How ‘mulching’ can help soil absorb rain water?

Beate Kubitz, one of our new colleagues at Slow The Flow Calderdale has written below about the process of mulching which helps the run off of rain water in a heavy rainfall event.  We thought this might prove useful to us all so please have a read.

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‘We agreed to help our friend Scot mulch the ground around his house. We were staying in Truckee, near Lake Tahoe, in California. The area is dry in the summer, and has deep snow in winter. It’s pretty high and mountainous and the slopes are steep. There’s quite a lot of erosion – and interest in natural management to slow winter snow melts, prevent flooding and preserve the soil.

Our mulching experience was more scientific than I expected. Scot’s friend Michael arrived with a truck load of well rotted mulch and his soil absorption measuring kit. This was a length of pipe with tiny holes drilled in it, like a short sprinkler called a runoff simulator and was equipped with a highly accurate flow meter. We attached it to an outdoor tap. The simulator delivered 4 gallons of water per minute – and the device spread the water along the soil surface in a straight line so you could measure its progress.

The soil in Scot’s garden was super compacted and on a slope. We measured about 6’ from the sprinkler, and set down a marker. It took just 14 seconds from starting the water flow for it to reach the marker. At 4 gallons per minute this means that the ground only took a gallon.

We then spent several hours spreading mulch over the garden to a depth of between 2” and 4”, and tilling it into the ground so that the hard surface was broken up and the mulch was turned into a soil amendment.

Setting up the runoff simulator device for a second time in the same spot, we turned on the water source. The difference was immediately apparent. Instead of flowing straight across the ground, the water soaked into it. We could see its progress, seeping into the air pockets and then trickling over the surface millimetre by millimetre, but after 10 minutes it still hadn’t reached the far marker. By this simple intervention, the soil could now hold over 40 times as much water. Where before a gallon had barely been absorbed by it, it was now holding over 40 gallons securely.

Scot’s friend Michael is a soil scientist who works as a consultant on water sheds in the Tahoe area of California. Amongst his work is ‘helping to keep Lake Tahoe clear’. He explained that he’d managed several soil management interventions to improve its capacity to hold water and stimulate the growth of appropriate vegetation, locking in a virtuous cycle that slowed down water flows and prevented soil and debris washing into the lake.

Although quite different from the Calder Valley, the experiments suggested that we should look at the different water retaining properties of land in the valley and encourage those that hold water above those that do not absorb it.’

More info can be found here – http://ierstahoe.com

Beate Kubitz

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