seepage,  etc.  The  remainder  represents  the  net  quantity  of  water
available  for  power  purposes,  The  summation  of  these  monthly
balances,  added  one  to  the  other  consecutively  can  be  platted  in  a
curve  in  which  the  abscissa  of  each  point  represents  the  total  time
from  the  beginning  of  the  period ;  and  the  ordinate,  the  total  quan-
tity of  water  available  during  the  same  interval.  The  scale  may
represent  inches  on  the  drainage  areas,,  cubic  feet,  acre  feet,  or
such  other  unit  as  may  be  desired.  Such  a  curve  is  represented  in
Fig.  396  by  the  irregular  curve  A-B-C-D-E-F.  The  inclination  of
the  curve  at  any  point  indicates  the  rate  of  the  net  flow  at  that  par-


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Calculations  for  Storage.  637

ticular  time.  When  the  curve  is  parallel  to  the  horizontal  axis,  the
flow  at  that  time  will  just  balance  the  losses  caused  by  evapora-
tion, seepage,  etc.  A  negative  inclination  of  the  supply  line  shows
that  a  loss  from  the  reservoir  is  taking  place.
In  a  similar  manner  the  curve  of  consumption  can  b*^  nlatt^H.
For  most  purposes  this  can  be  considered  a  straight  line  <
iation  in  the  use  of  power  from  season  to  season  is  a  refin
usually  warranted,  unless  the  uses  to  which  the  power  is
at  various  times  of  the  year  are  well  established.  In  ]
series  of  straight  lines  of  consumption  are  drawn,  repress
use  of  water  at  rates  of  100  to  600  acre  feet  per  day.  T
correspond  essentially  to  rates  of  from  50  to  300  cubic  fe<
ond.
