降水补给地下水过程中包气带变化对入渗的影响

Influence of unsaturated zone thickness on precipitation infiltration for recharge of groundwater

采用野外试验和室内试验方法,通过对不同降水量的入渗过程中岩土含水率和水势变化规律分析,发现入渗水流在包气带内下渗过程的岩土吸水、过水(即水分通量不变、且不等于零)和脱水的不同阶段,当岩土水势的梯度分别大于、等于和小于1cmH2O/cm(厘米水柱/厘米)时,岩土含水率分别表现为增加、稳定和减少。对包气带不同埋深的岩土含水率和水势变化特征分析表明,随着地下水位不断下降,包气带增厚对降水入渗补给地下水的影响程度和方式都发生改变。当包气带厚度小于潜水蒸发极限深度时,包气带的增厚使得岩土水分亏缺累积量增大,导致入渗速率和地下水获取的总入渗补给量减小;当包气带厚度大于潜水蒸发极限深度时,随着包气带厚度增大,入渗速率趋于稳定,无限时间内地下水获取的总入渗补给量不因包气带增厚而变化,但是有限时间内地下水获取的入渗补给量趋小。这是因为入渗途径的延长导致入渗所需时间增加,以至在有限时间内包气带内过剩的入渗水分尚未充分排出补给地下水。

The variation rule of soil moisture content and moisture potential gradient in the process of infiltration of unsaturated zone for various precipitations was investigated by means of field test and indoor experiment. It is found that the infiltration process consists of three stages： absorption, passing through and dehydration. The soil moisture content tends to steady if the potential gradient is equal to 1 cm H2O/ cm and tends to increase or decrease corresponding to the gradient higher or lower than this critical value. Following the continuous dropping down of groundwater table the influence of unsaturated zone thickness on precipitation infiltration for recharge of groundwater may change. When groundwater table higher than the limited level of groundwater evaporation the .increase of unsaturated zone thickness may result in the increase of accumulation of soil moisture deficit and leads to the reduction of infiltration rate and recharged water amount. Whereas, when ground water table lower than the limited level the infiltration rate will maintain a constant in spite of the increase of unsaturated zone thickness. The total recharged groundwater amount is not affected by the thickness of unsaturated zone in unlimited time period, but the recharged rate tends to decrease in a limited period due to the increase of the infiltration path length.