I will measure the rate of photosynthesis by counting the number of bubbles that are released by the stem of the pond-weed when the lamp is shone on it from different distances.
Prediction : I predict that as I increase the distance between the light source and the Canadian Pondweed (reducing the light intensity), the volume of oxygen produced within the time limit (the measure of the rate of photosynthesis) will decrease.
This is an interesting project. You have identified 2 critical variables in photosynthetic rate. Two others would be CO2 availability and plant species. This second variable is what complicates your question. Some plants can use a whole lot of light effectively. Other plants have evolved in shady conditions and can use only a relatively small amount of light before photosynthetic rate levels off.
I suggest that you measure this yourself. Try using an aquatic plant like elodea (any aquarium store will have it). Expose it to various amounts of light. You can measure it with a light meter or just use distance from the light source as your variable. You can measure photosynthetic rate by counting oxygen bubbles because the bubbles will all be about the same size, or you can capture the oxygen and measure the volume.
Adding different amounts of bicarbonate (baking soda) will allow you to see how adding CO2 will influence rate of photosynthesis. First boil and cool distilled water to drive off dissolved CO2 so that you can control this.
Obviously, I have not answered your question for dealing with the amount of water. This actually is a more difficult thing to test because measuring the oxygen produced is harder when you are not using an aquatic plant.
If you just want numbers to plug into your model, take a look at this scientific paper:
Look at figure 1 on page 3.
The water content is on the X (bottom) axis, while the Y (up-and-down) axis shows the rate of photosynthesis (measured by how much CO2 the plant took up). Note that the 2 different species responded differently. You can also see that there is a lot of variation, but you can use the average photosynthetic rate for each water level. The scientists also looked at the effect of CO2 and temperature. Temperature would depend on light intensity, so you could use that in your model.