PART VII — WATER RESOURCES, FORESTRY, AND AGRICULTURE 



others have claimed that this estimate 

 is greater than the true resistance 

 within a canopy. Presumably, this 

 question can be resolved by fluid 

 mechanics, energy budgets, and the 

 new porometers. 



Diffusivity Within the Canopy — 

 This is harder to measure. At present 

 it is estimated by measurements 

 of radiation absorption and tem- 

 perature and humidity gradients. 

 The method is susceptible to error, 

 produces estimates at variance with 

 the wind speed and employs the very 

 temperatures and humidities that one 

 would like to predict. A new method 

 of estimating diffusivities within the 

 canopy is required but none has yet 

 appeared. 



Microclimatic observation will un- 

 doubtedly continue in the future. If 

 the observations are to be most useful 

 in testing and improving our under- 

 standing, they should include the ver- 

 tical variation in leaf area and poros- 

 ity as well as radiation, temperature, 

 humidity, and ventilation. Since this 

 makes a formidable list of equip- 

 ment and tasks before a complete, 

 and hence worthwhile, set of ob- 

 servations can be made, microclimatic 

 and evaporation studies seem ideally 

 suited as testing grounds for coop- 

 erative or integrated teams of sci- 

 entists. 



Horizontal Heterogeneities — The 

 final remark concerning the aerial 

 portion of the problem must concern 

 horizontal heterogeneity and advec- 

 tion. Chimneys and sun flecks among 

 the foliage clearly render our ideal, 

 stratified models unrealistic. There- 

 fore, efforts to incorporate these 

 heterogeneities into the analysis are 

 welcomed, even if they only prove 

 that the ideal, homogeneous model 



gives the same average evaporation 

 and microclimate as the realistic 

 model. 



The larger heterogeneities con- 

 noted by "advection" are known to 

 be important, justifying the term 

 "oasis effect." Advection of carbon 

 dioxide has already been treated 

 simply in a photosynthesis model, 

 and incorporating large-scale advec- 

 tion into the existing evaporation 

 models seems manageable and worth- 

 while. 



Water Storage in Soil 



The transport of water to foliage 

 from soil has not yet been mentioned. 

 Relatively less can be said about it 

 in a systematic way. As a comple- 

 ment to the simulation of evaporation 

 from foliage, we need a comprehen- 

 sive simulator of this portion of 

 the path of the water that will tell 

 us how much water gets to the leaves 

 and, more important, how stomatal 

 resistance is modified. The simulator 

 concerning soil and plant is more 

 lacking in foundation than one con- 

 cerning plant and air. Nevertheless, 

 beginnings have been made by Cowan 

 and Raschke. 



Gaps in Scientific Understanding — 

 These primitive simulators reveal se- 

 rious deficiencies in our understand- 

 ing of (a) the relation between water 

 potential in the leaves and stomatal 

 resistance; (b) the conductivity of 

 different root regions; and (c) the 

 conductivity between soil and roots. 

 This last matter includes the dif- 

 ficult problem of root distribution 

 through the soil profile. The actual 

 storage capacity of the soil and rela- 

 tion between potential and content 

 seem fairly well established. The 

 effect of changes of temperature in 



time and depth is yet to be coped 



with. 



New instruments usable in the 

 field should help. The new porom- 

 eters have been mentioned already, 

 and the Scholander pressure chamber 

 promises to reveal water potentials, 

 even in roots. We are still left, how- 

 ever, to search for root distributions. 

 In the case of temperature differences, 

 on the other hand, the problem is 

 to improve our logic rather than our 

 observations. 



The next problem is the escape of 

 water from soil storage via a moist 

 surface or by leaching rather than 

 through vegetation. These two es- 

 capes greatly affect the loss from the 

 root zone of salts and nutrients that 

 pollute the water below. Evaporation 

 and land leaching from the soil have 

 been measured carefully in bare soil, 

 but the present challenge is to under- 

 stand the parameters sufficiently well 

 to estimate them when a canopy of 

 foliage is also removing water. This 

 is a fundamental problem of the 

 movement and loss of water from a 

 heterogeneous porous medium with a 

 variable and heterogeneous tempera- 

 ture. The research of the past has 

 not brought us a lucid understanding 

 of the system; at present, progress 

 seems most likely to come from de- 

 vising a better logical framework on 

 which to hang our measurements. 



A Final Word 



The reader may have noticed that 

 time has not been mentioned. That 

 is, analyses or simulators of an in- 

 stant only have been described. In- 

 tellectual satisfaction and eventual 

 utility requires that our understand- 

 ing and predictors be extended 

 through time, with the storage of 

 plant and soil as parameters. 



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