By far the greatest number of hydrological studies into the effects of climate change have concentrated on potential changes on streamflow and runoff. The distinction between “streamflow” and “runoff” can be vague, but in general terms streamflow is water within a river channel, usually expressed as a rate of flow past a point—typically in m³ s^-1 —whereas runoff is the amount of precipitation that does not evaporate, usually expressed as an equivalent depth of water across the area of the catchment. A simple link between the two is that runoff can be regarded as streamflow divided by catchment area, although in dry areas this does not necessarily hold because runoff generated in one part of the catchment may infiltrate before reaching a channel and becoming streamflow. Over short durations, the amount of water leaving a catchment outlet usually is expressed as streamflow; over durations of a month or more, it usually is expressed as runoff. In some countries, “runoff” implies surface runoff only (or, more precisely, rapid response to an input of precipitation) and does not include the contribution of discharge from groundwater to flow, but this is a narrow definition of the term.

This section first considers recent trends in streamflow/runoff and then summarizes research into the potential effects of future climate change.

4.3.6.1. Trends in Observed Streamflow

Since the SAR, there have been many notable hydrological events—including floods and droughts—and therefore many studies into possible trends in hydrological data. Table 4-1 summarizes some of these studies and their main results.

In general, the patterns found are consistent with those identified for precipitation: Runoff tends to increase where precipitation has increased and decrease where it has fallen over the past few years. Flows have increased in recent years in many parts of the United States, for example, with the greatest increases in low flows (Lins and Slack, 1999). Variations in flow from year to year have been found to be much more strongly related to precipitation changes than to temperature changes (e.g., Krasovskaia, 1995; Risbey and Entekhabi, 1996). There are some more subtle patterns, however. In large parts of eastern Europe, European Russia, central Canada (Westmacott and Burn, 1997), and California (Dettinger and Cayan, 1995), a major—and unprecedented—shift in streamflow from spring to winter has been associated not only with a change in precipitation totals but more particularly with a rise in temperature: Precipitation has fallen as rain, rather than snow, and therefore has reached rivers more rapidly than before. In cold regions, such as northern Siberia and northern Canada, a recent increase in temperature has had little effect on flow timing because precipitation continues to fall as snow (Shiklomanov, 1994; Shiklomanov et al., 2000).

Table 4-1: Recent studies into trends in river flows.
Study Area	Data Set	Key Conclusions	Reference(s)
Global	– 161 gauges in 108 major world rivers, data to 1990	– Reducing trend in Sahel region but weak increasing trend in western Europe and North America; increasing relative variability from year to year in several arid and semi-arid regions	– Yoshino (1999)
Russia
– European Russia and western Siberia	– 80 major basins, records from 60 to 110 years	– Increase in winter, summer, and autumn runoff since mid-1970s; decrease in spring flows	– Georgiyevsky et al. (1995, 1996, 1997); Shiklomanov and Georgiyevsky (2001)
– European former Soviet Union	– 196 small basins, records up to 60 years	– Increase in winter, summer, and autumn runoff since mid-1970s; decrease in spring flows	– Georgiyevsky et al. (1996)
Baltic Region
– Scandinavia		– Increase in winter, summer and autumn runoff since mid-1970s; decrease in spring flows	– Bergstrom and Carlsson (1993)
– Baltic states		– Increase in winter, summer and autumn runoff since mid-1970s; decrease in spring flows	– Tarend (1998)
Cold Regions
– Yenesei, Siberia	– Major river basin	– Little change in runoff or timing	– Shiklomanov (1994)
– Mackenzie, Canada	– Major river basin	– Little change in runoff or timing	– Shiklomanov et al. (2000)
North America
– United States	– 206 catchments	– 26 catchments with significant trends: half increasing and half decreasing	– Lins and Slack (1999)
– California	– Major river basins	– Increasing concentration of streamflow in winter as a result of reduction in snow	– Dettinger and Cayan (1995); Gleick and Chalecki (1999)
– Mississippi basin	– Flood flows in major basins	– Large and significant increases in flood magnitudes at many gauges	– Olsen et al. (1999)
– West-central Canada	– Churchill-Nelson river basin	– Snowmelt peaks earlier; decreasing runoff in south of region, increase in north	– Westmacott and Burn (1997)
South America
– Colombia	– Major river basins	– Decrease since 1970s	– Marengo (1995)
– Northwest Amazon	– Major river basins	– Increase since 1970s	– Marengo et al. (1998)
– SE South America	– Major river basins	– Increase since 1960s	– Genta et al. (1998)
– Andes	– Major river basins	– Increase north of 40°S, decrease to the south	– Waylen et al. (2000)
Europe
– UK	– Flood flows in many basins	– No clear statistical trend	– Robson et al. (1998)
Africa
– Sahelian region	– Major river basins	– Decrease since 1970s	– Sircoulon (1990)
Asia
– Xinjiang region, China	– Major river basins	– Spring runoff increase since 1980 from glacier melt	– Ye et al. (1999)
Australasia
– Australia	– Major basins	– Decrease since mid-1970s	– Thomas and Bates (1997)

However, it is very difficult to identify trends in hydrological data, for several reasons. Records tend to be short, and many data sets come from catchments with a long history of human intervention. Variability over time in hydrological behavior is very high, particularly in drier environments, and detection of any signal is difficult. Variability arising from low-frequency climatic rhythms is increasingly recognized (Section 4.2), and researchers looking for trends need to correct for these patterns. Finally, land-use and other changes are continuing in many catchments, with effects that may outweigh any climatic trends. Changnon and Demissie (1996), for example, show that human-induced changes mask the effects of climatic variability in a sample of midwest U.S. catchments. Even if a trend is identified, it may be difficult to attribute it to global warming because of other changes that are continuing in a catchment. A widespread lack of data, particularly from many developing countries, and consistent data analysis makes it impossible to obtain a representative picture of recent patterns and trends in hydrological behavior. Monitoring stations are continuing to be closed in many countries. Reconstructions of long records, stretching back centuries, are needed to understand the characteristics of natural decadal-scale variability in streamflow.

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