*2.3. Methods*

This study aims to develop the WEAP model for optimal allocation of water resources among competing sites. The WEAP model is one of the powerful tools used to evaluate the existing and planned water resources development in a given watershed. The model is used to identify water scarcity areas that cause conflict and can simulate a water allocation policy. Water allocation priority rules are set within the WEAP model based on either first come first served, or specific use or user, and/or making allocation proportional to demand. As it is generic, the model is not capable of capturing every fine distinction or detail of a water resource system and as such is best applied to scenario screening and pre and feasibility levels of analysis rather than to detailed design and permitting tasks. To allow simulation of water allocation, the elements that comprise the water demand–supply system and their spatial relationship are characterized for the catchment under consideration. It also helps to understand the available water resources demand for current and future development scenarios. The WEAP model also helps to provide a system for maintaining water demand and supply information in addition to the simulation of demand and flows. The hydrological systems in the WEAP model of Awash River Basin are depicted as nodes and links. The main river is drawn as a series of nodes, showing points of inflows from each catchment and river confluence linked to each other by river reaches.

It also calibrates and validates the streamflow in four gauging stations (Akaki, Hombole, Melka Kuntre, and Modjo) in the Awash River Basin. Three scenarios were established after developing the WEAP model to predict the water demands until 2050. The model was run on a monthly basis. The baseline or current scenario 1980–2016 was set to estimate the irrigation water demand. The WEAP model for the Awash River Basin was set up to simulate the current/base year (1980) condition and three subsequent scenarios: the reference scenario (1981–2016), the medium-term development (2017–2030), and the long-term development (2031–2050). For each development scenario, the outputs analyzed include the water demand satisfaction of the irrigation demanding sector and spatiotemporal variations in water shortage. The methodology of the study is shown in (Figure 2).

**Figure 2.** Methods of estimating water demands through the WEAP model.

The WEAP model provides an integrated assessment of climate, hydrology, water resources allocation, and watershed management [33]. It also addresses various issues such as water resources, water demands analysis in different sectors, provides priorities in water allocation, reservoir operation, and management. It solves the water allocation challenges at user-defined periods, either monthly or yearly based on linear programming structures [34].
