Joshua Gray
September, 2007
Introduction
The experiments outlined in this brief summary sought to investigate the eaffects of different soil types and depths on a number of different hydrological and ecological variables including: soil water content, leaf area index, evapotranspiration and outflow.
Experimental Setup
The BIOME-BGC for Excel model[A1] was used for the investigation. Meteorological data for the study area, Chapel Hill, NC, was obtained for a 25 year time period (1980-2004). This data included temperature (daily max/min), precipitation, solar radiation and vapor pressure deficit. Initialization files were constructed to simulate different soil conditions as summarized in Table 1:
Table 1
Simulation / Soil Depth (m) / % Sand / % Clay / % SiltSim.1: loam / 1.0 / 50 / 10 / 40
Sim.2: loam / 0.5 / 50 / 10 / 40
Sim.3: sand / 1.0 / 80 / 10 / 10
Sim.4: sand / 0.5 / 80 / 10 / 10
Sim.5: clay / 1.0 / 20 / 70 / 10
Sim.6: clay / 0.5 / 20 / 70 / 10
The spin-up functionality of the BIOME-BGC model was used to create a restart file for which the carbon variables were equalized[A2]. This restart file was used for all subsequent simulations. The model was used to simulate changes in the state variables over the 25 year meteorological record.
Results
Soil properties had a significant affect on soil water content, and, by consequence, leaf area index (Figures 1 and 2).
Figure 2 shows the simulated soil water content for the year 1980. Loamy and sandy soils 1m in depth both had higher soil water contents than their 0.5m deep equivalents, with loamy soils having a higher amount of soil water than sandy soils of the same depth. These results, along with the simulated outflow, indicate that loamy soils do not drain as quickly as sandy soils, and produces less outflow as a result. Figure 2 illustrates that the soil water content of both depths of clay soil was much higher than both loamy and sandy soils, and also that the 1m clay soil was much wetter[A3] than the 0.5m clay. This result is not surprising when we consider the small pore sizes characteristic of clay soils. In these small pore spaces the hydrostatic forces holding the water in place are much higher than those found in the much larger pores of loams and clays. Consequently, although there is higher soil water content for clays, less of that water is available for extraction by trees for growth. This effect can be seen in Figure 1 which shows lower LAI’s for the clays when compared to both sandy and loamy soils. Figure 1 also shows that higher LAI’s could be expected in shallower clay soils, even though there is less overall water when compared with the deeper soils[A4]. Presumably, this is due to the fact that the smaller amount of water found in shallow clay soils is more available to trees for growth than the water deep in the 1m clay. Another interesting observation from Figure 1 is that the 1m sandy soil supported the highest LAI, next the 0.5m loam, then the 1m loam. Again, this is most likely a consequence of the relationship between soil water content and how tightly that water is held in the soil[A5].
Conclusions
In general, clay soils support higher water content at all depths than do loamy and sandy soils. The depth of loamy and sandy soils is an important variable for determining soil water content, but higher water content can be found in loamy soils than sandy soils of the same depth. We can also conclude that soil water content alone is not indicative of the amount of water available for tree growth. For example, though clay soils generally have high water content, most of this water is held too tightly by hydrostatic forces to be utilized for growth.
It can be concluded that both depth and soil texture play important roles in the hydrologic regime of a forest. It is also clear that neither texture nor depth alone is sufficient in predicting variations in soil water content and outflow.Simulated evaporation and transpiration showed very little variation among soil types and depths. This is somewhat surprising considering the differences in leaf area index, that is, how much leaf is available to transpire water, and variable soil water. This phenomenon warrants further study[A6].
[A1]Give version number for completeness
[A2]Equilibrated, not equalized. Which soil was used for spin-up?
[A3]Deeper soil holds more water – but expressed as a SWVC, they may be much more similar
[A4]Need to see what vol water content and soilpsi is
[A5]Graph soilpsi
[A6]Need more than the one set of daily graphs to explain what you are seeing – specifically vol. soil water, soilpsi and ET would be useful to synthesize an explanation