Cistern size and wastewater fee

At first it seemed like a lot to me, then I did the math. It's only about 10 liters per square meter per day and for the current hot weather (or is it different in northern Germany for you?) not that much anymore.

It was freshly laid sod, not watering would have meant disposal. And I'm here in the middle of NRW (Münsterland). Or is that already north for you ;). And it was insanely hot and dry in the meantime (well, at least after today's heavy rain I don't need to water).

Viewed from here, Northern Germany starts roughly at Stuttgart ;) You haven't watered that much either, I just looked it up and at those temperatures about 20-25 l/m² per day are recommended. In any case, I still have to come up with something to ensure irrigation during the germination phase. A bit more than the cistern (not enough), "finished by Easter" (probably won't work ;) ), and hope that 2022 will be as rainy as 2020.

The 8-9m3 concrete cistern probably won't work now. Unfortunately, I can't connect the entire roof area. Now we are wavering between a flat tank or a round concrete cistern. Now I have a comprehension problem: The flat tank has the inlet about 20cm from the top edge of the shaft. Total height about 1m. But if I connect the supply line from a rain downpipe, I first have to go about 60cm into the ground and then continue there with a slope of 1-1.5%. That means I also have to install the flat tank deeper by this height difference? Thus, the flat tank would have to be installed at least 80cm deep in my case, which relativizes the supposedly "simpler" installation again? Or am I overlooking something? Currently, I even have a larger excavation with a flat tank. The flat tank lies deeper and I have to cover it with a lot of earth again, whereby a minimum overburden applies here because it is not as stable as concrete. Only one option remains: the flat tank as close to the downpipe as possible. Unfortunately, we have 4 downpipes + 2 garage. I don't think a flat tank will work there either...

Why do you have to go 60 cm into the ground?

Unfortunately, I can’t connect the entire roof surface.

What exactly is the problem? Too many connections? Ideally, you plan the cistern so that it is located at a lower point of the terrain but close to the buildings. We have it relatively centrally between the house, carport, and garden shed. The pipes can then be combined, where possible you use Y-pieces so that you don’t have to lay multiple lines in parallel. Since the water flows, you don’t need a frost-resistant depth and can lay it a bit higher.

Why do you have to go 60 cm into the ground?

I wasn't quite clear-headed yesterday. It was about the frost-free depth, but also about the fact that the pipes are somewhat overbuilt and so I don't damage anything while compacting.