Trends in Streamflow, Nutrients and Sediment in the Upper White River Basin, Indiana

This U.S. Geological Survey fact sheet summarises a study (Koltun, 2019) done with The Nature Conservancy, supported by the Nina Mason Pulliam Charitable Trust. Existing water-quality and streamflow data were analysed to measure how water quality in the Upper White River Basin of central Indiana has changed, and to find areas whose trends may call for conservation. The study focused on substances linked to hypoxia (low oxygen) and degraded stream habitat — nutrients and sediment the Conservancy wants to reduce in the White River, which reaches the Gulf of Mexico by way of the Wabash, Ohio and Mississippi rivers.

What was measured

Streamflow data from three USGS gauges on the river — at Muncie, near Nora and near Centerton — and water-quality data from the USGS, the Indiana Department of Environmental Management, the Muncie Sanitary District and Citizens Energy Group were used to:

1. estimate yearly mean concentrations and fluxes of nutrients and total suspended solids (TSS) over periods within water years 1992–2017 (a water year runs October 1 to September 30); and
2. assess trends in streamflow and in concentrations and fluxes — including flow-normalised changes between 1997 and 2017. Flow normalisation strips out year-to-year swings in streamflow while keeping seasonal and long-term streamflow trends.

Load is the total mass carried past a point; yield is load divided by the land area draining to it; flux is the rate at which load moves.

Figure 1. The Upper White River watershed and the three study gauges. Base from USGS digital data.

Concentrations and fluxes of TSS, total phosphorus (TP), nitrate plus nitrite (NO23) and total Kjeldahl nitrogen (TKN) were estimated with the USGS Weighted Regressions on Time, Discharge, and Season (WRTDS) method in the EGRET software.

Loads and yields

Loads of every substance grew from the upstream gauge to the downstream one — but yields did not always. TSS, TP and TKN yields were highest at Muncie, upstream; the NO23 yield was highest at Centerton, downstream, with Muncie's only about 70 percent of it.

Between Muncie and Centerton the drainage area grows about 10.1 times, but the TP load grows about 14.6 times and the NO23 load only 7.9 times. Nearly half the TP load and about 58 percent of the NO23 load passing Centerton came from the land between Nora and Centerton.

Figure 2. Loads and yields of total phosphorus and nitrate plus nitrite at the three gauges, water years 1992–2017.

Streamflow is rising

EGRET smoothing and Mann-Kendall tests both showed upward trends from water years 1978 to 2017 in annual maximum daily, mean daily and 7-day minimum flows at all three gauges. Only some were statistically significant at the 0.05 level: mean daily flow at Muncie, and maximum daily flow near Nora and near Centerton.

Table 1. Trends in annual streamflow statistics, water years 1978–2017; a filled arrow marks a trend significant at the 5-percent level.

The climate is part of the story. Since 1895, Indiana's average yearly precipitation has risen 5.6 inches, with more of it in heavy downpours, and the state has warmed 1.2 °F; temperatures are projected to rise about 5 to 6 °F above the 1971–2000 average (52.5 °F) by mid-century. Researchers have warned that heavier spring rain will wash more human-derived nutrients into Indiana's waters, and that warmer, nutrient-rich water means more algal blooms, murkier water and less oxygen. The Conservancy is interested in measures that help the landscape hold and absorb water.

Nutrient and sediment trends, 1997–2017

Bootstrap methods — repeated analyses of random resamples of the data — were used to judge the size, direction and likelihood of flow-normalised changes.

Despite rising streamflow, flow-normalised concentrations and fluxes most often went down. That is promising, but rising flows may partly cancel the gains from better land treatment and wastewater treatment, so more nutrient and sediment retention on the landscape may be needed to meet goals.

• Near Nora, every flow-normalised concentration and flux of TSS and nutrients decreased.
• At Muncie, increases in TSS and TP concentrations and fluxes were judged likely.
• Near Centerton, some increases occurred too.

Table 2. Directions of change in flow-normalised concentrations and fluxes, water years 1997–2017 (L, likely; ALAN, about as likely as not; HL, highly likely).

A puzzle at Centerton

Near Centerton, flow-normalised NO23 concentrations likely rose, while its flux very likely fell. The explanation lies in when concentrations were high:

• At daily flows below about 1,500 ft³/s, concentrations exceeded 4 mg/L more often and for longer.
• At flows above about 1,500 ft³/s, they exceeded it less often.

Yearly flux is dominated by the rarer high flows, where concentrations fell; the time-weighted mean concentration is dominated by the more common low flows, where they rose. Flows at Centerton were below 1,500 ft³/s more than 50 percent of the time over its 71-year record, and above 3,000 ft³/s only about 25 percent of the time. One possible reason: sporadic, runoff-driven non-point sources of NO23 fell, while steadier sources — wastewater treatment plants and/or tile drains — grew.

Figure 3. Estimated nitrate-plus-nitrite concentration in relation to streamflow over time, White River near Centerton, 1992–2017.

Sources

• Koltun, G.F., and Hauswald, C., 2020, Trends in streamflow, nutrients, and total suspended solids in the Upper White River Basin, Indiana: U.S. Geological Survey Fact Sheet 2020–3030. https://pubs.usgs.gov/publication/fs20203030
• The figures and tables come from the fact sheet's PDF. A photograph credited to The Nature Conservancy is not reproduced.
• Words on this page from people and organisations outside the federal government are paraphrased; rewritten in hubnx's own words.