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A strategic programme for NERC Lowland catchment research
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Influence of woodland on recharge

Title

The influence of woodlands on recharge in the Pang catchment

Overview of project

Comparative measurements of recharge rates under woodlands and grasslands in southern England have produced inconclusive results so far. This has caused a controversial debate about the actual evaporative water loss from woodlands and the impact of afforestation and deforestation on the catchment water balance. The few reliable data for evaporation come mostly from extensive, homogeneous woodlands, which are suitable for micrometeorological evaporation measurements. However, such woodlands are not representative of the English lowlands.

Field instrumentation

In order to assess the impact of woodlands on the catchment water balance the LOCAR woodlands project investigated evapotranspiration rates for various woodland types that are more typical for the region and which have been overlooked in previous research. The key technique used to measure woodland transpiration was the sap-flux method, which is applied at the tree-scale and is therefore suitable for any kind of woodland. Complementary measurements of the rainfall interception loss, as an additional component of woodland evaporation, were also made at some of the sites.

Aims

Primary aims of this project were to:

  • measure evaporation from major components of the woody vegetation in a lowland catchment;
  • derive descriptive models of woodland evaporation; and
  • use models to estimate the impact of woodland vegetation, and changes in it, on catchment recharge.

Main findings

The annual transpiration totals for the heterogeneous woodlands agreed with earlier studies made in homogeneous broadleaved woodlands. This would confirm the observation made by Roberts (J. Hydrology 66 (1983), 133-141) that forest transpiration is a 'conservative hydrological process'. However, a different picture emerged for the fragmented woodlands. The transpiration of the hedgerows and woodland edges was on average 60% higher and in some parts of the wet woodlands it was even twice as high. The stomatal sensitivity to the air humidity was species-specific. It was lowest in willow-dominated riparian woodland and highest in oak-dominated heterogeneous woodland. For the first time an analytical model of rainfall interception was parameterised for hedgerows, where rain shadows and rainfall concentration play an important role. At all woodland sites, the interception loss was noticeable in the winter, as well as in the summer, because the meteorological conditions partially compensated for the absence of the leaves.

Description of activities

The transpiration of the woody vegetation at six different sites in and near the Pang catchment was measured by using of the sap-flux technique. At each site, the measurements were run continuously for at least one complete growing season. The tree canopies investigated were: hedgerows, woodland edges, heterogeneous woodlands and wet woodlands. At three of the sites the rainfall interception evaporation was measured as well and all sites were equipped with automatic weather stations. The application of the sap-flux technique required surveys of tree species and stem diameters at each site and also a re-calibration of the technique in the laboratory. This additional information was necessary to scale-up the field data to the stand scale. For all sites, the canopy transpiration was compared to the potential evaporation and, for most of the sites, used to derive the stomatal conductance through the Penman-Monteith equation. The seasonal water-use and the stomatal behaviour were quantified for the first time for these types of woodlands, which had been neglected in previous research. The rainfall interception was measured as the difference between gross rainfall and throughfall plus stemflow. Automatic tipping-bucket raingauges connected to large collectors were used in order to facilitate an event based analysis of the data. From this the Gash model of rainfall interception was parameterised for the hedgerow site and the oak dominated woodland.

Areas of application

  • Modelling the water use of forested areas
  • Predicting the impact of afforestation or deforestation on the catchment water balance
  • Managing water resources in a changing climate.

Related work

So far the publications from the project refer to the findings from specific sites. It is intended that the results in the these papers will be integrated. In doing so, this could enable scientists to compare the water use and the stomatal behaviour across all sites investigated in the project and to quantify the impact of changes in the area and the type of woodlands on the water balance of the catchment.

Researchers' details

Principal Investigator:

Co-investigators:

Publications

All publications from this and other LOCAR projects are listed in the publications database.

Refereed journal papers

Herbst M., Roberts J.M., Rosier P.T.W. & Gowing D.J. (2006) Measuring and modelling the rainfall interception loss by hedgerows in southern England. Agricultural and Forest Meteorology 141, 244-256.

Herbst M., Roberts J.M., Rosier P.T.W. & Gowing D.J. (2007) Seasonal and interannual variability of canopy transpiration of a hedgerow in southern England. Tree Physiology 27, 321-333.

Herbst M., Roberts J.M., Rosier P.T.W., Taylor M.E. & Gowing D.J. (2007) Edge effects and forest water use: A field study in a mixed deciduous woodland. Forest Ecology and Management 250, 176-186.

Herbst M., Rosier P.T.W., Morecroft M.D. & Gowing D.J. (2008) Comparative measurements of transpiration and canopy conductance in two mixed deciduous woodlands differing in structure and species composition. Tree Physiology, 28, 959-970.

Herbst M., Rosier P.T.W., McNeil, D.D., Harding, R.J. & Gowing D.J. (2008) Seasonal variability of interception evaporation from the canopy of a mixed deciduous forest. Agricultural and Forest Meteorology, in revision.

Conference proceedings

Herbst M., Roberts J.M. & Gowing D.J. (2005) Transpiration and evaporation from hedges in southern England. Annalen der Meteorologie 41, 22-25.

Herbst M., Roberts J.M. & Gowing D.J. (2007) Der Einfluß von Randeffekten auf die Verdunstung fragmentierter Waldbestände. Berichte des Meteorologischen Institutes der Universität Freiburg 16, 117-122.

PhD thesis

Callaghan, N.C. (2007) Water loss from wet woodlands of the English lowlands. PhD thesis, The Open University, Milton Keynes, UK, 295 pp.

Recommended reading

Herbst, M., C. Eschenbach and L. Kappen. 1999. Water use in neighbouring stands of beech (Fagus sylvatica L.) and black alder (Alnus glutinosa (L.) Gaertn.). Ann. Sci. For. 56:107-120.

Komatsu, H. 2005. Forest categorization according to dry-canopy-evaporation rates in the growing season: comparison of the Priestley-Taylor coefficient values from various observation sites. Hydrol. Proc. 19:3873-3896.

Oren, R., J.S. Sperry, G.G. Katul, D.E. Pataki, B.E. Ewers, N. Phillips and K.V.R. Schäfer. 1999. Survey and synthesis of intra- and interspecific variation in stomatal sensitivity to vapour pressure deficit. Plant Cell Environ. 22:1515-1526.

Roberts, J.M. 1983. Forest transpiration: a conservative hydrological process? J. Hydrol. 66:133-141.

Roberts, J.M. and P.T.W. Rosier. 2006. The effect of broadleaved woodland on Chalk groundwater resources. Q. J. Engineer. Geol. Hydrogeol. 39:197-207.

Shuttleworth, W.J. and I.R. Calder. 1979. Has the Priestley-Taylor equation any relevance to forest evaporation? J. Appl. Meteorol. 18:639-646.

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