for infrastructure elements such as reservoirs, canals, and hydropower projects (Huber-Lee _et al._ , 2006; Purkey _et al._ , 2007; Yates _et al._ , 2005a, 2005b, 2008). Additionally, it has watershed rainfall-runoff modeling capabilities that allow all portions of the water infrastructure and demand to be dynamically nested within the underlying hydrological processes. This functionality allows the modeler to analyze how specific configurations of infrastructure, operating rules, and operational priorities will affect water uses as diverse as instream flows, irrigated agriculture, and municipal water supply under the umbrella of input weather data and physical watershed conditions. This integration of watershed hydrology with a water systems planning model makes WEAP ideally suited to study the potential impacts of climate change and other uncertainties internal to watersheds.

The SEDAPAL WEAP model represents Lima’s system through a series of demand and supply nodes, connected via transmission links representing either natural streams or engineered canals. The water demand in the Metropolitan area is represented by a demand node for each of Lima’s four main regions: Central Lima, Eastern Lima, Southern Lima, and Northern Lima and Callao. The model contains all existing water infrastructure, including five reservoirs in the Alto Mantaro, two groundwater basins, and all canals and transfer tunnels of the Marca I, II, and V,<sup>5</sup> including a trans-Andean tunnel. The model also contains the existing water treatment plants of Atarjea and Chillón. Additionally, each of the 14 proposed Master Plan projects can be modeled individually or in different combinations.

The WEAP model takes as input (i) monthly hydrological series, (ii) the depth, levels, and operations of the reservoirs, and their evaporation, (iii) the design capacity of all investments, and (iv) different demand scenarios, divided per demand region. Besides water supply to the different areas of the city, the model also reports monthly water levels in the reservoirs, groundwater extraction levels, and unmet demand. Figure 4-3. shows a screenshot of the schematic of SEDAPAL’s water management system within WEAP.

This is SEDAPAL’s first model of the water system supplying Lima Callao, and they will continue to use it for their planning after this project. Before the development of this model, SEDAPAL’s project prioritization was based primarily on experts’ considerations and the additional supply sought in different parts of the city. SEDAPAL did not have tools that allowed for an integrated and dynamic analysis of different projects. SEDAPAL’s continuous involvement in the model’s development helped both build confidence in the model and its results, and increase the likelihood of a successful technology transfer of the model to SEDAPAL for its future analyses.

We developed a code to evaluate the WEAP model on the Amazon cloud, allowing the simultaneous running of up to a hundred cases at once. First, we ran the baseline (i.e., without Master Plan) under the 300 water supply and demand futures we created, then the baseline plus one individual project at the time, and finally, the first selection of portfolios to identify which ones performed better across these futures.

> 5 The three Marcas indicate the sequenced construction of the investments: Marca I began in 1960s, Marca II at the end of 1990s, and Marca III-V in 2000s.

15