Study on the Recharge Mechanism and Development of Groundwater in the Inland area of Tanzania

在坦桑尼亚中部的Makutapora盆地，年平均降雨量约为600毫米，而潜在的蒸发量为2470毫米。因此，地下水补给高度依赖于降雨量和降雨模式。为了了解流域及周边地区地下水补给机制，选择了一个小流域，并进行了密集的水文观测。利用称重式蒸渗仪推导了实际蒸发量与土壤含水量的关系，并根据入渗量的测定计算了代表点的径流率。将这些结果结合起来模拟非饱和带的水量平衡，并计算出补给量为降雨量的15%。将Makutapora盆地的地下水分为浅层地下水和深层承压地下水。后者则根据氯离子浓度分为1型和2型。深层承压地下水的滞留时间一般在25年以上，可能几十年，而浅层地下水的滞留时间则短于20年。P_3<CO2>、氚浓度和稳定同位素组成也表明，1型是来自Chenene Hill地区的组分，2型是来自靠近盆地底部的上平面的组分。

In the Makutapora Basin in central Tanzania, mean annual rainfall is about 600 mm while potential evaporation is 2470 mm. Thus groundwater recharge is highly dependent on the amount and pattern of rainfall. To understand the mechanism of groundwater recharge in the basin and surrounding area, a small watershed was selected and intensive hydrological observations were carried out. The relationship between actual evaporation and soil water content was derived from weighing lysimeter and the runoff ratio of representative sites were calculated from the measurement of infiltration capacity. Those results are combined to simulate water balance in the unsaturated zone and recharge is calculated to be 15 % of rainfall.Groundwater in the Makutapora Basin is divided into shallow groundwater and deep confined groundwater. The latter is then classified into type-1 and type-2 according to chloride ion concentration. The residence time of deep confined groundwater is generally more than 25 years, probably a few tens years, whereas the residence time of shallow groundwater is shorter than 20 years. P_<CO2>, tritium concentration and stable isotope composition also show that type-1 is the component from Chenene Hill area and type-2 is the component from the upper plane near the basin floor.