Researchers show weather variability hinders ability to assess trends in Iowa’s water quality
AMES, Iowa – When can we expect to see reduced nutrient levels in our water if we make positive changes to the landscape? Research from New Iowa State University shows how complicated it is to give a valid answer to this question.
The research is presented in a recent article in the peer-reviewed Environmental Quality Journal, co-authored by Ph.D. student Gerasimos J. Danalatos, Professor Michael Castellano and Associate Professor Sotirios V. Archontoulis, Iowa State Department of Agronomy, and Systems Analyst Calvin Wolter Geographic Information Center at the Iowa Department of Natural Resources.
Their study used a modeling approach, combined with water sampling data from 29 Iowa row crop-dominated watersheds monitored since 2001, to estimate how long it will take before the state can reliably identify a 41% reduction in nitrate loss to waterways from non-point sources, which come primarily from agricultural land. The 41% goal reflects the state’s commitment in Iowa’s Nutrient Reduction Strategy to reduce nitrate loss in streams flowing into the Mississippi River, which contributes to the dead zone. hypoxic Gulf of Mexico.
The researchers found that the probability of measuring a statistically significant 41% reduction in flow-weighted nitrate concentrations over 15 years was as high as 96%. It sounds encouraging. However, results varied widely across the 29 watersheds. The probability of measuring the 41% reduction (if any) ranged from about 23% to nearly 100%. The challenge of measuring or observing a real reduction is mainly due to the interannual (year-to-year) variability of precipitation.
“With that kind of range, it’s really hard to make good predictions and gauge progress,” Castellano said.
Of the water quality parameters assessed, the researchers concluded that the flow-weighted nitrate concentration offered the best opportunity to measure actual reductions in nitrate loss.
“Focusing on nitrate concentration or total loss without flow weighting tells you more about precipitation over time than about nitrate trends due to changes in land use and management,” Danalatos said. .
To test their ability to measure nitrate changes, the researchers used a new approach, using a Monte Carlo simulator, a scenario generator that can quickly reflect the probabilities of many different situations. They have run over 5,000 scenarios with the simulator.
During the process, they also looked at several non-meteorological variables that they believe could help explain trends in nitrates, including changes in land use and agricultural management, and soil organic matter, a measure of soil quality. Some weak relationships emerged between these variables, depending on the size of the watershed. They also looked at differences in monitoring approaches, such as daily versus monthly sampling. Ultimately, properties associated with water flow explained most of the variation across the watershed over time.
“Our results are the first to quantify how important it is to account for interannual variability in nitrate loss when assessing long-term trends due to changes in land use and management.” , said Castellano.
“Our main message from the research findings is that the policy-engineered timelines we use to measure these trends do not correlate well with our ability to reliably track significant changes in water quality,” Castellano said. “This is mainly due to the impact of weather. Also, if rainfall patterns become more variable, the time frame to measure changes in water quality will only lengthen.
The researchers point out that good monitoring data from various watersheds over long periods of time is essential for drawing meaningful conclusions. They also point out that the measurement challenges identified in this study further underscore the need to rely on other relevant data when assessing water quality trends, particularly land-use change. and adopting conservation practices known to reduce nutrient loss.
According to Adam Schnieders, water quality resources coordinator for the Iowa Department of Natural Resources, the agency that funded the study, “The goal of the Iowa Nutrient Reduction Strategy is to achieve a 41% reduction in nitrogen and phosphorus loads leaving the state from nonpoint Sources. Seeing this reflected in Iowa’s water quality will be the ultimate measure of success. This work helps us to better understand how Iowa’s water monitoring data can be used to track progress toward these goals – and can help inform future efforts to account for the annual variability caused by weather and d other factors identified in this research.”