Lecture 1: The Hydrologic Cycle
Outline
Elements of the hydrologic
cycle (Fig. 15.5)
- evaporation from the ocean (~455 cubic km per year)
- residence time of water in the ocean (avg. evaporation rate 125 cc/sq.
cm; avg. depth 3900m)
- transport in the atmosphere (~46 cubic km per year to the land)
- precipitation (~409 cubic km per year back into the oceans) (~108 cubic
km per year onto land)
- evaporation from the land (~62 cubic km per year)
- transport by rivers (~46 cubic km per year back to oceans)
- transport by ground water
Water - the Miracle Substance
- Where did it come from?
- What makes it special?
- Where is it? (Fig. 15.3)
Evaporation from the ocean
- The evaporation rate varies with latitude
Transport in the Atmosphere
- Winds transport moisture (Fig. 18.19)
Precipitation
- Precipitation varies with latitude: see global distribution of deserts
for example (Fig. 18.20)
- Pattern over land not uniform (Map) due to
topography (Fig. 18.21 showing rainshadow effect)
- Pattern of US has maxima in northwest and southeast (Map)
Evaporation from the land
- Not all water winds up in rivers
- Relative proportion of runoff and
evaporation depends on temperature and can be quantified mathematically:
Transport by Rivers
- rivers come in all sizes, and form networks (Overhead Fig. 10.3 from
another book)
- Region drained by a river network called a drainage basin
- River's velocity, depth, and width vary regularly with its discharge
- see overhead
- Lag time between runoff and rainfall (Fig. 1, Perspective 15.2)
- Hydrograph as record of runoff (Fig)
- Floods have definite statistics (Fig. 2, Perspective 15.2)
Transport by ground
water
- Flow of ground water depends of permeability
and water pressure (head)
- flow velocity = permeability * (difference in head)
- Surface flow down slope of water table
- Deep flow channeled by permeable rocks into aquifers (Fig. 16.12)
