Using the Web Soil Survey to Evaluate Internal Drainage Potential

by • 3 years ago

The Web Soil Survey is a useful web application that provides information about the drainage potential at a site.

Using the Web Soil Survey to is a great tool to assist in your drainage planning. Before diving into this resource, I have said there are few things more important to growing healthy grass than good drainage. One of my favorite quotes is: “You need two things to grow good grass, common sense and drainage. If you are short on the former you going to need more of the latter.” Drainage is important because most grasses perform poorly in wet soils. Also, wet soils are more susceptible to compaction and most diseases and insects prefer wet environments.

There are two main types of drainage: surface and internal. Good surface drainage is hugely important, but we won’t spend much time talking about it here. It is relatively easy to see if surface drainage is adequate or not with a glance or a few easy measurements. Internal drainage, however, is much harder to visualize. Poor internal drainage can be created by two very different situations: low water infiltration rate and a shallow depth to the water table. If you have poor internal drainage, knowing the underlying cause is causing the problem is important for deciding how to correct it.

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Let’s start with the depth to the water table. The water table is the level where the soil is saturated with groundwater. If the water table is near the surface, the internal drainage is expected to be poor and growing healthy grass will be a constant challenge. The best solution for lowering the water table is to install drain tile. The depth of the drain tile will determine the new water table level. So if the drain tile is installed at 18 inches, the water table will be at that same level. Poor internal drainage because of a shallow water table can cause problems in any soil type including sandy soils. The proper spacing of the drain tile will be determined by the soil texture.

The second situation that causes poor internal drainage is a low saturated hydraulic conductivity. This is problem most common in soils with high clay content, and therefore these soils are also most susceptible to compaction. In this situation, installing drain tile and backfilling with the original soil material will not solve the problem because the water near the surface simply can move fast enough through the soil to get to the drains. In this case, a better solution would be to install slit drainage or slit trenches, where drain tile is installed but then backfilled with a sandy soil that can quickly carry water to the drain tile. Good surface drainage is also a very important factor for these impermeable soils.

The Web Soil Survey is a useful web application that provides information about the drainage potential at a site. Let’s take a look at an example. Below is a screen shot of an athletic field from the Web Soil Survey. The two colors represent the drainage class of the two soil types found on the site. From the legend we see that the North end is “somewhat poorly drained” while the South end is “poorly drained”. The first thing we should note is that it will be challenging to grow healthy grass on both of these sites unless something is done about the drainage. Good surface drainage is always important for athletic fields. We now want to figure out if this poor drainage is being caused by a shallow water table or by an impermeable soil.

Drainage Class Map

Drainage_Class_Map

The permeability of the soil can be judged in the Web Soil Survey by looking at the “Saturated Hydraulic Conductivity (standard classes)”. The saturated hydraulic conductivity is a measurement of how quickly water can move through the soil. Here is a map of the saturated hydraulic conductivity of the fields.

Hydraulic Conductivity Map

Hydraulic_Conductivity_Map

From the map, we can see that the saturated hydraulic conductivity is the same for the North and South ends, and listed as “Moderately High”. This suggests that our internal drainage issue is likely being caused by a shallow water table, and is not related to the soil’s hydraulic conductivity. The Web Soil Survey can estimate the depth to the water table at any site in the US. Shown below, we see that the North end has a higher depth to water table – 50-100 cm which is about 20-40 inches and pretty reasonable for most uses. The South end has a depth to water table of 0-25 cm, or about 0-12 inches, this suggests that internal drainage to lower the water table would be useful. Because the soil has a moderately high saturated hydraulic conductivity, slit drainage may not be necessary.

Depth to Water Table Map

Depth_to_water_table_map

The Web Soil Survey is a powerful tool that has many applications for site assessment and planning. These are just three maps of scores of different soil properties that you can view. Additionally, using these maps can be a powerful communication tool to explain to your golfers, parents, customers, board members, or supervisors about the challenges of growing turf on your site. There are many YouTube tutorials that explain how to use the Web Soil Survey and create maps like the ones shown above. I encourage you to explore and see what you can learn about your site.

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