Throughout most of the twentieth century, water resources planning was mainly in the hands of civil engineers. They planned, designed and built the infrastructure to monitor, store, control and manage water resources. It was essentially a “top- down” process with little stakeholder involvement, and still is in many countries. However, the impacts of water scarcity, increasing demand and competition among water- using sectors and society's growing awareness and interest in the natural environment have meant that many public and private sector organizations now engage in water- related issues and employ people from various disciplines, many of whom may influence decision- making in managing water resources. Each brings specific knowledge and experience to the table.

By definition, when water is scarce, there is not enough to satisfy everyone's needs. This inevitably brings conflicting demands for water from different sectors, making it an area ripe for misunderstanding and dispute over allocations. In such circumstances, it is vital that everyone understands and appreciates the needs of others, recognizes and agrees on priority allocations and knows when to compromise and negotiate trade- offs. Setting policy and decision- making in the water sector has become immensely challenging and complex, requiring knowledge and understanding of biophysical and socioeconomic systems and processes. This is the essence of IWRM, which is at the heart of the 2030 Agenda and SDG 6 (see Section 2.9).

This chapter offers a starting point for individuals who are relatively new to water resources management and would like to understand better and appreciate how others view water resources. We describe the roles of some key professionals involved in water resources planning and management and what water means to them: hydrologists, water resources planners, river engineers, freshwater ecologists, water engineers (supply and wastewater) and irrigation engineers, working in both developed and developing worlds.

Those already involved and perhaps focused on one aspect of water management may also find it helpful to dip into those water sectors with which they may be less familiar but which may impact on their decision- making. It may also benefit those who indirectly influence decision-making as water users to appreciate how others use water and avoid misunderstandings that can and have led to inappropriate plans with serious financial consequences (Perry & Steduto 2017).