Developing and Applying the Water Supply Simulation M&l Robert M. Clark and Richard AI. Males The Water Supply Simulation Model is a series of computer programs that allows for be described as a network of pipes, evaluation of the physical and economic characteristics of a water distribution system in a storage tanks, treatment plants, de- spatial framework. Development of the model and its application to the water supply system of mands, and other hydraulic elements. the village of New Vienna, Ohio, are described. Information concerning the network is Passage of the Safe Drinking Water Act has intensified interest in problems related to water supply and water utility management. Analysis of the regulations to be promulgated under the act indicates that some water utilities, particularly small ones, may be adversely affected economically.’ An often-suggested option is that small systems combine with a larger system to form a regional water supply utility. It is assumed that the economies of scale associated with a regional water system would benefit the customers of small systems. costing, conservation policies, operating improvements versus increased capital expenditure, user class subsidization, and fire protection capacity. The model, called the Water Supply Simulation Model (WSSM), incorporates a series of submodels to describe the various economic, demographic, and hydraulic aspects of a water utility. stored in a network data base, including descriptive or calculated information about each element, such as size of pipe, geographic location of each demand, population associated with a demand, and connectivity of pipes (how one pipe is connected to another). Model structure A characteristic of many production and transportation problems is the trade-off between the cost of building and operating facilities to meet demands for a product and the cost of transpor- tation.” High transportation costs and low facility costs indicate decentraliza- tion; the reverse situation indicates a few large central facilities. These costs must be considered in planning, design- ing, constructing, and operating water supply systems. It is possible to separate the water supply system physically into two com- ponents: (1) the acquisition and treat- ment function, and (2) the delivery (transmission and distribution) system.:’ Each of these components has a different cost function. The unit costs associated with treatment facilities are usually assumed to decrease as the quantity of service provided increases. The delivery system, however, is more directly af- fected by the characteristics of the area beingserved. Thecost tradeoffs between the two components determine the cost of delivering water to any portion of the service area. The WSSM (Figure 1) is based on the concept of a water utility as a network providing service to a series of demands that are spatially distributed. The model can be used to analyze the costs of delivering water to thesedemand points. The service network is modeled as a link-node network, and it is assumed that water enters and leaves the system only at the nodes, which represent treatment plants, junctions, demand locations, and storage tanks. Water is carried between the nodes through connecting links (pipes). Costs are allo- cated to the various facilities and system components based on the amount of flow carried by that component. As a data base management system, the WSSM utilizes files that are per- manently stored, as opposed to “ decks” of data cards. The items of primary interest are the nodes and links of the distribution system. Two files are main- tained-the node file, containingrecords for each node, and the link file, containing records for each link. The nodes and links define the network, and they are numbered as shown in Figure 2. The individual node and link numbers (identi- fiers or IDS) correspond to individual records in the link and node files. All records in a file are structured in the same general fashion (Figure 3). The WSSM therefore requires that the water delivery and treatment system The seven software modules used in the model are theestablishment module, the editing module, the display module, the listing module, the hydraulic analysis module, the system solver module, and theinput-output (I-O)module(Table 1). Establishing a data base for the WSSM requires the use of four of these modules. TABLE 1 Major elements of WSSM Element Network data base Description Separate but cross-referenced data for the link and nodal elements of the network Because few analytical instruments are available for study of the economics of water supply systems, the US En- vironmental Protection Agency’ s Drink- ing Water Supply Research Division initiated a program todevelop techniques and methodologies to evaluate the eco- nomics of regional systems. This article describes the development of a simula- tion model designed to aid in such an evaluation. The model can also provide insights into other water-related eco- nomic issues, such as spatial pricing and Standard and user-specified data items Data base access methods Routines designed for efficiency and ease of use Routines in standard FORTRAN, not strongly tied to a particular computer Hydraulic network analysis model Nonproprietary WATSIM model with both steady-state and dynamic simulation capabilities and ability to model a wide range of hydraulic elements Other physical and economic Linear cost allocation procedure models Display and reporting modules Data base establishment and edit modules General purpose physical system solver to solve problems involving multiple sources, travel time. and mixing at nodes General purpose listing programs for link and nodal elements Flextble pen plotter display program for producing annotated maps of network Establish comprehensive link and node files and check and cross-reference information in the two files, thus maintaining topological integrity of the network AUGUST 1986 R.M. CLARK & R.M. MALES 61 Copyright (C) 1986 American Water Works Association