Building earth dams in Tarragona, Spain

Hola from Spain!
We are on the hunt for a finca to live on and have a few on the short list in Tarragona province. I understand the region is very arid for most of the year with occasional rain events so we are looking at building a series of dams on the terraces. Quite by the way, we were looking for an above ground swimmimng pool and I came across several articles that said you need to apply for planning permission to erect one as it's considered a "major project". This got me wondering if there were rules about putting in dams. Anyone know?  
The legalities of buildings is another issue with many of the old casas not on the register and no new building allowed, does anyone know if building underground like the old cave houses requires planning? My concern with building anything that involves hiring an earth mover is that the villages are small communities. As foreigners, we stick out and may be the subject of gossip and anything we do becomes common knowledge -which in turn alerts the local planning office. I'd really like to do things by the book, but a) it may not be allowed if you are only allowed so much major work/acre/finca and b) can get expensive getting an architect to draw all the plans.

Also, we are looking for ideas and contacts for local resources for mulch-straw, bark chip etc. Anyone with a mini digger? Plant nursery? Stables, dairy where we can get manure?

I look forward to hearing from you

Hi Sarah,
me and my partner are on a similar situation, we started seriosly looking for a finca in january, so I can tell you what we've learned so far:
1) I don't know especifically about putting dams, but count on needing permits for everything xD
2) Any building better to do it legally xD (just to avoid having to take it down later, aines and so on) which means with permits again. It is possible to do some building but they are more strict now than they used to be, so it may be better to find an already existing building or something to restore (keep in mind that restoring something will come with its needs for permits again)
3) Another problem we have encountered is that you are going to have difficulties registering in a finca, there may be some municipallities that make exceptions, but generally you cannot register in a finca (rural land) unless it has a masia which is in a catalog (in which living, turism and a bunch of other uses are allowed). Those masies are a lot more expensive to buy. An alternative to this is to stay registered in your country of origin, or to ask someone you meet in spain to register in their place, and then live in the finca.
4)For manure, do not worry, that can be gotten cheap and even for free probably everywhere, there are plenty of stables and people with horses around (I think). For bark-chip/woodchips at least for now I haven't found any good resource, they sell them in places to be used in fireplaces or something like that, but it isn't super cheap if I recall correctly.
Personally, we have now decided to check if we can get more people involved in our project to be able to buy a masia and go from there because we want to do things properly and legally.

Hope that helped, we can chat more if you want

Regards,
Aina