Ian R. Calder

Impact of lowland forests in England on water resources: Application of the Hydrological Land Use Change (HYLUC) model

[1]xa0The U.K. Governments 1995 White Paper on Rural England [Her Majestys Stationery Office, 1995] proposed a doubling of the area of woodland within England by the year 2045. Questions were later raised concerning the possible impacts on water resources of such a large change in land use. This paper presents results of field study investigations of the water use of grass, heath, oak, and pine vegetation at Clipstone Forest, Nottinghamshire, United Kingdom, which were used to calibrate the water use model HYLUC and derive predictions of the impact of different vegetation types on recharge in this locality. Average annual recharge plus runoff (millimeters) over a 32.5 year period and the uncertainties due to spatial sampling, calculated with the HYLUC model, were 136 ± 11 for the grass site, 122 ± 3 for the heath site, 76 ± 5 for the oak site, and 34 ± 8 and 38 ± 3 for two pine sites. In this region of Britain the long-term recharge beneath pine is approximately one quarter that under grass and essentially only occurs in years of above average rainfall. Oak woodland is also predicted to have a significant impact by reducing recharge plus runoff by almost one half as compared with grassland.

Assessing the water use of short vegetation and forests: Development of the Hydrological Land Use Change (HYLUC) model

[1]xa0This paper describes the theoretical background and recent developments to the present version of the Hydrological Land Use Change (HYLUC) model which has been used to investigate the impacts of land use change, particularly changes relating to forest cover, on water resources in many countries of the world. The model makes use of the “limits” concept and the Penman equation to ensure parsimony in terms both of data requirements (daily rainfall, daily potential transpiration, and land cover data) and in terms of the parameters required to take account of the different evaporative responses of different vegetation types. The model is applicable both for research and land and water resource management purposes (where input data is generally very limited).

An evaluation of the impacts of energy tree plantations on water resources in the United Kingdom under present and future UKCIP02 climate scenarios

[1]xa0The Hydrological Land Use Change model was used to assess the range of water resource impacts associated with four potential energy tree species (Eucalyptus nitens, Eucalyptus gunnii, Nothofagus sp., and Fraxinus excelsior) at eight United Kingdom locations under present and future, Environment Agency Rainfall and Weather Impacts Generator, climate scenarios generated using UK Climate Impacts Programme 2002 (UKCIP02). Parameter values were derived using expert opinion and interpolation because of limited data. For Fraxinus excelsior, there are questions concerning the unusual, in a world context, published findings that evaporation from a tree crop is less than that from grass. Model predictions indicated that under the present climate all tree species, excepting Fraxinus excelsior, at all sites have greater mean annual evaporation, (8 to 84%) and reduced water yields (−6 to −97%) compared with grass. The predicted increase in tree evaporation arises from parameter values reflecting both increased rainfall interception and higher transpiration due to deeper rooting depths. Under future climate scenarios, (1) “potential annual yield” (difference between actual rainfall and potential evaporation) will decrease, becoming negative at all studied sites in England and Wales by 2080; (2) at drier sites and for species with highest evaporation rates, E. nitens and Nothofagus, evaporation rates will decrease; (3) at wetter sites and for all species, evaporation rates will increase; (4) at all sites and for all species, water yields will decrease; (5) differences between species remain the same, with evaporation rates increasing and water yield decreasing in the order Fraxinus excelsior, grass, E. gunnii, Nothofagus, and E. Nitens; and (6) there is an overall trend through time toward convergence in water yields from trees and grass. If higher water yield predictions for Fraxinus excelsior are proved correct, this would represent an attractive land use option for water and energy production. Field research is required to validate these predictions. Assuming future climate changes match those predicted, soil moisture deficits will occur for longer periods during the year and will become increasingly limiting for evaporation. The monitoring of soil moisture may then provide one of the most sensitive methods of both determining model parameter values and testing predictions of differences in evaporation between species and changes in evaporation over time.

Models, myths and maps: Development of the EXploratory Climate Land Assessment and Impact Management (EXCLAIM) tool

For many years maps have been used to convey concepts and ideas in an easily understandable form. The advent of computer based Geographic Information Systems (GIS) has allowed users to explore different scenarios using a variety of data. This paper describes the development of specialised GIS tools, via thin-client technology, to assist policy makers, donor organisations and Non-Governmental Organisations (NGOs) understand how the interactions between changing land use (particularly land uses involving forestry, irrigation and soil water conservation structures) and climate affect water resources and peoples livelihoods. The research brings together hydrological and socio-economic models with easy to use user interfaces. It is expected that the EXCLAIM tool will help to support the Bridging Research And Policy process and encourage the development of evidence based (rather than presently often myth based) land and water policies, especially in relation to watershed development projects.

Forests and floods—in support of an evidence-based approach to watershed and integrated flood management

In reply to the comment by Bronstert and Kundzewicz that “Even Calder & Aylward (2006) discuss several parallel issues, in addition to forests vs floods, yet they do not cover the important role of forests delaying snowmelt-caused floods, which are common in many areas” we would agree that this aspect was not covered in the paper but we would again caution against the implication of this comment that forests are necessarily always beneficial in mitigating snow melt floods. We understand from studies conducted in British Columbia, Canada, (Wei and Davidson, 1998 Wei, 2006, Personal communication) that temperature, as affected by altitude, is an important factor influencing snow-melt floods. In mountain watersheds, the snow-melt process usually begins at lower altitudes and gradually moves up to the higher altitudes. Melted water from the lower altitudes contributes to the lower and rising parts of hydrograph while the higher altitudes contribute more towards the peak flows. Forest harvesting at high altitudes can have the potential to increase floods through increased snow accumulation and faster snow-melting processes. But, by contrast, forest harvesting at lower altitudes may have the potential to reduce flood flows through flow de-synchronization. Here the snowmelt processes at low altitude clearcut sites would be expected to occur earlier and more quickly than from the uncut forest. These arguments would indicate against the claim that forests can always delay snowmelt-caused floods. Although it is likely to be true for forests at higher elevations, it may be misleading, or completely incorrect, to make this claim for forests at lower elevations in a mountain watershed.