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Making sense of hydrologic variability
Precipitation and streamflow are both incredibly variable aspects of the environment, often changing dramatically over short time scales and small spatial scales. If you look up the precipitation record for a location of interest (data freely available from the National Weather Service, Natural Resource Conservation Service, and many other outlets), you would see that events seem to happen 'randomly', often without an obvious pattern in the frequency, magnitude or duration. Take, for example, the precipitation record for Kingston, New York from October 1, 2010 through September 30, 2013 (Figure 1). There is an immense amount of variability from day to day. So what can we really say about precipitation from these data? July and August of 2011 appears to be a very wet time period, with many events clustered and one event reaching over 12 cm (nearly 5 inches)! Do you think that caused a flood? Precipitation was very sparse from mid-December 2011 to mid-February 2012. Do you think that was a drought?
It is essentially impossible to answer the questions posed above about July 2011 being a flood or winter 2011-2012 being a drought from the precipitation data alone because precipitation is not the only factor that causes floods and droughts. Processes of water use and transport occurring in a landscape also matter. For example, increased impervious surface associated with urbanization is known to dramatically increase runoff, resulting in much higher peak discharge (bigger floods) for any given amount of rainfall. In contrast, a large rain event occurring on dry soil will have a relatively small effect on streamflow compared with the same rainfall event occurring on very wet or saturated soils, because more of the rain would be absorbed by the dry soil. Landscape processes also influence droughts. While a prolonged lack of precipitation can initiate a drought, the severity of the drought is strongly influenced by the water demand, by vegetation and/or humans, throughout the landscape. So the immense variability that is inherent to precipitation is often amplified or dampened by processes occurring in the landscape. This greatly complicates the job of forecasting floods and droughts, requiring that hydrologists account for numerous factors that vary in time and/or space. However, harnessing an immense amount of real-time information from precipitation gages and radar systems, river discharge gages, and satellite imagery, entities such as the National Weather Service can make remarkably accurate and timely forecasts of floods and droughts. The National Weather Service maintains a network of 13 river forecast centers and 50 hydrologic service areas that provide real-time flood warnings throughout the US, which can greatly reduce loss of life and property damages.
Source: Patrick Belmont, data from NOAA National Climatic Data Center.