General Comments

In their manuscript “Extending the SWAT model to transport chemicals through tile and groundwater flow” Rathjens et al. et al. describe the implementation of chemical transport for groundwater and tile drainage in the Soil and Water Assessment Tool (SWAT). This functionality can be of interest for researchers involved in transport modelling on landcape-level, for example, for academia, authorities, environmental agencies, water suppliers and other stakeholders. Therefore, the topic is appropriate for Hydrology and Earth System Science since this kind of research contributes to the understanding of hydrology and transport in the environment.

However, some points have to be considered before the manuscript can be published. Specifically, the calibration for the exemplary studies sites and how parameterization interacts with the modifications made have to be explained in more detail. This would add value to the justification of the modifications. 

Therefore, minor revisions are suggested. Please find more detailed comments in the following paragraph.

Detailed Comments

Application (chapter 3; lines 171 ff): It would be desirable to get more detailed information on the calibration process and the selected parameters:

1. Are both model parameterizations (with and without modification) similar for each test site? Please clarify.
2. What are the parameters involved in the calibration? What is their impact on groundwater and drainage contribution to streamflow? Please indicate in more detail with specific focus on code updates made.
3. A “multi-metric” calibration is mentioned. Besides visual inspection, was there any weighing used between streamflow and concentration metrics for calibration or, resp., a multi-objective calibration?

This manuscript is well written and suitable for publication in HESS. Tile drains and potential groundwater flow to streams are two important pathways in many agricultural watersheds. Adding these processes to the well-established SWAT model will undoubtedly expand the model utility for watershed managers, researchers, and ag-engineers and practitioners in the farming community to develop and assess best management practices and stewardship programs that support sustainable and more environmentally friendly agriculture. Two specific comments are below.

1. Section2.4 Lines 110-115. Was the general term "soil profile" defined as the same as "root zone" or the layer between the bottom of the root zone and shallow ground water? A clear description of the "soil profile" is needed to understand Eq 5.
2. Section 3 Lines 165-170. The author(s) should provide the specific values of the pesticide use rate and basic environmental fate parameters such as soil half life and Koc for both parent and metabolite. It is disingenuous by only providing qualitative descriptions of "readily degradable" or "moderate", etc., unless the model can take such qualitative inputs for a simulation.
3. Section 3 Line 171. Did the "multi-metric calibration” include pesticide use and fate parameters of the parent and metabolite? if yes, what are the final calibrated values?