If runoff is still too severe, then embark on a system of swales and water features which collect runoff and hold it on the slope. This held water sinks into the ground for the most part. A small amount evaporates. Most of these swale systems hold very ephemeral water during peak rain events. Every swale and every water collection feature needs a safety valve to safely lead water away if it becomes full. This is often a rocked waterway with a lip slightly lower than the top of the berm (or dam). These overflows usually lead to a natural stream channel or to diversion drains to fill lower systems of swales, flooded fields, ponds and lakes.
These systems would have many consequences. This evens out water flows. Peak flows are lower and summer flows are higher. The water is mostly filtered through the soil and substrata and emerges through springs. This means the water is cleaner all the way from the source and continuing downstream. There is less mud in the water. Flooding and rapid runoff leads to lots of contaminants and soil getting into the water. It is expensive to clean this water up. Agriculture pollution from agorochemicals and livestock is bad for water cleanliness and safety. Livestock can be raised in salmon-safe ways that keep manure out of waterways. Fence off waterways. Take water to stock, never let stock foul the water. Pastures and rangelands should be keylined or swaled if necessary to prevent water runoff. All water should sink into the land. Riparian zones should have a belt of dense-rooted vegetation on both sides of the stream (for the most part). Preferably trees and shrubs which make a dense net of roots which soaks up the below ground flow of nutrients before its gets to the stream. And turns the nutrients into marketable crops that are good for wildlife at the same time. This keeps the water cleaner in the streams and keeps it cooler from the shade.
Water naturally picks up nutrients as it moves downstream. This background nutrient flush would be much lower than today’s situation. If we stopped dumping toxins into the system with oil breakdown products, herbicides, pesticides, acid rain, etc than our water supply would be oh so much cleaner than today. Hence it will be much easier to clean up water for domestic consumption. Hold the maximum rain events on site and safely lead any excess runoff off the property safely. The amount of financial loss each year from flooding in the USA is huge. Doing watershed rehabilitation as outlined here would reduce flooding by magnitudes. This payback is measured in the billions annually.
There would be many follow-on affects as well. This would sequester the carbon out of the air to a pre-industrial level. Solving our climate change crisis. Furthermore, the oxygen column would be at a much higher pressure and this pushes more oxygen into the higher atmosphere where the oxygen forms ozone. So this will also repair our ozone thinning problems. Wood production would go up significantly in the world, allowing for a renaissance in wood culture and related economies. In other words the new (or enhanced) forests would be a significant resource. Combined with a switch to restoration forestry (instead of today’s exploitative forestry), the world’s forests could grow larger and taller while providing a large harvest of high-quality wood in perpetuity. Instead of killing the goose that lays the golden egg, we will hatch and nurture many new golden geese. In the form of forests. The reforestation of South Korea after the Korean war is one of the world’s best models.
The above strategy can be adapted to urban areas as well. The higher catchments are the roofs. We put water collection on all buildings and collect the water in tanks for future use, or put into rain gardens and swales in the yard as well as in the soil base itself. Yards can include rich, deep beds for food and ornamental plantings. Hugekulturs are biomass-filled raised-beds which become like giant sponges. The water collection features are built into every yard, every block, every street. Street water runoff goes into swales thru curb cuts. Seattle has some very successful examples of this. So each yard holds all, or most, of the rainfall that falls onto it and uses it for irrigation or sub-irrigation. This means they have to rely less on city water for their landscaping. This infiltrated water keeps the local aquifers topped up and cleans the water which pops out as springs lower down slope (or is pumped out). Rainfall events in cities often lead to overfilling of the drainage systems. Sewage treatment plants cannot do as good a job or have undesirable discharges. A lot of the runoff is quite contaminated. This system briefly outlined here would solve all these problems. An important reference on this is the book The City Forest: The Keyline Plan for the Human Environment Revolution by P.A. Yeomans. Yeoman is the Australian who invited the Keyline System of Soil & Water Management. This is a system of controlling water in the landscape. In City Forest Yeomans applied his ideas in the urban landscape.
Doing this worldwide would solve most of the world’s water problems. We don’t have to treat all parts of all watersheds. Some parts of the landscape are healthy, and some can regain health easily if we stop beating nature over the head.
Each property needs it own design. Solutions are adapted to local cultures and benefit the people who install the systems and do the maintenance. Locally appropriate species of plants are used.
The cost of doing all this is minimal and has a great payback. I have gone into great detail on how many people it would take and how much it would cost in my 2010 paper
A Carbon Sequestration Proposal for the World - Based on Reforestation, Improved Ecosystem Management & Increasing Soil Carbon Levels in Farm Soils
There is also a great deal of information in my 525-page book Restoration Forestry: An International Guide to Sustainable Forestry Practices. 1994, Kivaki Press. Out of print. I have a few copies for sale. $40 postpaid.