Prior to the creation of the global Aqueduct Water Risk Atlas, indicators (Table 1) were developed and tested in a number of river basins worldwide. The results of these basin studies helped inform and shape the global Aqueduct Water Risk Framework. Complete guidelines and processes for indicator selection, data collection, calculations, and mapping techniques are described fully in the Aqueduct Water Risk Framework.1 This study focuses on the specific characteristics of the indicator data and calculation in the Orange-Senqu River Basin (ORB).
The data selection and validation process for the Orange- Senqu River Basin Study involved three steps: (1) a literature review, (2) identification of data sources in the public domain, and (3) the compilation and expert review of selected data sources. Calculation of 6 of the 14 indicators required the creation of original datasets to estimate water availability and use at a sub-basin scale. The hydrological catchments used in the exercise are hydrozones developed by WRP Consulting Engineers for the Orange-Senqu River Commission (ORASECOM). Computation of the original datasets was completed by ISciences, L.L.C.
Two measures of water use were used in this study: total withdrawal, the total amount of water abstracted from freshwater sources for human use, and consumptive use, the portion of withdrawn water that evaporates or is incorporated into a product thus is no longer available for further use. Withdrawals for the basin are courtesy of the South African Department of Water Affairs (DWA) and WRP Consulting Engineers. The withdrawal data was then georeferenced to the hydrological catchments.
Two metrics of water supply were computed: total blue water and available blue water. Total blue water approximates natural river discharge and does not account for withdrawals or consumptive use. Available blue water is an estimate of surface water availability minus upstream consumptive use. Modeled estimates of water supply were calculated using a catchment-to-catchment flow accumulation approach developed by ISciences, L.L.C., which aggregates water by catchment and transports it to the next downstream catchment. Water supply was computed from runoff (R), the water available to flow across the landscape from a particular location and calculated as the remainder of precipitation (P) after evapotranspiration (ET) and change in soil moisture storage (ƒ¢S) are accounted for (i.e., R = P . ET . ƒ¢S). The runoff data is courtesy of the German Agency for International Cooperation (GIZ - Deutsche Gesellschaft fur Internationale Zusammernarbeit) and ORASECOM, and computed by WRP Consulting Engineers. Runoff is the output from a rainfall-runoff calibration model, using observed rainfall, stream flow, and land use to calibrate parameters and simulate runoff records similar to observed records in the Orange-Senqu River Basin (ORB) over a common time period. Rainfall and the calibrated parameters were used to generate runoff values for 1920 to 2004.
The remainder of this document contains definitions, formulas, and data sources for the Orange-Senqu River Basin Study.