---

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 threshold 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

simplest approach: use worst event on record

envelope curves for maximum flood discharges

use the past record as key for the future? Statistical techniques

the maximum annual discharge rate of the river in a period of 100 years is called the 100y flood

the probability to exceed this value is in a given year is 1%, the exceedence probability is Pe = 0.01

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

if data are normally distributed (or can be transformed so that they are normally distributed) the discharge rate equivalent to the 100y flood can be determined

there are differnt ways how to do this

1. determine the rank of the maximum annual dischareg rates of the time series, then calculate the exceedence probability Pe: =rank/(number of measurements+1), and plot the dischareg rate or ln(discharge rate) vs Pe on Probability paper (Example ) and extrapolate the linear fit to the data to the recurrence interval of interest
2. determine mean and standard deviation of the maximum discharge rates and determine the discharge rate that is exceeded in 1% of the cases using the normal disrtibution

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

the problem of this approach: 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.