London: Blackwell Scientific Publications. - Solbe, J. F. (1986) “The effect of agricultural land use on the hydrological cycle in Britain.” In: Effects of forest management on hydrological processes. (Ed. E. F. Bruenig). pp. 21-29. Proceedings of the International Symposium held in 1986 at the University of Hamburg, Hamburg, Germany. - Soutar, C. A. (1989) “Water resources, forests and the environment: a global perspective.” Environmental Management, 13 (6), 667-682. - Speirs, D. C. & Frost, K. T. (1985) “Runoff generation on cultivated soils under different tillage regimes.” In: Proceedings of the conference on Surface Hydrology. (Eds. V. P. Singh & J. D. Kalma). pp. 404-412. New Delhi: Allied Publishers. - Szolgay, J. (1991) “Hydrological modelling of the Danube River in Hungary.” In: River conservation and management. (Eds. P. Calow & G. E. Petts). pp. 361-373. London: Blackwell Scientific Publications. - Swanson, F. J. (1973) “Forest Hydrology: A Review of the Current Status.” In: Proceedings of the International Symposium on Forest Hydrology. pp. 143-185. Oxford: Pergamon. - Thorn, C. E. (1966) “The river system and the hydrological environment.” In: British rivers. (Ed. I. S. McLaren). pp. 1-7. London: Macmillan. - UNESCO (1990) Water resource management. Paris: UNESCO. - UNEP/UNESCO (1990) “The precautionary approach in water resources management.” In: Major watershed management projects of the world: An international evaluation of their performance. (Ed. J. D. Kalma, J. G. Arnold,