The Effects of Hydraulic Conductivity on Growth

Gretchen Carlson

October 31, 2007

Exercise 2

The Effects of Hydraulic Conductivity on Growth

Statement of the Problem

The investigation explores how changes in the surface hydraulic conductivity (k), the decay of hydraulic conductivity with depth (m), and the soil depth alter the stand growth, streamflow, and nitrogen uptake.

Experimental Setup

All model runs were performed using RHESSys files for Coweeta, NC. During 5 simulations, hydraulic conductivity, both at the service and as it decays with depth, and soil depth were altered. The initial run maintained default levels and the latter runs adjusted 1 parameter each run. Each run began on October 1, 1999 and ended September 3, 2003. Results were analyzed by monthly averages. The table below shows the multiplier during each run for the 3 variables.

Worldfile = world_ws18.Y1999M9D30H24.state

Tecfile = tec1999 -g -st 1999

Run / M / K / Depth
1 / 0.417 / 211.8 / 1
2 / 0.417 / 300 / 1
3 / 2.0 / 300 / 1
4 / 2.0 / 300 / .5
5 / .5 / 300 / .5

Results

The first 2 runs, with the default variables and increased K, produced similar results for LAI , PSN, uptake, and total stream outflow with predictable trends of increasing productivity in the summer months and no productivity during winter months. Both runs also showed increased levels of stream run off during the spring and early summer months as precipitation increased. The year 2003 showed significantly more stream runoff as overall precipitation increased for the year.

Using run 1 as a standard to compare the other runs, both runs 3 and 4 showed the same pattern suggesting that the change in the soil depth did not play a significant role on the productivity or streamflow. Only runs 3 and 4 showed increases in de-nitrification and nitrification as shown in figure 6. Both runs showed a lower value of LAI in the growing months as seen in figure 1. Productivity shows significantly more variability during runs 3 and 4 with dips throughout the summer months as productivity to fluctuated as shown in figure 2. These same fluctuations are observed with the nitrogen uptake in figure 3. Figure 5 demonstrates the relationship between streamflow and m. As m increases, streamflow stabilizes. After the spinup balances the values, there is a continued decrease in streamflow despite and more precipitation in 2003.

The final run examines the same conditions 4 with only slightly increased m. This shows similar trend as runs 1 and 2 in all figures with slightly less LAI, productivity, uptake, and streamflow.

Figure 4 explores the relationship between PSN and uptake and shows a strong positive relationship. This reflects the similar trends in uptake and PSN in figures 2 and 3 for all runs.

Conclusions

• These 5 model runs demonstrate the relationship between thedecay of hydraulic conductivity with depth and productivity and streamflow. As m increases and the water table becomes lower, streamflow is modulated by providing a steady flow to streams. In both runs 3 and 4 the water table is likely below the drainage.

• Increased m also leads to decrease in productivity and LAI as the uptake of Nitrogen is affected by hydraulic conductivity.

• Small increases in K did not alter the productivity significantly; however increasing k by an order of magnitude would likely have more influence on the uptake of Nitrogen and productivity.

• The dependence of LAI and productivity to precipitation is not clearly shown in these figures. It is surprising that productivity does not increase during 2003. There may be a relationship the following year as the ground water table is finally restored.

Figure 1: Leaf Area Index for each model run. Notice run 3 and 4 show decreases in LAI

Figure 2: Net Photosynthesis for each model run. Notice run 3 and 4 show lower values

Figure 3: Vegetation Nitrogen Uptake for each model run. Lowest values are for runs 3 and 4.

Figure 4: Nitrogen Uptake vs PSN for runs 1 and 2. Both plots show a strong correlation between uptake and psn.

Figure 5: Total stream outflow for each model run. Notice the stable runoff production for run 3 and 4.

Figure 6: Denitrification for runs 3 and 4. These were the only 2 runs with denitrification.