Estimating watershed residence times in artificially-drained landscapes and relation to nutrient concentrations.
A research project funded by the Indiana Water Resources Research Center through the U.S. Geological Survey’s 104B annual base grants (section 104 of the Water Resources Research Act of 1984, as amended).
Start Date: 2018-03-01 End Date: 2019-02-28
Total Federal Funds: $15,000 Total Non-Federal Funds: $30,014
Nutrient runoff from agricultural lands leads to Harmful Algae Blooms and eutrophication in freshwater ecosystems including the Great Lakes and the Gulf of Mexico. Best Management Practices (BMPs) implemented over the last few decades aim to reduce nutrient transport to streams and rivers. Evaluations of their effectiveness have found mixed results in reducing nutrient concentrations. This could indicate that BMPs are ineffective in certain areas, or simply that the residence time of water and nutrients in the watersheds are long and the effect of BMPs won't be seen for decades.
This project investigated the relationship between nitrate concentrations and a proxy of watershed travel times derived from water stable isotope ratio variability in the Wabash Sampling Blitz study area (Region of the Great Bend of the Wabash River watershed) . This study did not directly test BMP effectiveness, but provided a new context to examine the role of water age on nutrient dynamics.
Figure 1. Region of the Great Bend of the Wabash River watershed.
- Classify watersheds into short and long residence times of stream water using stable isotope variability from Wabash Sampling Blitz collections. Streams that show low stable isotope variability over repeat sampling are assumed to have longer watershed travel times.
- Confirm strong ground water influence in select watersheds in which we suspect long residence times from stable isotopes using additional sampling for isotopes and radon concentrations. Preliminary radon measurements were conducted, but Covid-19 research protocols greatly reduced the sampling that could be completed.
- Investigate differences in nutrient concentrations of common land cover types and surface water residence times. Statistical analysis was used to explore relationships in the dataset.
Major Conclusions & Significance
We hypothesized that drainage areas with high likelihood of tile drain usage would tend to have shorter travel times and this hydrologic 'short-cut' could result in higher nitrate concentrations.
- The stable isotope variability of the waters suggested that faster flow pathways were associated with higher watershed percentages of potential tile drained agricultural areas and higher spring nitrate concentrations.
- Stream nitrate concentrations were highest in smaller watersheds with high percentages of tile drain area.
- The largest watersheds had nitrate concentrations of ~5 mg/L which was near the average of small watershed variability observed, showing the integrated influence at larger scales.
Seasonality and antecedent soil moisture variability have been found to influence stream nitrate concentrations previously.
- The highest stream nitrate concentrations were observed during the wet spring events, followed by drier spring events and then fall sampling events were the lowest.
- These results indicate different degrees of hydrologic connectivity of agricultural soils to area streams during wet and dry conditions.
The hydrological impact of tile drains is hard to model because of uncertainties in density, depth, and landscape position of tile installations in addition to underdeveloped understanding of tile flow initiation.
- Our attempts to estimate watershed travel time distribution based on soil properties and topography showed the opposite pattern we expected from the stable isotope distribution.
What Does This Mean For Indiana?
Best Management Practices are critical to the health of Indiana waterways, especially since Indiana has a significant amount of agricultural land that drains into the Mississippi River and ultimately to the Gulf of Mexico, where the nutrient concentrations lead to hypoxia (low oxygen levels in water). This study highlights the spatial and temporal variability of nitrate concentrations in local streams. It shows that at the smaller watershed scale, the intensity of tile drainage and inferred speed of water moving through the watersheds are predictors for stream nitrate concentrations. Findings suggest that BMPs should be located in those small watersheds with intensive agricultural activity and hydrologic conditions favorable to nitrate export. Further effort is needed to better understand interactions between soil water and shallow groundwater in the area.
Training The Next Generation
One of the missions of the Indiana Water Resources Research Center, and all Water Centers, is to train the next generation of water scientists. This project successfully funded research for one Masters student within Dr. Welp’s lab, trained three undergraduate researchers, and provided science outreach experience for eight additional graduate and undergraduate students to engage with community members about land management influence on water quality.