Hydrogeological map of Lower Saxony 1: 50 000 — Average annual groundwater regeneration for the 30-year period 2021-2050 in the hydrological winter half-year, climate protection scenario (RCP2.6)

The map shows the modeled average annual groundwater regeneration for the 30-year period 2021-2050 in the hydrological winter half-year (Nov.-Apr.) in mm/a calculated with the “climate protection” scenario (RCP2.6). Groundwater is a raw material that can regenerate and renew itself. The main supplier for the groundwater supply is precipitation water leaking in Lower Saxony. It ensures that the groundwater deposits of the storage rocks are replenished in the underground. The groundwater formation is particularly high in winter, as at this time a large part of the rainfall in the soil is leaking. In the warmer seasons, on the other hand, much of the precipitation already evaporates on the surface or is absorbed by plants. The new groundwater formation is widely distributed in different areas. It depends on the distribution of precipitation and evaporation, the characteristics of the soil, the land use (growth, degree of sealing), the relief of the land surface, the artificial drainage by drainage, the groundwater fluid level and the properties of the near-surface rocks. Since these parameters differ significantly in the smallest space in Lower Saxony, groundwater formation is also subject to large lateral fluctuations. In order to determine the new groundwater formation, there are different methods. The available maps show the area-differentiated designation of the mean groundwater formation, which was calculated using the mGROWA method (short for “monthly large-scale water balance”). The model mGROWA was developed for the large-scale simulation of the water balance at Forschungszentrum Jülich in cooperation with the LBEG (Herrmann et al. 2013) and updated methodically for Lower Saxony since 2016. In addition, a series of new input data has been used to provide an up-to-date data base for water management planning and water approval procedures. Daily and monthly climate projection data were used as climatic input data. The climate projection data represent the results of an ensemble of different climate models (the Lower Saxony climate ensemble AR5-NI v2.1 see Hajati et al. (2022)). The data was provided by the German Weather Service. The basis for this is the EURO-CORDEX Ensemble (Jacob et al., 2014). As part of the BMVI expert network, the DWD saw a downscale from a 12.5 km grid to a 5 km grid. The climate models are driven by the “climate protection” scenario (RCP2.6). This is a scenario of the IPCC, which means significant efforts in climate protection and low emissions. The results of all climate models are equally likely. Therefore, in addition to the mean, which shows a tendency, the upper (maximum) and lower (minimum) edge of the result bandwidth can be retrieved via the maptip. For better regionalisation, the climatic input parameters precipitation and potential evaporation with bilinear interpolation were scaled down to a 500 x 500 m grid for mGROWA22.本地图展示了基于“气候保护”情景（RCP2.6）计算得到的、2021-2050年共30年时段内，水文冬季半年（11月至次年4月的模拟年均地下水补给量，单位为mm/a。 地下水是一种可自我再生更新的自然资源。下萨克森州地下水补给的主要来源为入渗降水，其确保了储水岩层的地下储水量得到持续补给。冬季地下水补给量尤为充沛，因为此时土壤中的大部分降雨可渗入地下；而在暖季，多数降水会在地表蒸发或被植物吸收。 新增地下水补给量在不同区域广泛分布，其受降水与蒸发的空间分布、土壤特性、土地利用方式（植被覆盖度、地表密封程度）、地表地形、人工排水措施、地下水位以及近地表岩层性质等诸多因素影响。由于下萨克森州内上述参数在极小空间尺度上差异显著，因此地下水补给量也存在显著的横向波动。 为测算地下水补给量，学界存在多种方法。本次展示的地图为分区域标注的平均地下水补给量，该数据通过mGROWA（monthly large-scale water balance，即“月度大尺度水平衡”）方法计算所得。mGROWA模型由于利希研究中心（Forschungszentrum Jülich）与LBEG合作开发（Herrmann等，2013年），并自2016年起针对下萨克森州完成系统性更新。此外，本次研究采用了一系列全新的输入数据，为水资源管理规划与取水许可审批流程提供了最新的数据库支撑。 气候输入数据采用逐日与逐月的气候预估数据，该数据源自多气候模型集合的模拟结果（下萨克森州气候集合AR5-NI v2.1，详见Hajati等，2022年），由德国气象局（DWD）提供，其基础数据集源自EURO-CORDEX集合（Jacob等，2014年）。作为BMVI专家网络框架下，德国气象局将原始数据从12.5km的网格尺度降尺度至5km网格。 该气候模型集合以“气候保护”情景（RCP2.6）驱动。RCP2.6是政府间气候变化专门委员会（IPCC）提出的情景，代表着大力推进气候保护、实现低排放的发展路径。 所有气候模型的模拟结果权重均等。因此，除了体现整体趋势的平均值外，还可通过地图提示（maptip）获取结果区间的上限（最大值）与下限（最小值）。 为实现更精准的区域化表达，针对mGROWA22模型的气候输入参数（降水与潜在蒸发量）通过双线性插值法被降尺度至500×500米的网格。