Ive been thinking of doing this for a while... have some huge bush around 2hrs drive with perfect places for a set up like this.
A few things i would advise on... dont use a car battery, you need a deep cycle battery... get 105AH or so.
Also get a larger volume pump... say the sureflo 13gpm 55psi
Put your panel and pump on the river side so there is like a 10mtr or so run into the river.
Run the pipe up to your plants and have a holding tank that all the water is pumped to, then your standard hose timer from your tank to drippers or soakers on your plants.
At your solar end use a larger panel if you want to hide it, green shade cloth on a small 63w panel should still allow at least 10w of power to run, or place your panel (depending on how the sun is in your area) under a tree so either only morning or evening sun will hit it.
Still even if a 63w panel is under a tree and hidden from the air you should still get enough power to keep your battery topped up.
At your battery you will need a 12v timer and a solar controller to stop the battery over charging.
then just run the pump for say 10min every morning should get a 55gal drum filled no problem and soak to your plants over an hour or so.
If you were really serious about doing stealth solar you could design and make your own panels.
each cell is 0.5v and have 36 in series to make 18v to charge a battery... this is so in low light it will still put out 13-14v to keep charging a battery.
So you basically want to design a low light panel... if you ran 40 cells in series that would make it 20v output so it would be a higher voltage output in a lower light situation and extra charging amps from the added cells.