Encountering with loop error in PSpice

Hi
I encountered with following error in simulating the circuit shown below in PSpice. 

ERROR -- Voltage source and/or inductor loop involving E_U2
You may break the loop by adding a series resistance

Could any one help me please?

http://uupload.ir/files/8xbv_opamp1.png

You need to clean up the schematic for a start, you have nodes that are unused(remove the unused tie points on the wires), resistors and voltage sources shorted, etc.
You also have opamps driving each others outputs, which is not OK.
If you wish, for example, to add the outputs, then you need a summer, I think you need to look at a bit of opamp theory before you blindly start using spice, Spice is capable of doing stupid things at the best of times, even when everything is wired correctly,
cheers,
Richard

Thanks for your time and consideration.
Yes! you're right. maybe as I am a hydraulic student, I've designed my circuit somewhat blindly. I'll try to re-design it with summing opamps.
regards,
Atousa

I notice that you have a lot of RC time constants on the inputs. My question is, are you needing intergrators, if so, you can put then around an op amp as well.
It would help if I knew what you were wanting to achieve. Are you, for example, trying to set the whole thing up as an analogue computer,
cheers,
Richard

I am trying to simulate the rainfall-runoff process in a basin that is divided to five sub-basins connecting each other as shown. The voltage sources are as the rainfalls.

Dear Richard,

I used summing opamps, and the output is as shown in image. Would you please tell me why two of these voltage generators (V1 and V2) show negative values whereas the specified values for all of them via text files are positive.

Rigards
Atousa

http://uupload.ir/filelink/Qd1ZhCoodloW/qx10_schematic1_page1.pdf
http://uupload.ir/files/xar6_output.png

your summing amps are wrong, you have positive feedack around the op-amps.
Look up summing amplifiers, taking special note of the configuration of the inverting (-) and non-inverting (+) inputs of the op-amps.
There can also be a common mode voltage problem unless you use inverting summing amplifiers, you can put an inverter on the output of the summing amplifiers to get the voltage back to the way you would expect it. The common mode problem means that you should use inverting amplifiers for every amplifier, and use inverters to get the sense of the voltage correct at the inputs and outputs of the various summing amplifiers.
You also have a number of the voltage sources around the wrong way(V1 and V2), I assume you want the positive outputs driving the amplifiers. In some cases you may need a negative sense on the voltage source if you use an inverting summer. However, inverting amplifiers are "cheap" in a simulation, so keep the voltage sense +ve and use an inverter with a gain of one where necessary, I have appended a file of at a quick glance what I think the system should look like,
cheers,
Richard