The image above is a screenshot from a part of NASA's Treasure Map for Water Ice on Mars.
Within 'the only two regions' I mean the uppermost parts of the two coloured extensions to the north into the Hellas basin.
Could not a ditch onto the subsurface ice on a north facing slope in both regions let the water flow downhill into a natural or artificial basin in the southern summertime ?
I think it could, because:
- The image shows a blue/green region in the east of Hellas basin which means there's an ice layer there that starts at 0.3 to 0.6 m below the surface.
- That region lies below the region in Gale crater where the REMS pressure sensor measured the values in this diagram from which we can conclude, when using the Steam Tables calculator, that there's enough pressure in that region to have liquid water up to at least 5⁰ C.
- With the aid of the JPL Horizons website I calculated that at Dec.1, 2012 (sol 115) the max. solar radiation at Gale crater was 685 W/m², and on this diagram we can see that on that day the max. ground temperature was about 10⁰ C.
This image shows that in the southern summer season for the north facing slopes on two craters in the blue/green region in east Hellas basin the max. solar radiation was even higher, so with the same thermal inertia of the surface the ground temperature would be well above zero there.
Such high solar radiation values are not possible for the subsurface ice regions in the northern hemisphere because Mars is near aphelion in northern summertime, so no downhill flowing water there, not even on the steepest south facing slopes.
11 degrees was the steepest I could find for a north facing slope of an upper crater's rim in the blue/green region with Mars Trek, likely because the elevation tool is just not accurate enough, so it could be that some smaller craters have steeper slopes.
A problem could be the reflectivity of the ice causing less heat available to melt it, but when digging the ditch to the level where the ice layer starts, a thin dust layer on it can be left that would much help in warming it up.
The great advantage over digging for the ice in a horizontal layer will be that the ditch dug on a slope would naturally grow deeper and deeper (and possibly wider and wider ?) into the ice layer when water would flow through it.
Image: NASA/JPL/University of Arizona
Part from ESP_017287_01325
Click on the image, and then again, for a closer view.
Indeed, it looks like there's subsurface ice in this region.