Three Weeks Away from a Drought?

A common saying in Pennsylvania is that you are always three weeks away from a drought. Is there truth to this statement? Well, it depends on a lot of factors, including the atmospheric conditions, the crop water demand, and the soil properties.

Let's take corn as our example. Corn water use varies by growth stage (Table 1). Early during its growth, corn water use is predominantly due to evaporation from the soil surface. The rate of evaporation can vary from 0.1 to 0.2 inches per day, depending on the wetness of the soil surface and residue cover. While you cannot control when it rains, you do have a handle on residue cover. Residue cover helps to get water into the ground by reducing soil sealing from raindrop impact, and then it reduces evaporation. This is where no-till with high residue cover shines. In its early development, the corn root system is still developing and therefore doesn't draw moisture from deep depth. This continues until about 60 days after emergence, when the plant reaches its maximum rooting depth. Still, most moisture is drawn from the surface of the soil. Irrigators use the 4:3:2:1 rule, which states that 40% of water is drawn from the top 25% of rooting depth, 30% from the second 25%, and so forth.   

During its development, corn water use continues to increase until it peaks at about 0.32 inches per day. You can do the math, corn will need about 2 inches of water per week to avoid water stress in the summer. Does it rain 2 inches per week in Pennsylvania? No - The average precipitation is about 1 inch per week, so the difference has to come from the soil. In addition, we don't deal with averages in the real world, so if the soil does not store the balance, you might run into a drought situation quickly.

* For South Central Nebraska (from NebGuide G1850, University of Nebraska Cooperative Extension).

Once you understand the water needs of a crop, soil properties explain whether you are three weeks away from a drought. In the US, we have the benefit of having great soil information at our fingertips. You can download the SoilWeb app (PDF guide) developed by USDA-NRCS on your smartphone or access it on the web to get instant data for your field. The information includes the available water in the top 60 inches of the soil.

As an example, I went to an area in Chester County and found there were different soil types in the same field. The area I looked at had significant portions of Glenville, Manor, and Glenelg soil that vary dramatically in their available water holding capacity. 

The Glenville is a soil with silt loam surface texture and loamy subsoil. At first sight, you would think this is great because loamy soil has higher water holding capacity than sandy or clay soils. However, the soil has significant amounts of rock fragments and a fragipan starting at about 30 inches deep.

Fragipans are quite common in Pennsylvania. They are naturally impermeable layers so dense that roots don't grow into them. They cause a shallow water table in the spring, and when they dry out, they have limited capacity to hold water. So the soil profile of the Glenville soil only holds 5 inches of available water.

Manor soil does not have a fragipan, but it has a more sandy texture. Despite bedrock being at a deep depth (starting at 60 inches), it has moderate available water: 8.8 inches.

The Glenelg, on the other hand, is a deep soil with loamy texture to more than 80 inches, and its available water capacity to 5 feet is 10 inches. Now, crops already start to experience some water stress when the available water is depleted by 50%, so you can see that indeed it is quite accurate to say you are always three weeks away from a drought if you have soil with low water holding capacity.