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11.8 Good Records and Their Uses 373
still air, too, causes little melting, because the thermal conductivity of air is small, but there
is condensation of moisture and rapid melt when the vapor pressure of the air is higher
than that of the snow. Because the heat of condensation of water vapor is 1,073 Btu/lb at
32 F (596 kg-cal/kg at 0 C) and the heat of fusion of ice is only 144 Btu/lb (80 kg-cal/kg),
condensation of 1 in. (25.4 mm) of moisture results in 1,073>144 7.5 in. (190 mm) of melt.
Rain falling on snow produces relatively less melting by comparison, namely, (T 32)>144
for each inch of rain (25.4 mm), T being its temperature in F and equaling the wet-bulb
o
temperature of the air. Even when T 60.8 F (16 C), for example, an inch of rain (25.4 mm)
induces only (60.8 32)>l44 0.2 in. (5 mm) of melt. In more general terms, each degree
day above 32 F(0 C) can result in 0.05 to 0.15 in. (1.3 to 3.8 mm) of melt. This is the degree-
day factor.
For snow packs on mountainsides, an areally weighted average temperature can be
computed for the melting zone. The lower boundary of this zone is drawn by the snow line;
the upper boundary by the contour lying above the observation station by about 1000 per
three times the difference between the observed temperature and 32 F(0 C). The assump-
tion is made that the temperature of the atmosphere drops by approximately 3 F (1.7 C) for
each 1000-ft (305-m) lift in elevation.

11.7.4 Measuring Runoff

No studies of surface-water supplies, storm and combined sewerage systems, and waste-
water disposal can advance to the design stage without thorough evaluation of pertinent
runoffs and their magnitude and variability. Stream gauging itself is based on an under-
standing of open-channel flow. Measuring devices are many: current meters, floats,
weirs, and surveying instruments; measuring techniques involve chemicals, radioactive
tracers, and persistent dyes. Each of the devices and techniques has its own advantages
and disadvantages, and each has its peculiar range of usefulness. Once established at a
stable cross-section, and rated by suitable means, gauge height need be the sole measure
of record, automatic records being traced by vertical movements of a float in a stilling
well. Occasional checks of actual runoff will reinforce the validity of the rating curve,
which relates measured discharge to measured gauge height.

11.8 GOOD RECORDS AND THEIR USES

Collection of representative hydrologic information should be encouraged at every oppor-
tunity if the water potential of a river system is to be fully understood and put to use.

11.8.1 Rainfall Records
Although precipitation is the ultimate source of all water supplies, it should be understood at
the outset that water supply studies should be based, whenever possible, on direct measure-
ments: runoff records for surface-water supplies and groundwater flows for groundwater sup-
plies. Nonetheless, hydrologists will and should continue to look for useful relations between
rainfall and runoff and between rainfall and infiltration; and engineers will and should put
these relations to work in the absence of direct measurements. However, the links in the chain
of hydrologic sequences still remain too weak to be placed under much stress. If this is kept
in mind, rainfall records need not be eschewed by engineers. Instead they can be put to use to
good purpose in the business of water supply and drainage by providing a better idea of the
variations to be expected. For example, they illuminate the possibility of the drought that