CE322 Basic Hydrology

Jorge A. Ramirez

Computation of Precipitable Water - Example

Compute the precipitable water for a saturated column of atmosphere extending 10 km up from the ground surface. Assume the surface pressure is 101300 Pa, the surface temperature is 30 C and the ambient lapse rate is 6.5 K/km.

Precipitable water:

The solution is presented in tabular form. The columns of the table are computed as indicated below.

Column 1: Elevation in km

.

Column 2: Linear vertical temperature distribution.

Column 3: Vertical pressure distribution assuming hydrostatic conditions and linear temperature distribution:

Column 4: Saturation vapor pressure:

Column 5: Moist air density

Column 6: Specific humidity

Column 7: Average specific humidity

 (1) Elevation (2) Temperature (3) Pressure (4) Vapor Pressure (5) Density (6) Specific Humidity (7) Average Specific Humidity (8) Dp/g (9) -qv Dp/g (km) C K Pa Pa kg/m3 qv kg/m2 0.00 30.00 303.00 101300.00 4310.66 1.15 0.03 0.02 -2126.62 44.05 2.00 17.00 290.00 80437.86 1934.12 0.96 0.01 0.01 -1757.16 20.10 4.00 4.00 277.00 63200.08 804.92 0.79 0.01 0.01 -1438.96 8.50 6.00 -9.00 264.00 49083.90 307.27 0.65 0.00 0.00 -1166.85 3.30 8.00 -22.00 251.00 37637.08 106.16 0.52 0.00 0.00 -935.99 1.16 10.00 -35.00 238.00 28454.98 32.66 0.42 0.00 W=77.10
W is in units of mass (M) per unit area (kg/m2) which can be expressed also in units of volume (V) of liquid water per unit area.

W = 77.10 kg/m2 = 77.10 mm