Irrigate Wisely to Protect Groundwater

Author: Chrissy Lucas

Publish Date: Summer 2010

Protecting groundwater should be a concern for every Oregon resident. Most rural residents rely on groundwater for drinking, watering livestock, and irrigation of gardens and/or agricultural crops. Thereby, it is important to follow some best management practices to reduce non-point source pollution. Non-point source pollution is contamination that cannot be attributed to one large source but many small sources that individually seem insignificant but cumulatively can result in large impacts on the environment.

 Nitrate is a common non-point source pollutant that is being found in more and more groundwater supplies throughout Oregon. Nitrate is a form of nitrogen that moves easily through the soil with water. You might wonder where the nitrate comes from; fertilizers, pesticides, and animal waste are some of the most common sources. Northern Malheur County, Lower Columbia Basin, and the Southern Willamette Valley all have established Groundwater Management Areas to address high levels of nitrate contamination found in rural drinking water supplies. Nitrate levels above the EPA limit of 10ppm or mg/L for public drinking water supplies are considered a health risk. Levels below 10ppm are considered safe for people of all ages to consume, however, if the level is approaching 10ppm pregnant or nursing women and infants are not encouraged to drink the water. No matter what area in Oregon you reside following a few simple irrigation management practices will help protect groundwater.

Schedule irrigations using available soil moisture and crop water use requirements.

Don’t over-irrigate! This sounds relatively simple but most folks would rather err on the side of overwatering than underwatering. Over watering can cause stress to the crop and also nitrogen deficiency from leaching. Take the time to evaluate and graph soil moisture. Soil moisture sensors are readily available or you can even use a shovel and your fingers. In addition, the Agri-Met system ( pn/agrimet/), an online tool, provide daily charts showing the ET (evapotranspiration) rates and is a fairly accurate estimate of crop water use. Combine the tools together to help you plan a tailored irrigation schedule.

Time irrigation to keep water in the rooting zone.

Know your soil! Irrigation shouldn’t be used to fill the soil to field capacity. By knowing your individual soil’s available water holding capacity you can vary the length of time you irrigate. Using a more frequent but lighter irrigation schedule helps keep water in the root zone where plants can use it, and reduces the amount of nitrate that can leach. Don’t forget to account for the weather and time of year. For example, manage your irrigation applications to keep any stored water to a minimum in the fall.

Adjust application rates to meet crop requirements during different growth stages.

Know your crop! Each crop consumes various amounts of water from planting to harvest. Certain stages of growth may require more or less water, understanding the growth stages of your crop is crucial to providing the right amount of water. Most crops can tolerate a reduced level of available water capacity that won’t affect the crop quality or yield. The typical range is between 40-60% so keeping an irrigation schedule that fulfills the crops' water need and doesn’t allow the soil to dry below that level is imperative.

Keep applications uniform and accurate.

Make sure that whichever irrigation systems are used have a consistent and even flow being applied across the entire area being irrigated. Without this consistency, you may be overwatering and leaching nutrients in one area of the field while another is being underwatered – affecting total crop yields.

Follow label directions when using fertilizers.

Label directions should always be followed, as well as working with guidelines from a current soil test. It can be a balancing game of applying the right amount for the crop at the right time. The excessive application creates a higher level of probability of leaching contaminates into the groundwater, and in addition wastes the money invested in the product. While under application can show little or no effect on the overall crop quality and yield, taking time to incorporate a few irrigation basic management practices into your system can help protect Oregon’s groundwater, the health of your family, conserve water, save money, all while producing a high-quality crop.