known as the  water table.  The depth of the water table-or the distance from the ground surface to the water level—varies considerably with the amount of rainfall.  During  droughts,  the  water  table  may  be lowered, but during a rainy season it will probably rise. As you should understand from your studies of soil formation,   the   stratum   over   which   groundwater accumulates is an irregular, rather than a continuously flat,  plane.  Therefore,  unless  the  water  is  confined,  it flows horizontally over the irregular stratum and is nearer the surface in some places than it is in other places.  Where  this  underground  water  flows  near  the surface and the ground area is low, the water may flow out as a spring. Or, it may seep out and create a swampy area.  The  underground,  flowing  water  also  may  become entrapped between impervious layers. In this case, enough water pressure may buildup to create an artesian well if the strata is penetrated by drilling or by a natural opening. In some regions of the world, there is not enough surface water or groundwater available to support the need  for  water.  In  these  areas  alternative  sources  are necessary. Rain, itself, can be an alternative source. In some locations, large catchment areas are constructed to collect rain and store it for future use. These catchment areas are usually constructed on the side of a mountain or a hill facing the prevailing direction of rainfall. In other areas, snow and ice may be used as alternative sources. Another source, although costly to develop for use,  is  seawater  that  has  had  the  salt  removed  by desalination. Selection and Development of Water Sources When selecting a water source for development, the engineer must consider three primary factors: water quantity,  water  reliability,  and  water  quality. The quantity factor considers the amount of water that is available at the source and the amount of water that will be required or demanded for use. The amount of water that maybe available at the source depends on variables, such as the amount of precipitation, the size of   the   drained   area,   geology,   ground   surface, evaporation, temperature, topography, and artificial controls. Water demands are estimated using per capita requirements and other controlling factors, such as water  requirements  for  fire  protection,  industrial  use, lawn  sprinkling,  construction,  vehicles,  and  water delivered  to  other  activities. The reliability of a water supply is one of the most important factors that the engineer considers when selecting a water source. A reliable water source is one that will supply the required amount of water for as long as needed. To determine the reliability of the water source, the engineer studies data, such as hydrological data, to determine the variations that maybe expected at the water source. Geological data should be studied since geological formations can limit the quantity and flow  of  water  available.  Also,  legal  advice  may  be necessary when selecting a water source since the laws regulating  and  controlling  water  rights  may  vary considerably from state to state and country to country. The third primary factor the engineer must consider when selecting a water source is the quality of the water. Practically all water supplies have been exposed to pollution of some kind. Therefore, to ensure that water is potable and palatable, it must be tested to determine the existence of any impurities that could cause disease, odor, foul taste, or bad color. In most cases, the water will   require   treatment   for   the   removal   of   these impurities. In water treatment, the water is subjected to various  filtration  and  sedimentation  processes,  and  in nearly all cases is disinfected using chlorine or other disinfecting  chemicals. Once the water source has been selected, develop- ment of the source can begin. Developing a water source includes  all  work  that  increases  the  quantity  and improves the quality of the water or makes it more readily available for treatment and distribution. In developing   a   source,   the   engineer   may   use   the construction  of  dams,  digging  or  drilling  of  wells,  and other improvements to increase the quantity and quality of the water. For a more detailed discussion of water source selection,  development,  and  treatment,  you  should  refer to  chapter  9  of  the  UT1  TRAMAN.  For  NAVFAC guidance, you should refer to Water Supply System, MIL-HDBK-1005/7. Now that you are familiar with water sources, let us move onto water distribution. DISTRIBUTION SYSTEM ELEMENTS AND ACCESSORIES The elements of a water distribution system include distribution  mains,  arterial  mains,  storage  reservoirs, and system accessories. These elements and accessories are described as follows: 1. DISTRIBUTION MAINS. Distribution mains are the pipelines that make up the distribution system. Their function is to carry water from the water source or treatment works to users. 2.   ARTERIAL   MAINS.   Arterial   mains   are distribution mains of large size. They are interconnected with smaller distribution mains to form a complete gridiron  system. 2-16