WATER EVALUATION AND PLANNING SYSTEM MODEL ASSESSMENT FOR OPERATION OF HARDAP DAM AND WATER RESOURCE MANAGEMENT STRATEGIES IN ITS CATCHMENT, NAMIBIA

Abstract

Management of water supply and demand remains a major challenge in most developing countries, especially in a semi-arid country like Namibia, where surface water supplies are limited mostly during drought periods. Given the extremities, the Hardap catchment is exposed to, water supply and demand management is a challenge. This results in water resource allocation problems, which threaten the sustainable water resource management in the catchment. A baseline study was conducted to develop and assess a calibrated WEAP model for the Hardap Dam operation and water resource management strategies. Water demand and supply availability were evaluated for three water use sectors, namely, domestic (tourism and livestock included), aquaculture, and irrigation. The WEAP model was structured according to four scenarios with a current account (1999) and reference period (2000-2021). These scenarios are as follows: Scenario 1: 10% demand saving management for Mariental Town and improved irrigation efficiency by changing from the current flood irrigation method to the drip irrigation method. Scenario 2: increased irrigation area for the Hardap Irrigation Scheme. Scenario 3: hydrological seasons for high and low flows which evaluated the change in the hydrological regime; very wet, wet, normal, dry and very dry years. Scenario 4: the discharge of water made from Hardap Dam in 2017 for the downstream demand, Neckartal Dam. The modelling results show that the operation of the Hardap Dam is challenging and the change from 100% to 70% operation rule of full supply capacity has greatly reduced the availability of water supply. The water management strategy results indicate that the 10% demand saving management for Mariental Town has reduced the domestic water demand by 10% hence no impact on the water availability of the Hardap Dam. Improved irrigation efficiency has a positive impact of about 30% reduction in water demand and 13% increase in water availability, with a significant impact of improvements during the drought seasons. Under the increased irrigation area scenario, the irrigation demands are increased by 3.8 Mm3/m for an area of 213 ha and little impact is observed on the storage volumes during dry years. However, the model forecasted a critical and an extreme threshold of 300 and 1000 ha respectively. This has a major impact on demand coverage which was reduced to 90% for the 300 ha. While the 1000 ha has a significant impact on the water supply which depleted the dam levels to below the dead zone. The hydrological seasons were evaluated and the results show that there is a major effect during drought years, due to an increase in storage volumes. The forecasted inflows of 2015 and 2016 were within the observed water year type of dry and very dry. The result of the discharge of 10.7 Mm3 for Neckartal Dam from Hardap Dam in 2017 showed no threat to the water demands and supply as both demands are met.