James Todd
James Todd

As the drought continues for many of us across the country, water becomes an even more precious commodity. No doubt that irrigation management will be a factor in 2013 if current weather patterns continue. There is even quite a bit of interest in irrigation in many parts of the country that have not irrigated in the past due to persistent drought conditions.

There are many ways to determine crop water needs and monitor soil moisture. I will go over a few of the more prominent ones here. Gypsum blocks have been used for quite awhile to measure soil moisture content and provide a very reliable indication of soil moisture by reading blocks with a meter. Soil capacitance probes have gained popularity in the past several years and are sold by several manufacturers. These probes can be set up to remotely transmit data and provide “real time” soil water information.

But perhaps the most reliable, cheapest, and most efficient way to measure soil moisture is by using a 3-foot probe with a 1-foot bucket and measuring the moisture at each foot “by feel.”  Determining soil moisture by feel can be done easily if you know your soil classification. The following website gives the basic guidelines for doing so: http://www.mt.nrcs.usda.gov/technical/ecs/agronomy/soilmoisture/index.html.

Whether you use gypsum blocks, soil capacitance probes, or the “feel method” to determine soil moisture content, you should have a fairly good idea of how much water is available to the crop.

Once you know how much water is available in the soil, then you need to determine how much water the crop is using.

This is easily accomplished by using PET (Potential Evapotranspiration) of the crop given the crop stage and the ET (Evapotranspiration rate). Local government weather stations usually report the ET for each given day. The PET can be calculated by multiplying the ET by the given crop coefficient. The PET represents how much water the crop is using each day. This can be tracked like a checkbook.

If you know how much water is available in the soil and how much water the crop is using, triggers can be established to initiate irrigation once soil moisture content reaches a certain level. These triggers vary based upon soil type, crops, regions and irrigation capacity.

This gives just a basic overview of irrigation scheduling. There are no doubt hundreds of ways to determine how much and when to irrigate. But the basic premise boils down to knowing how much water is available to the crop and how much water the crop is using.

If a grower tracks these two variables and knows how much water is being applied and how efficiently it is applied, irrigation scheduling can be an almost exact science. Even in dryland conditions, I have found that monitoring soil moisture levels can be an effective tool in making input decisions.