What is the fine-grained topic of the following text: This 4 Panel graph shows the effects of varying input parameters -- effective tile drain diameter, depth to tile drain from surface, depth to impervious layer, and hydraulic conductivity correction factor -- on the objective function drain discharge in a 2D, variably-saturated flow model. Only return the topic.
The effects of varying input parameters on drain discharge in a 2D, variably-saturated flow model.