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Study Tracks Raindrops

As part of the Environmental Engineering Seminar Series, Jeffrey McDonnell, Ph.D. presented a lecture on Friday, Jan. 10  in the McDonnell Douglas Engineering Auditorium entitled “Tracking Raindrops: Basic Science for Understanding Global Change Impacts on Fresh Water.” Catering to mainly engineering students and professors, the lecture went into detail about recent research and different techniques for tracing the flow of rain and watersheds and how that impacts global freshwater sources.

Guest speaker McDonnell is the professor and associate director of the Global Institute for Water Security at the University of Saskatchewan in Canada. His findings presented on Friday mainly consisted of western North America, particularly the Cascade Mountain Range where he conducts much of his research, although his case studies also included Mexico, New Zealand and the Southeast United States. The increasing scarcity of freshwater is the biggest concern when investigating watershed and rain flow, and the results from McDonnell’s studies indicate a different approach to analyzing and collecting data than that of the past.

“Understanding storage, flow and residence time is key for wrestling many of the modern water resource engineering problems,” McDonnell said.

Land use ¾ the way that humans use land with agriculture, buildings, industry ¾ affects the amount of runoff caused by rainfall. Runoff is the flow of water over land, which can be caused by rain, snow melt or other sources. Runoff is affected by the soil’s ability to absorb rainfall, but also by the slope of a hillside and the bedrock underneath. Once the amount of water that saturates the bedrock reaches a certain threshold, the hillside releases water. A variety of factors of this nature make it difficult to predict how much groundwater exists, what condition it is in (as related to pollutants) and how old it is.

Stationarity is the idea that there will always be approximately the same amount of water available to us in a given region, despite fluctuations in climate, seasonal changes and long-term climate change. However, McDonnell cited an article entitled “Stationarity is Dead,” published in “Science” in February 2008, to argue that this may not be true and that the Earth is indeed in danger of suffering major losses to its water supply. Drastic changes to land cover, like deforestation and urban expansion, make stationarity an unlikely theory and predictions for water availability even more difficult.

McDonnell introduced a fairly new method of tracking water with isotopes, stable molecules that naturally exist in water. In the past, it was very difficult and expensive to do isotope research, but with updated technology the data is easier to gather and from it scientists can glean much more about the age of water from a certain source, also referred to as residence times. Previously, data from rainfall and runoff would be graphed to form a correlation, but the isotope approach presents a more accurate transit time from water through catchments, or drainage basins.

Residence times show how long the water has been in ground versus how quickly it is released through watersheds by rainfall, which is what comprises the transit time. Typically, the deeper the bedrock, the older the water is because it can saturate further and take longer flow-paths than more shallow bedrock. Using residence times to see how old the water is, we can then predict the amount of water in the ground and when certain substances known to be present, like nitrates, will show themselves. The size of the basin, the slope length and gradient of the hill are also contributing factors to the mean residence time of the water.

“What this is really pushing us towards is thinking about a new conceptual approach for watersheds that includes storage, flow paths and residence time,” McDonnell said.

McDonnell is conducting further research to probe this subject of the transit time at a catchment outside Nanjing, China this summer. He will also be conducting other field work next winter.

Above all, McDonnell sees his work as a different approach for analyzing mountainous watersheds and the problem we face in predicting our water supply and scarcity with changing environmental factors. He urged the undergraduate and graduate research students in the audience to look to using isotopes for another interpretation of residence and transit times.