The impact of broadleaved woodland on water resources in lowland UK: II. Evaporation estimates from sensible heat flux measurements over beech woodland and grass on chalk sites in Hampshire

摘要

The impact on recharge to the Chalk aquifer of substitution of broadleavedwoodland for pasture is a matter of concern in the UK. Hence, measurements ofenergy balance components were made above beech woodland and above pasture,both growing on shallow soils over chalk in Hampshire. Latent heat flux(evaporation) was calculated as the residual from these measurements ofenergy balances in which sensible heat flux was measured with an eddycorrelation instrument that determined fast response vertical wind speeds andassociated temperature changes. Assessment of wind turbulence statisticsconfirmed that the eddy correlation device performed satisfactorily in bothwet and dry conditions. There was excellent agreement between foresttranspiration measurements made by eddy correlation and stand level treetranspiration measured with sap flow devices. Over the period of themeasurements, from March 1999 to late summer 2000, changes in soil watercontent were small and grassland evaporation and transpiration estimated fromenergy balance-eddy flux measurements were in excellent agreement with Penmanestimates of potential evaporation. Over the 18-month measurement period, thecumulative difference between broadleaved woodland and grassland was smallbut evaporation from the grassland was 3% higher than that from thewoodland. In the springs of 1999 and 2000, evaporation from the grassland wasgreater than that from the woodland. However, following leaf emergence in thewoodland, the difference in cumulative evaporation diminished until thefollowing spring.