Water pressure is the force exerted by the water stored in the reservoir on the upstream and the water depth at the tail of the dam. 

1. External Water Pressure Load 
   External water pressure can be calculated by the law of hydrostatics according to which in a static mass of liquid the pressure intensity varies linearly with the depth of liquid and it acts normal to the surface in contact with the liquid. For the non-overflow section of the dam water pressure may be calculated as follows 
   FH = horizontal component of hydrostatic force, acting along a line 1/3 H above the base = ½ wH2 
   w = Unit weight of water (=10 kN/m3) 
   Fv = Vertical component of hydrostatic pressure 
   = Weight of fluid mass vertically above the upstream face acting through the center of gravity of the mass. 

2. Internal Water Pressure (Uplift Pressure) 
   Internal water pressure is the force exerted by water penetrating through the pores, cracks and seams with in the body of the dam, at contact surface between the dam and its foundation, and with in the foundation. It acts vertically upward at any horizontal section of the dam as well as its foundation and hence it causes a reduction in the effective weight of the portion of the structure lying above this section. 
   The computation of internal pressure involves the consideration of two constituent elements, i.e, 
   Hydrostatic pressure of water at a point 
   The percentage C, area factor, of the area on which the hydrostatic pressure acts Both these elements are discussed below. 

3. Hydrostatic Pressure 
   In practice dams are usually provided with cut-off walls or grout curtains to reduce seepage and drain to relieve pressure downstream from the cutoff. Actually cutoff and grout curtains may not 
   be perfectly tight and hence fail to dissipate the head (h1 – h2) 
   Usually a distribution like 1-2-3-4 is used with 3-4 a straight line as shown in Figure 3-3. 
   Opinions about the value of uplift reduction factor,  (Zeta), are varied, the tendency is to take: 
   = 0.85 (for normal loading cases) 
   = 1.00 (for exceptional loading cases like earthquake Uplift pressure distribution for perfectly tight cutoff walls.

Uplift area factor, C 
The value of area factor for concrete has been determined experimentally by several investigators. However, for the foundation rock the value of area factor is not determinable experimentally and hence the same has been estimated on the basis of theoretical considerations. 
Some of the earliest investigators recommended, for both concrete and rock, a value of area factor ranging from one third to two-thirds of the area to be considered as effective area over which the uplift pressure acts. However, Harza, Terzaghi and Lelivakey have indicated that, for both concrete and rock, the value of area factor is nearly equal to unity. 
Values suggested for uplift area factor are Value of C Suggested by 
0.25 to 0.40  Henry 
1.00  Maurice Levy 
0.95 to 1.00  Terzaghi
As such the present practice followed in the design of dams is that the uplift pressure is assumed to act over 100 percent of the area with in the body of the dam as well as its foundation. Hence, under all conditions, the value C = 1.00 is recommended.