Geohydrological Assessment of the Iishana Sub-Basin,Namibia

本文研究区位于纳米比亚中北部的Iishana分流域,区域内的盆地属于半干旱环境,其特点是低洼相互连接的短暂排水网络。在过去的十年中,由于城市发展造成的天气异常和天气模式的变化导致该流域经常发生干旱和洪水事件。城市的发展需要增加对不断增长的居住处的供水。然而,盆地的供水量有限,大多数干净的饮用水通过运河从安哥拉-纳米比亚边境的Calueque大坝供应。因此,本研究重新确定了使用需要最少经济投入的有效方法来调查潜在地下水区的必要性,并采用不同的方法来检测和估计潜在的地下水区域并评估地下水水质。从ALOS DEM中自动提取排水流线来更新现有的排水网络,通过确定已知地下水可用性的参数来估算潜在的地下水区,进而评估人类和牲畜饮用的地下水质量。研究区内的数据受限于水管理战略决策,仅指示主要的已知短暂河流网络,因此需要确保所使用的数据是准确和最新的。具体研究内容如下:(1)通过目视解译结果来验证自动提取的排水网络,利用CLC度量来统计计算提取网络的百分比,将网络叠加在Landsat 8图像的湿地上,并得到自动提取与描绘的排水网格之间匹配度为91%。从ARVI指数与Sentinel-2数据集中获取植物物种分布,编制了一个主要由裸露土壤、植被和水三个主要的土地覆盖单位组成的地形图。采用从土地覆盖中提取的植被单元与田间采集的植被物种绘制物种分布图,并使用CAC指标得到土地覆盖分类结果的准确度为70%。得到的植被物种分布图未经验证,因为该区域被发现具有在田地中观察到的混合植被物种分布和由此产生的地图。(2)整合能够确定地下水可用性的专题图来评估地下水可用性。在ArcGIS软件中采用WoE方法来单独评估专题图,使用加权叠加应用工具进行数据集成,利用协方差统计方法评估由此产生的潜在地下水区,以确定哪些参数对确定潜在的地下水可用性具有很大影响。(3)评估地下水质量,并在研究区域内插入其化学元素,以确定哪些潜在的地下水区域具有适合人类和牲畜消费的水质。

The study was conducted in the Iishana sub-basin located in north-central Namibia.The basin is a semi-arid environment which is characterized by low-lying interconnected ephemeral drainage networks.In the past decade,the basin has experienced frequent droughts and flooding events due to recent weather anomalies possibly enhanced by the sprawling urban development and the change in weather patterns.The enhancement of urban development necessitated a need to increase water supply to growing settlements.However,water availability is limited in the basin with most clean drinking water being supplied through the canal from the Calueque Dam across the Angola-Namibia border.The study,therefore,recognized a need to investigate for potential groundwater zones using cost-effective methods that require minimal inputs.Henceforth,different methods were employed to detect and estimate potential groundwater zones and assess groundwater quality.Firstly,was to update the existing drainage networks by automatically extracting drainage flow lines from the ALOS DEM.Secondly,was to estimate potential groundwater zones by identifying parameters that can determine groundwater availability.And thirdly,determining the quality of groundwater for drinking both for human and the area which only indicates major known ephemeral river networks making the data limited for effective decision-making on water management strategies.,and therefore a need to ensure that the data being used are accurate and up-to-date.Automatically extracted drainage networks were validated firstly by visual observation of data overlay,whereby the networks were superimposed on the Landsat 8 imagery of the wet secondly by statistically calculating the percentage of extracted networks using the CLC metric.A 91% match between extracted and delineated drainage networks was obtained.Furthermore,the vegetation species distribution was generated from the ARVI index in conjunction with the Sentinel-2 dataset.First,a landcover map was developed with three main landcover units mainly;bare soil,vegetation,and water.the vegetation unit was then extracted from the landcover and used together with field collected vegetation species for species distribution mapping.The landcover classification obtained a 70% accuracy using the CAC metric.The resulting vegetation species distribution map was not validated as the area was found to have a mixed vegetation species distribution as observed in the field and the resulting map.Last but not least,the potential for groundwater availability was assessed by integrating thematic maps that have the potential for determining groundwater availability.Thematic maps were individually assessed prior to the integration.A WoE method was employed in the ArcGIS software using the weighted overlay application tools for data integration.The resulting potential groundwater zones were assessed with a covariance statistical method to determine which of the parameters has a high influence in determining potential groundwater availability.Finally,groundwater quality was assessed and chemical elements thereof interpolated across the study area to determine which potential groundwater zones have suitable water quality for human and livestock consumption.

排水网络; 地下水可用性; ALOS DEM;

Drainage network; groundwater availability; ALOS DEM;