A8 Change in Hydrological Regime

Topic

A basin's hydrological regime can undergo changes in its statistical structure—specifically its mean flow, variability, and seasonal flow distribution—due to both human and climatic factors: land-use changes (such as deforestation or urbanization), dam construction, groundwater extraction, and variations in the temperature and precipitation the basin receives.

Urbanization-induced surface sealing, in particular, reduces the time it takes for rainfall to concentrate in the river channel, increases peak flow, and decreases base flow by suppressing much of the natural infiltration that previously occurred; a basin's runoff coefficient—which might hover around just 20 percent of rainfall in a rural area—can reach up to 85 percent in a densely built-up urban area.

Dams, for their part, alter the hydrological regime differently: they smooth out the river's natural hydrograph, trap a significant portion of the sediment that would otherwise be transported downstream, and modify the temperature of the water eventually released.

Changes in snow-related seasonality represent a third way the regime is altered: when snow begins to melt earlier in the year, this shift causes the peak flow—traditionally occurring in spring during the thaw—to move toward winter.

Formally detecting these changes in the hydrological regime relies on two complementary tools: the Mann-Kendall test, which identifies statistically significant trends in flow series over time, and the double-mass method, which makes it possible to distinguish how much of the change is attributable to climatic factors versus human-induced causes. Budyko sensitivity models—the same framework previously used to describe hydrological drought—also make it possible to estimate how much a basin's discharge changes in response to a specific shift in precipitation or evapotranspiration; thus, from the alteration of statistical structure with which this document begins to the discharge sensitivity with which it concludes—and encompassing factors such as urban paving, dams, and changes in snow seasonality—a basin's hydrological regime ultimately reflects, in the very pattern of its discharge, every human and climatic pressure acting upon it.

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