What's needed for land to soak up water? Multiple factors that influence flooding and drought.

Considerations when making decisions about your garden, farm, ranch, neighborhood, or whole region. (Want to discuss this more? We are having two international online gatherings, September 16 & 19.)

I write often about the soil sponge. To me, it is the foundational infrastructure that makes life on land possible. However, there are many other factors that influence how much, and how quickly, water soaks into the ground—versus running off and creating both flooding and drought, and creating many other problems. If we want to take a living systems view of a place, we need to consider more than just a single factor.

Why it matters whether rain soaks in, or runs off

Consider this initial list of just a few of the things that are affected by a place’s ability to soak up water—whether rain, melting snow, floodwaters, or irrigation water—and hold it in the ground:

• Flood resilience

• Drought resilience

• Fire resilience

• Groundwater recharge and clean drinkable water, for all of life

• Flow of rivers and streams

• Soil erosion and mudslides

• Food sovereignty and security, for all of life

• Temperature:

  • Immediate local cooling via transpiration

  • Long-term climate cooling via photosynthesis turning CO2 into life and stable carbon

• Weather

  • Pressure zones that attract (or repel) atmospheric rivers and rain clouds

  • Small water cycling for formation of dew, mists, clouds, and rain

• Biodiversity

• Conflicts and wars over water and fertile land

Important, right? So let’s see what we can actually do, versus wasting our time complaining about what we can’t easily affect. Think about where you are, as you read through each of the following factors. (If you want to discuss this with a group from around the world, join one of our upcoming gatherings!)

Factors that influence how quickly water soaks in, and how much can be stored underground, for how long.

• Slope, terrain, topography: Is it flat, gently sloping? A steep mountain side? A deep valley? Gravity is powerful. How quickly does water flow away before having a chance to sink in?

• Depth to impermeable layers:  Is rain hitting a roof, paved road, or other impermeable surface? How deep is it to bedrock/ledge? Has the topsoil all slid off of the hillsides leaving bare rock because people cut all the trees 150 years ago? (like where I live in Vermont!)

• Soil texture: What is the ratio of different sized mineral particles? (Those tiny pieces of broken-down rocks called sand, silt and clay). Water infiltrates most quickly into sand, but has a harder time staying at the root zone (think of the beach, or a sand box). Water infiltrates most slowly into clay but stays there for a longer time once it has entered. (This is often taught as the primary consideration in infiltration rates, but the next factor can dramatically change things, no matter what your soil texture is.)

• Soil sponginess/structure/porosity/aggregation: Is the structure of those mineral particles arranged more like flour or like bread? Does the soil have pore spaces that water can move into, and structural integrity to keep those pores from collapsing? Have plants, microbes, fungi and other soil life turned the mineral portion of soil into little bundles called aggregates, (with open pores and tunnels between them) held together with biological glues, root hairs, and fungal hyphae to prevent wind and water erosion?  

• Amount of organic matter: How much biological material is in the process of decaying—leaves, mulch, dead microbes, dung, etc.—that can now absorb water into its dried cell spaces? This is often confused with the above (sponginess), but they are not the same.

• Compaction: Is there a layer of compaction or plow pan (from repeated plowing/tillage), where roots cannot penetrate?

• Permeability and openness of pores on soil surface: How easily can water enter the soil surface? How much surface sealing is there? Has silt moved sideways and filled in the little pore spaces on the surface? (This happens easily when silty soils aren’t protected by plants). If it’s a road, is the pavement or road surface semi-permeable? 

• Tunnels/burrows/cracks: Have animals (or cracking clay) created larger pathways for water to quickly enter the surface and prime underground flows? These can be very important when rain is infrequent. (Think about how hard it is for a completely dry sponge to soak up water.)

• Amount of vegetation, and ability to quickly take up water, and release it back into soils: How much vegetation will take up water through its roots (or structure, like mosses) to reduce flooding and pooling? How quickly do those particular plants or trees do that? Some plants also release extra water into soils at night, when they aren’t photosynthesizing.

• Rate of evaporation: How quickly does water evaporate? Is the soil shaded and protected from heat? How much mulch, plant litter, and plants are on the surface? What is the soil chemistry and salinity? What is the current air temperature, humidity, and wind speed?

• Saturation: How much rain has fallen recently? How many of the pore spaces in the soil are already full of water? (Consider that a totally wet soggy sponge cannot soak up any more water.)

• Soil moisture: Is the soil completely dry? Or is there enough moisture to prime the flows and wick the water along, when water enters? (Think about how hard it is for a totally dry sponge to soak up water.) Is there vegetation or mulch protecting the existing soil moisture?

• Rate of water arriving: How much water is arriving? Is it in the form of mist or large raindrops? A tiny trickle of melting snow? Massive floodwaters moving downhill? Is it being sped up by culverts and straightened rivers?

• Form/phase of water arriving: Is the water arriving as liquid or solid? Rain, snow, or hail? Does it have time to slowly melt or does the land need to soak it up immediately?

Plants can intercept moving water and give water a chance to soak in

Plants and plant litter/mulch can help to slow the movement of water in ways that give it more time to soak in. Plants can:

• Intercept vertical water movement —Plant leaf structure, density, diversity, and complexity all help to capture rainfall before it reaches the ground. This “gentles” rainfall and irrigation, giving it more time to infiltrate and reducing runoff. It also protects the soil surface from the compacting influence of hard rain.

• Intercept horizontal water movement — Water can be dangerous when it is uninterrupted. Water molecules join together and gather speed and power as the water moves downhill and across land. (Swales and riparian buffers are often ineffective against these larger more powerful flows.) Water also sticks to loose, unaggregated soil particles as it moves, creating surface sealing, erosion and mudslides. Plants and plant litter protect against both of these, acting as billions of “micro dams” that make water swirl and slow down rather than gathering speed.

How quickly can we influence infiltration rates?

Some factors are relatively stable and permanent (e.g. the physical properties of water). Others can change (slowly or quickly.) These factors all interact and influence each other, and vary in unpredictable ways, which can affect the time frame as well. The medium-term factors are the ones that humans can influence through their land management.

➜Medium-term factors that humans are able to influence through management within a few months or years:  soil structure/porosity/sponginess (biological influences on soil aggregation); percentage of organic matter; compaction; size of pores and cracks; presence or absence of living plant cover and plant litter; diversity of leaf structure, presence or absence of roots; tillage; cracking; surface crusting; animal and insect burrows, smaller scale topography (influenceable via swales, keyline plowing, ponds, ditches, etc.), light surface sealing from silt entering pores.

Short-term factors: saturation, amount of recent rainfall.

Very long-term factors (unlikely to influence without extreme intervention): soil texture (relative percentages of sand, silt, and clay), large-scale topography and slope of land, depth to bedrock.

(Relatively!) Permanent factors: gravity, physical properties of water (molecular adhesion and cohesion; ability to be in different forms in phases of freezing, liquid, vapor, and biological/EZ). Distance from equator.

What do we do?

We can influence the ability of a place to soak up and hold water by following Soil Health Principles. Here are a few simple ones: [maybe add links to more in depth articles you’ve written]

• Keep soil covered with living plants, plant litter, and mulch.

• Disturb the soil as little as possible so that the spongy structure can develop.

• Allow diverse plants to grow everywhere—so they can do all the amazing things they do, including feeding the biosphere that does all the work! Even if you have to build raised beds on top of pavement, or have window boxes on your balcony or rooftop, it makes a huge difference to have diverse plants.

• Whenever possible (and in all areas of your life) transition away from biocides/pesticides—herbicides, insecticides, antifungals, antibiotics, de-wormers, etc.—and other chemicals that make life harder for the biosphere. Consider this in your farm, your ranch, your lawn, your golf course, your playing field, your garden, your body, your pet’s bodies, your cleaning supplies, etc.

All of life is involved in the water cycle, and in creating the Earth’s structures and processes that transport water.

Do you want to discuss this? We are having two international gatherings on the topic!

Choose from Monday, September 16th, 2024 at 6 PM EDT, OR Thursday, September 19th at 10:30 PM EDT. Sign up here to join live or watch the recording. It’s by donation (or free, if you can’t afford a donation). We will look at our own places from around the world as examples, as we consider exactly what happens when rain, melting snow, or raging floodwaters move through the land. We will discuss in more depth which factors we can change, and then go deep on strategies, focusing in on one or two case studies from among our participants.

If you want to know more, consider signing up for one of our other courses and checking out our Seed Media project resources at the Land and Leadership Initiative.

Photo © Didi Pershouse 2024