RCES 



Figure VII-2 — THE HYDROLOGIC CYCLE 



\d, 



EVAPORATION FROM THE SEA 

 367 



Domestic and 

 Industrial needs 



(Units of measure in cc) 



S — Surface Runoff 

 P — Percolation 

 U— Uptake 

 R — Residual 



This is an idealized version of the water cycle. The numbers attached to the various 

 processes are relative units of measure. Note that the truly important parts of the 

 cycle are evaporation from the sea, precipitation, and evapotranspiration. 



1956, Penman showed how evapora- 

 tion from abundant foliage suffici- 

 ently wet to have wide stomata could 

 be calculated from the net all-wave 

 radiation available above the canopy. 

 The recent advance is, therefore, in 

 understanding how foliage condition 

 can decrease evaporation below Pen- 

 man's potential and how the evapora- 

 tion and consequent temperature and 

 humidity within the canopy are 

 changed. The total evaporation from 

 the canopy, according to our new 

 understanding, is affected profoundly 



by the leaf area and, more subtly, but 

 still considerably, by the stomatal 

 conductivity or porosity of the foliage 

 for water. 



Future Observations and Experi- 

 ments — This understanding has been 

 arrived at by means of mathematical 

 simulation. To make a substantial 

 improvement in our understanding — 

 or even to test our present under- 

 standing — future measurements of 

 evaporation from crops and trees 

 must include observations of leaf 



area and porosity as well as weather 

 and evaporation. Fortunately, since 

 the invention of a simple, portable 

 porometer by Wallihan in 1964 and 

 the subsequent calibration of several 

 modifications, porosity can easily be 

 measured. 



Earlier hydrologic observations sug- 

 gested that different vegetation con- 

 sumed different amounts of water 

 in evaporation. The simulators men- 

 tioned above, along with experiments 

 with sprays that shrink stomata, have 

 now established that evaporation can 

 be changed by modest changes in 

 the canopy. During the coming years, 

 therefore, one can expect a variety of 

 experiments seeking the most effec- 

 tive and least injurious ways of con- 

 serving water in the soil through 

 treating or modifying the vegetation. 



Microclimatic Measurements 



Turning to the distribution of 

 evaporation, temperature, and hu- 

 midity within the canopy — in con- 

 trast to the sum of evaporation 

 discussed above — one finds that a 

 greater number of parameters can 

 be effective. The changes in tem- 

 perature and humidity along the path 

 conducting water and sensible heat 

 out of the canopy depend on the 

 boundary layer around the leaf and 

 the turbulence of the bulk air within 

 the canopy. These two factors gen- 

 erally are of smaller magnitude than 

 the stomatal resistance and hence 

 are relatively ineffective, we believe, 

 in changing the sum of evaporation. 

 However, when we turn to the 

 distribution of temperature and hu- 

 midity within the canopy — the mi- 

 croclimatic question — these param- 

 eters are influential. Scientists do 

 not yet know how to measure them, 

 however. 



Boundary-Layer Resistance — For- 

 merly, this was estimated from a 

 conventional fluid mechanics equa- 

 tion, employing the square root of 

 leaf dimension divided by wind 

 speed. Recently, however, Hunt and 



201 



