My land, el Herrén del Cerrado, is in the mountains near Madrid (Spain). We have a tiny valley formed by the meeting of two tiny, soft hills. The valley collects water for nearby lands which means that when it rains, it's got water running through the center (we have Ranunculus sp.) but on the sides of the hills the soil is dry and has a lot of erosion. For pictures, go to: http://unasuertedetierra.blogspot.com.es/2014/06/erosion-en-el-prado-grande.html.
I want to do some kind of intervention to spread that water to the hills but I have no money to do keyline.
So, I've decided to so something line the stone lines they do in Burkina Faso but with tree trunks we have on site. Instead of placing the trunks on level, I want to place them so the cut the level line. I think some people here have done this with swales.
My doubt is about the % deviation form the level. When you build a deck, here in Spain, they recommend 2 % inclination so water runs off the deck. I've taken this value as reference.
However, I do not know if with 2% inclination water would travel too fast to infiltrate the soil. Should I use 1%?
My first question is why are you choosing not to place them on contour? If your purpose is to get more water soaked into the ground and less erosion and runoff to the valley bottom , that is what swales on contour do.
we will not be doing swales because we barely have soil over the mother rock, that peaks here and there. So, there is no soil to soak water in and when it rains, it quickly saturates and runs off the mother rock off the property. Also, it is very hard to excavate a ditch without meeting rock every few feet.
As for contour, we are going to do that in places where there are hills. But here two hills meet, making a small valley. This valley collects water from other properties so the center has running water all winter. This water is not coming from the hills on both sides, but from the other properties. It's a lot of water. The soil is in anaerobic conditions in the winter, but it dries in the summer. I want to spread that water to the hills and plant a forest of trees that will thrive in the water in winter/spring and then survive the draugth in summer.