By Sjoerd Willem Duiker
At a field day last week we used an infiltration ring and observed an infiltration rate of 6.67” in less than an hour. A nearby farmer measured infiltration of 8”/hr on his farm. These dairy farms used continuous no-till and cover crops. The numbers suggest that these farms would never generate runoff because it is extremely rare to have this type of rainfall intensity in Pennsylvania. Nonetheless runoff is observed occasionally from these fields. Why is this? To understand this better we need to analyze the infiltration process and compare our measurements with what happens naturally.
First, pouring water in a ring on a piece of saran wrap and letting it infiltrate is not the same as receiving that water as rain in the field. The big difference is the impact of the raindrops. Raindrop size depends on rainfall intensity – a classic study from the 1940s showed that the maximum raindrop diameter was 0.24 inches. It also showed that the heavy, high intensity rain had the largest raindrops - if the intensity was 3.5 “/hr, the median raindrop diameter was 0.1 inch and the largest drops 0.24”, but if the intensity was 0.5”/hr, the median drop diameter was 0.07” and the largest drops 0.18”. The larger raindrops were observed to fall faster – the largest raindrops fall at a velocity of 30 feet per second, while small drops fall at less than half that velocity. You might remember from physics class that kinetic energy is ½mv2, so the impact of raindrops depends on their size and their speed. The bottom line? – high intensity rain has very high raindrop impact, while gentle rain does not. The large drops fall on the soil surface as little bombs that destroy the aggregates if they are not stable. It illustrates the importance of soil cover because if the raindrops fall on crop residue or (cover) crop leaves, they will not impact the soil immediately and instead trickle into the soil. Secondly, it illustrates the importance of stable soil aggregates that do not readily break apart under raindrop impact.
A couple of years ago a graduate student of mine did a rainfall simulation study comparing continuous no-tillage with cover crop with chisel/disking without cover crop. Manure was applied in the spring (before tillage was done in Chisel/disking). Corn was grown for silage. The fields had about 8% slope. Rainfall intensity was 2.4 inches/hour. Plots were rained on for 30 minutes before runoff (if any) was collected for 1 hour. Rainfall simulations were done in end of May, July, and September. We did this two years on a well-drained and a somewhat poorly drained soil. On average, runoff from no-till averaged 15% of rainfall (across both soils, and all events), while it was 34% from chisel/disk. Therefore, in this experiment, average infiltration was 2”/hr in the no-till plots, and 1.6”/hr in the chisel/disk plots. What is the explanation for higher infiltration in no-tillage? Soil cover is maintained – either from crop residue or living cover; biological activity is improved and that boosts higher macro- and micro porosity so more opportunity for water to percolate into the soil; and near-surface aggregate stability is improved limiting sealing of the soil surface.
Second, infiltration depends on the soil moisture conditions and profile characteristics. Last week the soil was quite dry and the subsoil not saturated. This is likely to change as temperatures decrease and precipitation continues in the fall and winter. Come spring, the subsoil may be completely saturated. If precipitation falls then, infiltration is likely to be lower because the soil has a high moisture content. Subsoil characteristics also become much more important a that time. Some of our soils have fragipans at shallow depth that are almost impervious to water so the water just doesn’t percolate into them. Other soils have heavy clay subsoils that have low percolation. Therefore, it is more common to observe runoff in the spring than in the summer. Finally, the soil may be frozen and at that point no infiltration will occur – that explains runoff from snowmelt or rain on frozen soil.
So in a nut-shell, infiltration depends on a lot of factors, such as rainfall intensity, pre-rain soil moisture conditions, impermeable layers in the soil, soil texture, slope, biopores, aggregate stability, mulch rate, vegetation cover, and more. Some factors are affected by management, others are not. I hope this shows that infiltration is a lot more complex than one would think at first. However, it is very clear that soil management practices demonstrated at the tour make a big impact towards improving infiltration. The soil cover provided, the biopores and stable aggregation help increase infiltration and keep soil in place.
Source:psu.edu