SWAT - overland water balance, irrigation efficiency and surface runoff ratio

[Umesh]

Hello SWAT users,

I have not seen many people calculate the SWAT overland water balance.

In this project, I am using auto-irrigation method to supply irrigation from shallow aquifer to agricultural areas in a subbasin. I use soil moisture content as my trigger. Irrigation efficiency is 100%. I use different irrigation surface runoff ratios as shown in table below. I have calculated all the inputs and outputs as summarized below. All units are in mm of H2O.

I also calculated the shallow aquifer water balance as shown below. Baseflow is the shallow aquifer discharge. Recharge is calculated as total aquifer recharge minus deep aquifer recharge.

Here are my questions:

1) How can we calculate overland input/outputs? Is there anything missing on the overland water balance as shown in table above?

2) SWAT only reports the net amount of irrigation water (net-irrigation) applied that does not include the amount of water that is converted to runoff (irrigation-runoff). If we estimate the irrigation-runoff of from net-irrigation using the runoff ratio, the numbers do not add up.

3) Looking at the shallow aquifer water balance, when runoff ratio is increased, less amount of water is withdrawn from the shallow aquifer. However, the shallow aquifer is still balanced. Where does the water that is converted to surface runoff come from?

I have done similar studies for irrigation from deep aquifer, reach and reservoirs. When the irrigation surface runoff ratio is not zero and irrigation efficiency is not 100%, the numbers do not add up.

If anyone has done similar study, please share your thoughts.

Thank you

Umesh