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358 Chapter 11 Hydrology: Rainfall and Runoff

Snow
Precipitation

Evaporation from
Transpiration land and water surfaces
Infiltration
Surface runoff
Groundwater table (G.W.T.)
Spring
Percolation Swamp
Lake River
Groundwater Ocean
G.W.T.

Runoff or stream flow surface runoff
and groundwater runoff

Figure 11.1 The Water Cycle

11.2 COLLECTION OF HYDROLOGIC DATA

Without adequate quantitative information on Earth’s water resource, its use and develop-
ment become an economic uncertainty and an engineering gamble. The collection of perti-
nent data is, therefore, an imperative social responsibility that is generally assumed by
government. When and to what extent the government of the United States has accepted
this responsibility is summarized in Table 11.1.
Of the different hydrologic parameters, annual precipitation is a measure of the max-
imum annual renewal of the water resource of a given region. About one-fourth to one-
third of the water falling on continental areas reaches the oceans as runoff. The balance is

Table 11.1 Nationwide Collection of Hydrologic Data for the United States
Beginning Number Areal Density
Measurement and Apparatus Date of Stations per 100 mile 2
5
Rain and snow 1870 10 4
Nonrecording rain gauges 2 ⁄3 of total 2.7
Recording rain gauges 1 ⁄3 of total 1.3
Storm-tracking radar 1945 — —
3
Snow courses 1910 10 annually
Stream flow
4
Stream gauges 1890 10 0.5–11
Groundwater 1895 — —
Evaporation Several hundred
2
Conversipon factor: 1 mi 2.59 km 2