Fatih I wouldnt give up on watershed just yet,while I am far from one of the forum experts I do have a bit of experience close to your region.My wife is from Bulgaria and we spend 6-8 months a year here.
I've fished quite a few times with my wife's uncle in Cyprus and the reservoirs there receive about the same rainfall(some 100mm more others 1-200mm less per year) as your region,although they are in the mountains for the most part they do well.
Also I have a friend who is from southern Bulgaria on the border with Turkey,we usually visit his village 2-3 times a year to go fishing.While we usually focus on the river there are a few man made ponds that even held water and fish this year,one of the driest on record without using a well.The ponds where built there because they couldnt use the land for farming.
As far as using clay to seal the pond,I know of one fish farm here in BG that used shotcrete(sprayed concrete)instead of clay as none was available to seal their ponds................granted they used EU funding for the project,but there are other ways to do things.
one last thought,if you don't already own the land,I will add every pond I have scene in BG is in a place that was less then ideal for farming..........To me it seems like they put ponds to raise fish where they cant easily farm if that make any sense.A lot of times land with little value for farming but might be viable for a pond is not listed for sale but might be available through some digging and at a good price.
