|
<br />1-
<br />
<br />1-.
<br />
<br />1-.
<br />
<br />L.
<br />
<br />L
<br />
<br />L
<br />
<br />..
<br />,-."-",,",,.-,-..,-,.,.::-..-. -,
<br />.:'-...._._-.:.-."..............
<br />..........-.:::-;<;.:.-..-. .....-...
<br />":": ,-'",:;::::::::)':,-,::'-::::-"::::':"-::'::':
<br />
<br />
<br />rll"
<br />
<br />r'{/:~>;.:::::;::,;:=:: ':~~)~
<br />~;.}}tW?\::;:-. ..-::~}}~
<br />>"'.''''''::'0::''':'- .....-. ......
<br />;:;.:.:::::;:.:f{::
<br />:/::::'::;:::'::::::::::':-;;':-:::_~:'<::?-:;:;::
<br />
<br />-
<br />
<br />",.:,: ::.;:.~: -':-.- "-. :.: ,-, ..;...:.-.;.,
<br />
<br />R,ainfall IJ~';l~;~:bs
<br />
<br />.::::;:.'::
<br />
<br />. ".;-.
<br />c.,',"':'-',';,",,""'"';
<br />." ':" ".: ,::::::::,:-::~.~
<br />.-..........:<,
<br />.-.-...-,.......
<br />
<br />11~!1 General
<br />
<br />a
<br />
<br />II
<br />
<br />III
<br />
<br />~
<br />,
<br />i
<br />
<br />l-
<br />
<br />.lIl..
<br />
<br />,-
<br />
<br />1-
<br />1~
<br />1~
<br />.,~
<br />.,-'
<br />
<br />~~:::-':-.:;:::'::.:'::::-:>
<br />1!;li;j;;~~ Rainfall excess is that portion of the total williall depth that dra'ms directly from the
<br />land surface by overland flow. Bya mass balance, rainfall exc'~ss plus rainfall loss
<br />equals precipitation. When perfonning a flood analysis using a rainfall,nrnoff
<br />model, thl? determination of rainfall excess is of utmost import2mce. Rainfall excess
<br />Integrated over the entire wat~rshed resull'; in runoff volume, and the temporal
<br />distribution of the rainfall eXCf!SS will, along with the hydraulics of runoff, deter-
<br />mine the peak discharge. Therefore, the estimation of the magnitude and time
<br />distribution of raillialllosses should be performed with the best practical technol-
<br />ogy, considering the objective I)f the analysis, economics of th,~ proj",-'i, and conse-
<br />quences of inaccurate estimate",.
<br />
<br />Rainfall losses are generallycoliSidered to be the result of evapora lion of wa ter from
<br />the land surface, interception of rainfall by vegetal cover, depr.?ssion storage on the
<br />land surface (pavee, or unpavt!d), and infiltration of water into the soil matrix. A
<br />schematic representation of rainfall losses for a uniform intensity rainfall is shown
<br />in Figure 4.1. As sho\\'l1 in the figure, evaporation can start at an initially high rate
<br />depending on the land surface temperature, but the rate decreases very rapidly and
<br />would eventually reach a low, steady,state rate. From a practical standpoint, the
<br />magnitude of rainf"llloss that can be realized from evaporation during a storm of
<br />sufficient magnitude to cause flood nmoff is negligible.
<br />
<br />Interception, also illustrated in Fii511re 4.1, varies depending upon the type of
<br />vegetation, maturity, and extent of canopy cover. Experimental data on intercep-
<br />tion have been collected by numerous investigators (Linsley and others, 1982), but
<br />little is known of the interception values for most hydrologic problem,;. Estimates
<br />of interception for various vegetation types (Linsley and others, 1982) are:
<br />
<br />":":"":;':"-;':'-':":'.
<br />
<br />.."....--......._---.
<br />;.-..-....-_...<<......
<br />
<br />.,:.",,::;:
<br />
<br />37
<br />
|