Electrical conductivity (EC) is a fundamental piece to SWAT MAPS. EC layers are one of the first layers we look at when ground-truthing new fields as they show the field’s water dynamics and salts. It is also an important layer we look at when assessing problem spots in a field.
Although EC is all about water, data will not change with frost or snow when mapping later into the season. EC readings are quite deep within the soil to 40cm and 90cm. So, if soil moisture is “consistent” across a field, there should be no concerns with the EC readings. For example, if we begin mapping a field when it is dry and finish the rest of the field after a significant rain – then data may not be consistent and remapping the entire field is required. Other than this, mapping into the late fall is not an issue for the SWAT BOX.
High electrical conductivity areas typically fall into lower zones (6-10) on a SWAT MAP. Depressions, salinity, heavier clay, and peat will commonly be placed in these zones as they are usually higher in EC. Low EC areas usually are upper zones (1-5) on a SWAT MAP. Coarser textured soils, like sand ridges, will show up as low EC spots in a field. The “trick” when ground-truthing is to make sure these areas get separated as different zones. It is also important for us to determine how much EC should be factored into the final SWAT MAP when driving through the field and in relation to other layers. Below are a few different scenarios in relation to EC seen out in the field this fall.
Case 1: “Typical” EC
90% of the time, the EC layer has a strong correlation with the topography layer. High EC spots will be depressions while low areas will be placed on hilltops. One of many examples of this can be seen in the photo: on the EC layer, high EC is purple/blue while low is red. Same goes with the elevation layer – highest elevation is purple/blue and lowest is red. When ground-truthing fields like this, we need to make sure that different soil textures get placed in different zones. We also need to watch that severities of knolls and depressions are sorted out into zones accordingly.
Case 2: Erosion
Most of the time, a depression should read as a high electrical conductivity area. However, there are cases where it reads low due to erosion. These types of fields prove why ground-truthing is such an important step to building the correct SWAT MAP. An example of a field like this is pictured below. In these types of scenarios, it is important to make sure the low EC depressions get placed as lower zones but also that these spots get put into a different zone than the high EC depressions.
Case 3: Bathtub Ring Salinity
Similar in fields dealing with erosion, fields with bathtub ring salinity will also have low EC depressions. However, these areas will also show the classic “ring” surrounding the depressions. This is seen in the picture above. Again, when ground-truthing fields with low EC depressions, we need to make sure that these depressions are lower zones but separated from the high EC depressions and especially salinity.
Case 4: Low EC Range
In rare instances, the EC layer might have a very low EC range. If this is the case, the EC map may not provide “value” as a weighted portion of the SWAT MAP. When ground-truthing fields like this, a SWAT MAP may be chosen that is weighted with a combination of elevation and topography. An example of a field like this is seen below. Here, the water runs showed significant differences in the field and so an elevation and topography map matched these areas and the field quite well.
These are just a few of many scenarios involving electrical conductivity that we see throughout the sampling season. Just looking at the EC layer alone shows how important the process of ground-truthing is. When we talk about ground-truthing, there are a lot of different ways that electrical conductivity can interact with elevation and topography layers and ultimately contribute to a SWAT MAP. When choosing the best map for a field, we determine the weighting of these interactions.
If you are interested in discussing more about electrical conductivity or SWAT MAPS, please don't hesitate to contact me at the email address below.