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372 Chapter 11 Hydrology: Rainfall and Runoff
Pumping intrudes into the natural regimen of subsurface waters. Recharge, discharge,
and storage are forced to seek new equilibria. Lowering the water table may decrease and
even stop natural discharge, but increase natural recharge, especially in soils bordering on
surface streams. How much water can be salvaged economically by lowering the water
table through pumping depends on the cost of lifting water from increasing depths. If the
water table is to remain at a designated level, average rates of withdrawal and recharge
must be alike under the conditions generated.
Among other factors affecting groundwater levels are
1. Seasonal variations in evaporation and transpiration
2. Diurnal fluctuations in transpiration
3. Changes in barometric pressure
4. Passage of moving loads, trains for example, over artesian formations
5. Land and ocean tides
6. Earthquakes.
Fluctuations in levels registered by a continuous recording gauge on an observation
well seldom have one cause only. Records must, indeed, be analyzed with much care if un-
derground hydrologic cycles are to be fully understood.

11.7 RUNOFF
Water derived directly from precipitation flows over the ground into water courses as
surface, storm, or flood runoff. However, the amounts of water actually reaching streams
are reduced by infiltration, evaporation, and other losses along the way.

11.7.1 Dry-Weather Flows
Water flowing in streams during dry spells, or when precipitation falls as snow without
melting, is known as dry-weather flow or dry-weather runoff. It is composed of water
stored in the ground and impounded in lakes, ponds, swamps, and other backwaters.
Accordingly, the dry-weather yield of streams comes both from natural surface storage
and natural ground storage. In some river basins with headwaters at high altitudes, much
of the summer runoff is derived from melting snowfields. In the absence of snowmelt or
surface storage, streams lying above the groundwater table at all stages of flow are
ephemeral (short lived); streams lying above the summer groundwater level intermittent.

11.7.2 Runoff from Rainfall
Runoff from rainfall is influenced chiefly by (a) the intensity, duration, and distribution of
precipitation; (b) the size, shape, cover, and topography of the catchment area; and (c) the
nature and condition of the ground. Some of these factors are constant, others vary season-
ally. Generally speaking, conditions that tend to promote high surface runoff—high rates
of rainfall, steep slopes, frozen or bare and heavy soils, for example—are also conditions
that tend to reduce dry-weather flows.

11.7.3 Runoff from Snowmelt
Streams fed by snowfields and glaciers possess certain unique runoff characteristics
because of the following subsidiary factors: (a) heat melt, (b) condensation melt, and
(c) rainfall melt. Fresh, clean snow reflects about 90% of the incident sunlight. Warm,