We use two comprehensively instrumented field observatories to understand groundwater recharge processes in African drylands. The observatories are located on crystalline basement geology in semi-arid parts of Ghana and Burkina Faso, aridity indices 0.43 and 0.29, respectively, and we report 2017–2019 observations. Groundwater recharge was quantified by inverse water table fluctuation models using specific yield estimates derived from magnetic resonance soundings. Evidence for recharge drivers and mechanisms comes from high resolution meteorological observations, soil moisture (logged hourly and weekly along hillslope transects), overland flow plots, river stage, and stable isotopes of O and H in rainfall events and groundwater. Groundwater recharge varied between 87 and 175 mm/y, i.e. 7–15 % of annual rainfall. Rainfall was twice the volume of water lost via actual evapotranspiration across the four peak months (Jun-Sep) of the monsoon. This seasonal water surplus of ∼ 350 mm/y is not characterised by the annual scale of the aridity index. Overland flow was rare and soil moisture deficits were overcome at all monitoring locations. Large rainfall events only produced appreciable recharge when the antecedent soil moisture was close to field capacity, yet always produced large responses in river stage. Stable isotopes of O and H in groundwater indicate no evidence of evapotranspiration prior to infiltration and their composition is akin to depleted isotopic rainfall at the monsoon peak. Stable isotopes indicate recharge season timing and not a relationship between intense rainfall and groundwater recharge. We contend that the mechanism for groundwater recharge is predominantly diffuse in these semi-arid African settings.

中文翻译：

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半干旱非洲旱地的地下水补给是分散的：来自高度仪器化观测站的证据

我们使用两个设备齐全的现场观测站来了解非洲旱地的地下水补给过程。这些观测站位于加纳和布基纳法索半干旱地区的结晶基底地质，干旱指数分别为0.43和0.29，我们报告了2017-2019年的观测结果。地下水补给量通过逆地下水位波动模型使用磁共振探测得出的具体产量估计值进行量化。补给驱动因素和机制的证据来自高分辨率气象观测、土壤湿度（沿山坡横断面每小时和每周记录一次）、地表流量图、河流水位以及降雨事件和地下水中 O 和 H 的稳定同位素。地下水补给量在 87 至 175 毫米/年之间变化，即年降雨量的 7-15%。季风四个高峰月（六月至九月）的降雨量是实际蒸散损失水量的两倍。每年约 350 毫米的季节性水盈余并不能用干旱指数的年度规模来表征。地表径流很少见，所有监测点都克服了土壤水分不足的问题。大降雨事件仅在前期土壤湿度接近田间持水量时才会产生明显的补给，但在河流阶段总是产生较大的响应。地下水中 O 和 H 的稳定同位素表明在渗透之前没有蒸散的证据，其组成类似于季风高峰期的贫同位素降雨。稳定同位素表明补给季节时间，而不是强降雨和地下水补给之间的关系。我们认为，地下水补给机制主要分布在这些半干旱的非洲地区。