Rainwater Harvesting Updates

Until the recent atmospheric river drenched the Central Coast, the outlook for the 2021 grazing season was looking bleak.  The soil was bone dry, the remaining vegetation in the fields was crisp, hollow, and yellow, and the animals subsisted on alfalfa hay.  Although the majority of Octobers have at least one light rain, October 2020 was a dry month, without a single drop, and as one dry month led to another, the urgency of the situation grew steadily.

Even before the fall, signs were on the horizon that this winter would be subpar.  Forecasters predicted a moderate La Niña event, and we knew that this would be the winter that we implemented the water harvesting strategies we have been researching over the years.  The two primary tools we are implementing are called subsoil ripping, and swales.  This month we'll discuss subsoiling, and next month we'll cover swales.

Subsoil ripping is performed by dragging a deep shank with a small blade behind a tractor.  The blade runs deep below the soil, and lifts the column of soil above.  This lifting action creates a pocket of air deep in the soil, and breaks up hardpan and clay that would otherwise restrict the flow of water into the soil.

The lifting action also promotes a cracking and "opening" of the soil, allowing for water penetration, where water might otherwise run off the surface.

Subsoiling is a form of vertical tillage, which represents a vast departure from tillage systems of years ago.  Early tillage systems not only penetrated the soil, but also turned the soil over, and many tillage methods also pulverize the soil, destroying soil aggregates, breaking up soil organic matter, and oxidizing organic matter and soil humus.  As the soil organic matter "burns off" (oxidizes), the structure of the soil degrades, and eventually soils become rock hard, and almost devoid of organic matter.  A crust forms on top of the soil, and rain water no longer infiltrates the soil.  Rather, it puddles and later evaporates, or sheets off into drainage ditches.  The organic matter loss makes soils under traditional tillage regimes one of the largest contributors to global CO2 emissions, making agriculture the largest contributor of global CO2 emissions of all economic sectors.  Agriculture, Forestry, and Other Land Uses (AFOLU) account for 24% of global emissions according to the 2014 IPCC report, and destructive tillage practices are largely to blame:

With subsoiling, although it does disturb the soil in a meaningful way, it is also more gentle on the soil than most other tillage practices.  Subsoiling is a gentle "lifting" action that does not invert the soil, and leaves the majority of soil aggregates intact.  It breaks up hardpan layers, and allows air and water to reach deeper into the soil.  This allows plants to send roots deeper into the soil, collect water and minerals from deep in the soil, and subsequently leave their organic residuals behind.  This promotes better water holding capacity, more soil life deep in the soil, and the development of fungal networks that may efficiently move water and nutrients on an underground "superhighway" of fungal hyphae (fungus "roots").

In this way, when used correctly, subsoiling may begin a virtuous cycle that allows soils to absorb more water during rain events, grow more organic matter, build soil carbon through plant growth at depth, which then allows for and promotes more plant growth, and so on.  All of these phenomena are self-reinforcing, allowing the virtuous cycle to continue.

Fortunately, in preparation for the recent Atmospheric River that soaked the Central Coast, we subsoiled several important pasture areas, and we were able to capture the majority of the water that fell on our property.  It was a massive rain event, and we received 5" of rain in the span of 3 days.  Although this is still only a fraction of the rain we get in a normal year, the fact that it was all stored in the soil means that the small amount we got will go a long way toward a successful 2021 grazing season.