The effect of broadleaved woodland on Chalk groundwater resources

The impact of broadleaved woodland on chalk groundwater resources, in Hampshire, UK, was assessed with a comparative study of beech woodland at Black Wood near Micheldever and grass at Bridgets Farm near Winchester. At both sites, parallel estimates of evaporation and detailed studies of the changes in soil water content and the direction of water movement were made. Beneath both sites, a reduction in soil water content was observed during the summer to a depth of 6 m. However, total water potential measurements showed that upward movement of water was only occurring in the upper 2 m of the profile, whilst drainage was occurring below this point. Drainage continued even when soil moisture deficits existed in the upper part of the profile. Seasonal differences between woodland and grass do exist, however annual differences in evaporation and drainage are insignificant. The similarity of evaporation from and drainage below the broadleaved woodland and grass sites contrasts markedly with several other studies that show greater drying below woodland. Unlike other comparisons, this study was made within a large block of woodland and both sites were on chalk. The relevance of woodland size and chalk substrate are discussed.

本研究针对英国汉普郡地区阔叶林地对白垩纪地下水资源的影响开展评估，以米切尔德弗附近布莱克林地的山毛榉林与温彻斯特附近布里奇兹农场的草地作为对照对象。在两处研究站点中，同步开展了蒸发量并行估算，并针对土壤含水量变化与水分运移方向进行了精细化研究。两处站点下方的土壤含水量均在夏季出现下降，下降深度可达6米。然而总水势测量结果显示，水分的向上运移仅发生在土壤剖面0~2米的上层区域，该深度以下则发生向下渗漏。即便剖面上层存在土壤水分亏缺，向下渗漏过程仍持续进行。阔叶林地与草地间确实存在季节性差异，但二者的年蒸发量与年渗漏量差异并不显著。阔叶林地的蒸发量及其下方的渗漏量，与草地站点的对应指标高度相似，这一结果与其他多项研究结论形成鲜明反差：后者均观测到林地下方土壤干燥程度更高。与其他对照研究不同，本研究的林地属于大型连片林块，且两处站点均位于白垩地层之上。本文最后讨论了林地规模与白垩基质的相关性。