Red River

I recently mentioned that the timing of snowmelt runoff affects drought conditions in the American southwest. This harkens back to early this year, when I had communications with Pat Neuman, a retired hydrologist working in the midwest. He communicated to me that his studies of snowmelt runoff timing in the Red River (and other midwestern) basin indicated that it was coming earlier and earlier in the year. I even posted on the topic.

I thought I’d show an example, using streamflow for the Red River as measured at Fargo, ND. This is not the Red River of movie fame, it’s more properly called the “Red River of the North,” and forms the boundary between North Dakota and Minnesota. For each year, the data are the day of the year for the beginning of snowmelt runoff measured at Fargo, ND.

It looks like over the last few decades, snowmelt runoff has been beginning earlier. If we perform a linear regression, we can indeed confirm that the day of beginning snowmelt has been getting earlier, and the result is statistically significant.

The changes are a lot easier to see if we plot moving averages, showing the average for each ten-year interval spaced one year apart:

From this it looks like the changes in snowmelt runoff timing have themselves changed. In fact it rather looks like the average timing was reasonably steady until the mid-1960s, but has been getting rapidly earlier since then. A good way to characterize this is to find the moment in time at which fitting two separate straight lines (connected at their endpoints) gives the overall best fit:

For the first time interval (1910 to 1964), the trend rate is not statistically significant, so it gives evidence of no change. For the second time interval (1965 to 2003), the trend rate is strongly significant. It appears that up to 1964, snowmelt runoff timing for the Red River at Fargo, ND, had its ups and downs but remained constant over the long haul. Since 1965, snowmelt runoff into the Red River has been happening sooner. In fact, the slope of the line for the recent data indicates that snowmelt is coming earlier by about 0.62 days per year. From 1965 to 2003, its timing is about three and a half weeks earlier.

That’s not a trivial difference, it’s quite significant. It affects the supply of water to human society, and to native living things. In southern California, it affects both the severity and the length of the summer drought season. And it’s not just affecting the Red River of the North; such changes have been documented in New England, the northern midwest, and extensively in the American West (see here for some references).

One of the most worrisome aspects of global warming is that it will bring undesirable changes in the hydrological cycle. This doesn’t just mean changes in rainfall amounts and distribution; it also depends on how the timing of the seasons is itself being affected. Fortunately, some of these changes have been studied and can be partly planned for. Let’s hope we can limit the amount of global warming the planet experiences, so the changes don’t overwhelm us.