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Hydrological processes within a river basin

Drainage basin

• definition of a drainage basin (also often: watershed, river basin, or catchment): area that topographically appears to contribute all the water that passes through a given cross section of a stream
• widely recognized as the natural unit of water management and for the scientific study of hydrological processes
• example: major basins in NY state (Fig) State lines are shown in white, county lines are shown in light gray, streams are shown in light blue, and river basin boundries in orange (http://h2o-nwisw.er.usgs.gov/nwis-w/NY/)
• water balance of a drainage basin: Runoff = Precipitation - Evaporation +/- delta Storage (terms are always positive) (Fig)
• runoff: total amount of water leaving the system as surface water or groundwater
• hydrograph: timeseries of the discharge rate of a drainage basin

Floods

• flood occurs when a river overtops its banks and flows across the floodplain
• for a hydrologist, a flood is a discharge rate that execeeds some threhold value
• in rivers, floods and low flows are expressions of the temporal variability in rainfall or snowmelt interacting with river basin characteristics (basin form, hillslope properties, channel network properties)
• two features determine the occurrence of a flood: volume of direct surface or near-surface runoff, and synchroneity of runoff events within the basin
• flooding may also be the result of sudden release of water from dams or lakes, ice jams
• floods cause the biggest natural hazard damage in the US, example: Mississippi flood, 1993

Estimating the magnitude and frequency of floods

earliest approaches: estimate of maximum sized events, then frequency as important as magnitude: statistical approaches), today: 'worst scenario'

simplest approach: use worst event on record

envelope curves (enclose maximum values)

relationship between depth and duration of rainfall events

Envelope curves for maximum flood discharges

Past record key for the future? Statistical techniques

an event with a frequency of 1 in 100y has a return period (recurrence interval) of 100 years

discharge versus percentage of time discharge is exceed

frequency, F, for annual floods: F=m/(n+1), m: Rank order of event, n: number of years

annual maximum series, used to predict the 100 year flood

steps: determine rank and frequency and plot discharge against probability using some statistical distribution

normal distribution works often well with precipitation data, log normal for discharge

problems: not deterministic, based usually on non-adequate data, climate and terrestrial environment is variable

Resources

• Dingman, S.L. (1994) Physical Hydrology. Prentice Hall, Englewood Cliffs, 575pp.