Where Does the Water Go?

With all the rain in May and June, and higher than normal soil moisture levels, one can start to wonder where all the water could be going?
Where Does the Water Go? - Articles


Ponding in soybean field. Credit: Claire Coombs

Weekly, in Field Crop News, there is a weather outlook and every week early in the season, rain seemed to be the main subject. Lately, warmer weather is expected, however thunderstorms seem to be prevalent across several counties. With the most recent heatwave, any ponding in fields should disappear and tile drains should slow down, but where did all that water go? Are our crops reaping the benefits of all this water? Is this water leaving our field and taking valuable nutrients with it?

To understand where all the rain is going, we have to return to elementary school and review our water cycle. When it rains, the water only has a few places that it can go. The majority of all rain will return to the atmosphere through the process of evapotranspiration. This process accounts for about 80% of all water in the cycle and does not contribute to the stores of in our surface water and groundwater. The remaining 20% of the water not returned to the atmosphere through evapotranspiration will go into surface water flow (streams, lakes, rivers) via overland flow and only a small percentage of all rain water will infiltrate into the ground.

The water that flows over the surface into streams, ponds, creeks, and eventually our larger bodies of water (rivers, lakes and ocean) has the potential to carry valuable nutrients and topsoil with it, if flowing over a field. Water naturally flows downhill, and with any degree of slope on a field, the water will follow the natural slope of the field. It is important that we do everything we can to ensure that water flowing downhill is taking as little topsoil as possible with it. By maximizing infiltration (to be discussed in next section) and keeping the soil covered, the amount of water flowing downhill can be reduced. Implementing practices to prevent rill-erosion can help ensure that with all the recent rains, water is infiltrating into our fields and not creating rills in our fields, eroding our land and removing topsoil.

Water that does not evaporate, or flow overland into surface water, infiltrates into the ground. This small percentage of all precipitation is what is going into the soil to recharge our aquifers. Prior to infiltrating to the deep depths of the water table, the water is available to plant roots for uptake. This is what we are relying on to provide all the moisture the crop needs for the growing season.

At different points during the growing season, the moisture demand from crops is different. Additionally, depending on the potential yield, the total amount of water needed will vary (with higher yields needing additional water). For example, corn generally requires 20-30 inches of water of the growing season to produce a high yielding crop. So far, at a field I scout in Mercer county, we have had 9.02” of rain recorded. This suggests we are almost half way to our required 20-30” for high yielding corn, however that is assuming all the rain actually infiltrates into the soil and stays at a spot in the soil that is available for root uptake. Looking back to 2017 and 2016, the same field had 20.77” and 20.68” inches of rain from May 1 – November 1, respectively. To produce a high yielding corn crop, all the rain would have to be available to the crop, and used by the crop during the growing season. It is safe to assume that some of this rain resulted in overland flow, evaporated or infiltrated too quickly to subsoil to be available to a crop. Consequently, it is important to capture as much moisture as possible and store it for crop uptake.

With the crop needing as much rain as possible, everything should be done to ensure the soil can hold as much water as possible so it is available when needed. Efforts should be made to both increase water infiltration and water holding capacity. Increased water infiltration will allow the water to enter the soil and sit in the pore space to be available to plant roots. The soil texture (how much sand, silt or clay) will inherently affect water infiltration, however the amount of crusting and compaction in a field can influence water infiltration rate. Practices such as no-till can help reduce compaction and crusting by allowing the soil to be protected against erosive forces. Additionally, reduced wheel traffic, and not plowing at the same level every year, will help reduce compaction. With less compaction, the pore spaces between soil particles will be larger, allowing for more water to sit in these spaces and be available for root uptake. These practices will not only allow for increased water infiltration, but will also increase the water holding capacity of the soil, again allowing for water to be available when needed.

Finally, increased organic matter levels will be key to ensuring a healthy soil that can resist compaction and crusting, while maintaining a high water holding capacity. Overall, the physical properties of the soil will have a large effect on the availability of water in the field. For more information on soil physical properties and the relationship with soil health, there is a soils   video  on the Penn State Extension website.

The variable weather this year has created issues when planting and in the early growing season. While it may seem that we have had too much rain, it is still needed going forward in the growing season to make sure water is available all season for our crops. Furthermore, with thunderstorms and patchy rain storms predicted going into July, it is important the soil can handle intense rain fall. Ideally, regardless of intensity, the rain should be able to infiltrate into the soil and be available for plant uptake. Finally, this year, and going forward, practices should be put in place that help improve soil health to increase water infiltration and prevent overland flow to ensure our soils can capture all that water to grow the highest yielding crops possible.