698 



PLANT GROWTH AND PLANT COMMUNITIES 



The potential may be expressed as specific free energy, as aqueous 

 vapor pressure, as the pressure or suction in the Hquid phase, or as the 

 DPD ( di£Fusion-pressure deficit) of the hquid phase in plant tissues. 

 The curves in Figure 1 show that the volume-fraction of water is highly 

 dependent on the potential, and that the texture of the porous material 

 strongly affects the relationship. The latter effect results because of the 

 effect of grain size on the size distribution of the voids that occur be- 

 tween the particles. The relationships shown in Figure 2 arise because 

 the water flow occurs only through the water-filled voids; hence it fol- 

 lows that the transmission coefficient of unsaturated soils also will be 

 highly dependent on the volume-fraction and on the potential of the 

 water. The capacity of an unsaturated soil to serve as a source or as a 

 sink for moisture is summarized in Figure 3. The curves show that the 

 capacity to absorb or yield water in response to a unit change in po- 

 tential is highly dependent upon both the texture and the potential. 



The fact that C, k, and S is each markedly influenced by changes 

 in the potential of the fluid being transmitted greatly increases the 

 complexity of the flow equations needed to analytically describe fluid 

 flow in unsaturated porous media. The problem is made even more 

 difficult if the defining equations are also time- or space-dependent, as 

 is the case for transient flow or flow through anisotropic media. A 

 further complication is introduced by the fact that the relations shown 

 in Figures 1, 2, and 3 all exhibit hysteresis; consequently, different 

 equilibrium values of the dependent variables C, k, or S are obtained 



MOISTURE CONTENT 



SATURATION 



Figure 2. Idealized curves showing the effects of texture and moisture con- 

 tent on the transmission coefficient. 



