Assessing the effects of non‐stationarity on reservoir yield estimations: A case study of the Southern Okavango Integrated Water Development system in Botswana

Abstract

AbstractStreamflow data used for water resources planning should ideally be stationary, and any non‐stationary behaviour is taken into account. However, with limited time series data, the influence of non‐stationarity is often hidden and can result in unreliable estimates. This paper examines the impact of non‐stationarity on the Southern Okavango Integrated Water Development (SOIWD) project that was carried out with streamflow collected between 1969 and 1989 against an extended time series from 1990 to 2019. To achieve this, (a) the statistics of these inflows and (b) the reliability of satisfying water demands from the proposed reservoirs were tested based on the two periods (1969–1989 and 1990–2019). The results show that average monthly flows for July, August and September significantly change when peak outflows from the delta occur. Given the expected variability of the flow regime, an uncertainty approach utilising flow perturbation from ±5% to ±90% was also used to investigate the system's response to changes in the driving flow conditions. The increasing availability of flow data from 1990 to 2019 has shown that the SOIWD system would not have satisfied the water demand as it would not be operationally viable. This confirms the importance of accounting for non‐stationarity in reservoir yield estimation and reemphasises its importance in hydrological studies.