The impact of broadleaved woodland on water resources in lowland UK: I. Soil water changes below beech woodland and grass on chalk sites in Hampshire

摘要

The possible effects of broadleaved woodland on recharge to the UK Chalkaquifer have led to a study of evaporation and transpiration from beechwoodland (Black Wood) and pasture (Bridgets Farm), growing in shallow soilsabove chalk in Hampshire. Eddy correlation measurements of energy balancecomponents above both the forest and the grassland enabled calculation oflatent heat flux (evaporation and transpiration) as a residual. Comparativemeasurements of soil water content and soil water potential in 9 m profilesunder both forest and grassland found changes in soil water content down to 6m at both sites; however, the soil water potential measurements showed upwardmovement of water only above a depth of about 2 m. Below this depth, watercontinued to drain and the soil water potential measurements showed downwardmovement of water at both sites, notwithstanding significant negative soilwater potentials in the chalk and soil above. Seasonal differences occur inthe soil water content profiles under broadleaved woodland and grass. Beforethe woodland foliage emerges, greater drying beneath the grassland is offsetin late spring and early summer by increased drying under the forest. Yet,when the change in soil water profiles is at a maximum, in late summer, theprofiles below woodland and grass are very similar. A comparison of soilwater balances for Black Wood and Bridgets Farm using changes in soil watercontents, local rainfall and evaporation measured by the energy balanceapproach allowed drainage to be calculated at each site. Although seasonaldifferences occurred, the difference in cumulative drainage below broadleavedwoodland and grass was small.