When it rains hard and fast, so much of the water runs off, especially where there are few plants. Over time, this can form arroyos and remove the top soil from the land, which will decrease plant-life and worsen the situation. To be fair, our land is in pretty good shape these days, but we are continually trying to improve it. We are in the process of building swales across the contours, to help keep the water in place. When these swales are level, the water doesn't run, but instead is absorbed by the soil. This will help water all the hundreds of little trees we're planting.
The trench in these pictures is not one of those! On occasion, you want the rain to run, especially if you have a huge pond to fill! We had very little rain last year, and most of the rain in July (which wasn't that much) was slow and didn't run, so our pond was low.
This trench that we've just dug runs across several hundred feet of our neighbor's property. The land that the trench cuts across is fairly overgrazed and so run-off has increased. By digging the trench just off-level, not only have we slowed down the run-off (which should improve the soil and plant-life in place), but we are also directing the flow to where we want it to go, our pond.
Once the ground was saturated with an inch or two of slow, soaking rain, we then had a fast inch. You can see how much the pond came up in just that inch. And, of course, the trench has become the kids' new favorite playground. If it rains during the day, you can pretty much guarantee that the kids will run outside, to splash up and down the trench!