Effective Rainfall

Weather averages are useful but they will never predict the weather in Texas. When average rainfall statistics state we will get more rain in one month and less rain in another, along comes a drought or flood and throws the averages out the window.

I was once hired by a client in Comal County who had interviewed several people for a landscaping job. She told me the reason she hired me was the answer I gave to “How much rainfall do we get here in a year?” All other consultants gave the standard answer, “We get 32” on average.” I said the same, but I added, “But there is only 18” of effective rainfall.”

That told her that I understood the need for efficient water use and that my recommendations would be based on installing a landscape and landscape management program that would take all water use into consideration. Having a foot less water available should change any management approach.

When ranchers believe that it will rain “X” number of inches a year, they often plan around those averages. They plan to apply fertilizers at a certain time of the year based on average temperatures and rainfall. They plant fall or spring seed based on average rainfall events. Too many farmers lose money when the rain doesn’t fall. Worse still, the land suffers from the toxic fertilizers and dry land plowing.

In our area, rainfall of 20” a year can all go into the soil. Or rains can fall at 32” a year and only 16” soaks into the soil to become available for plants. What happens to the rest? It flows off the property where plants get no benefit. Some water might flow into a pond, and that can be helpful. But that water rarely helps pasture plants. And too much of it evaporates.

Many ranchers have semibare ground. Even with sufficient rainfall, continuous cattle grazing reduces leaf area so that cattle have trouble getting a mouthful in one bite. When rain finally falls, it dislodges the soil particles, then smaller particles fall into the open spaces blocking the pathway for water to filter into the soil. Then the soil compacts and it’s harder to grow grass.

With good rotational grazing and taller grass reserves, rain drops slide down the grass leaves and filter into the soil. There, it can follow open channels created by earthworms, dying roots, sponge-like organic matter and soil with more living organisms than the semi-bare dirt. The soil is held in place by the two to ten-fold root mass rather than that of the semi-bare dirt. Taller grass generally equals more water in the soil and aquifers.

Effective rainfall can be measured by observing the land and how water penetrates into the soil. When you observe the land after a rainfall event, how much rain remains in puddles? Puddles can indicate soil compaction and a lack of water infiltration. Puddles that stay for days indicate severe compaction, or anaerobic soils. Soils with good subsoil drainage will not puddle for more than a few hours, even after a hard rain.

Can you hear water squishing under your feet days after a hard rain? This also indicates lack of subsoil drainage even with tall grass. When subsoil is compacted, grass dries out and dies in droughts.

How much debris do you see accumulated? When large amounts of loose organic matter collect, it can indicate a lack of tall grass to keep it in place or rapid movement of water flow. A good growth of grass and good soil drainage will prevent large accumulations of organic debris.

If the debris contains substantial amounts of soil, you have a severe problem with bare soil. Soil should not accumulate after a rainfall event. There are exceptions (like hog damage areas), but this should be a rare exception.

Five days after a rain event, dig a few holes in the pasture. How easy is it for the shovel to penetrate the soil? Does the resistance come from grass roots or the soil? If it is hard to push the shovel into the ground more than an inch or two, your pastures need help. A shovel that goes in a foot or more may indicate effective water penetration.

Check for depth of water penetration. Good penetration from a one-inch rain may go as deep as a foot or more. Severe lack of water penetration indicates too much runoff. Even on ground with a slope, there should be good water penetration.

Native clumping grasses are generally better at retaining water than introduced grasses. They are often better for the aquifer. Allowing for good recovery time before regrazing will insure good water penetration into the soil, no matter what the grass.

Aquifers benefit from keeping the rainfall on the land where it falls. When runoff is slow, aquifers fill from water above. Aquifers that fill from runoff into streams and rivers take much longer to fill unless there is a gravel or porous sand layer.

Getting water into the soil is important but keeping it there is another story. With effective rotational grazing, you are on your way to improving your pastures and increasing weight gain.

David E. Will is a local rancher and consultant. He can be reached at 830-6299876.