I got to thinking about the swale I built and wondering if it's doing anything. Let me explain.
I'm halfway down a hill. After it rains, the swale fills up. For weeks after (depending on rain amount ) it can stay full. Looking close, I can see the water seeping out the side, going downhill. Not overflowing , but seeping out.
This leads me to believe it's seeping in at the same level it's seeping out. So in essence, my swale is like a guage, letting me know that seepage is occuring below grade. Once it's dry, no more seepage is occurring.
I would say that as long as the water is slowed down, then there is going to be increased infiltration. Even if it's not 100%, it's something. I've noticed that the longer I have had my swales installed, the less I have to water downhill from them. In the first few years there was some seepage, but as time has gone on, that's become very minimal. All of the plants and trees below these swales are certainly benefiting not only from the water that gets trapped, but also from all of the biomass (fallen leaves, chicken manure etc) that collects in them. I have one very active swale that collects about 5 inches of new "soil" every year. It's a mix of leaves, dust, erosion material from the settling pond and mulch. I usually dig that material out in the spring and add it to the top of the swales and garden beds. It's working out pretty good so far. I think the thing to keep in mind is that swales do more than just collect water. There's a lot going on in that little ditch. Just think of all the microbial life that gets a longer chance to work while the swale is full. Of course there's mosquitoes...so, yeah. Everything in balance eventually.