First Assignment
Engineering Hydrology
University of Isfahan
Water Balance
1. An area is being irrigated by a stream with a drainage area of 300 km2. The drainage area
contribution is 0.1 m3/s/km2. Determine the discharge of the channel and the area irrigated if 0.37
m3/s are required per 1000 hectares.
2. The average monthly precipitation in a watershed of 4500 km2 is 46 cm.If cumulative losses are
20% of precipitation,determine the area of exercise 1 that can be irrigated with the remaining
water. Also calculate the channel discharge.
3. The storage in a river reach at a specified time is 2 hectare-meter.At the same time the inflow to
the reach is 15 m3/s and the outflow from it is 20 m3/s .One hour later the inflow is 20 m3/s and
outflow is 20.5 m3/s.Determine the change in storage that occured in the reach.Is the storage at the
end of the hour greater or less than the initial value?
4. There are two reservoires, A and B,connected in series. A is an upstream reservoir. In 1 year
this reservoir received rainfall of 100 cm,discharged 30 cm of water,and experienced a change in
storage of 20 cm.Compute the evaporation from this reservoir asuming that other abstractions are
vanishingly small.Reservoir A contributes its runoff to reservoir B in which storage remains
constant.compute the runoff from this reservoir if its rainfall and evaporation are the same as for
reservoir A.
5. Runoff from a 250-square mile catchment drains into a reservoir that has an average surface
area of 4500 acres. The reservoir is to be used for a municipal water supply.
Annual rainfall over the watershed is 40 inches and the annual runoff is 2 inches.
Evaporation from the reservoir averages 40 inches per year. If the surface area of the
reservoir is roughly constant with depth, how much will the reservoir depth change
over one year? (For the exchange students: 1 inch = 2.54 cm, 1 acre = 4.05×103 m2,
1 mile = 1609 m.)
Evaporation
1. The following information are available from a wheather station. Estimate the daily
evaporation using Penman method. The incoming solar radiation and outgoing earth
radiations are 30 and 10 MJ/m2/day, respectively. Find the monthly evaporation using
Thornthwaite’s equation prodided that the data are monthly.
Precipitation
1- Estimate the mean areal rainfall using the isohyetal method.The area is 3000 hectares,and area between
isohyetal lines area are as follows:
Isohyetal Interval (cm)
Enclosed Area (ha)
0-1
1-2
2-3
3-4
4-5
150
900
600
450
900
Assume that within each enclosed area a rain guage is located whose measured rainfall is the average of
isohyetal interval. Estimate the mean areal rainfall by the arethmatic mean method ,and compare this estimate
with the one obtained from isohyetal method.
2- Assume that rainfall is not known at the guage located at the point (3,3) in figure .Compute the rainfall
at this point using the normal ratio method. The normal precipitation of the three neighboring gaging
stations are as follows:
Station Coordinates
(1,2.5)
(4,1)
(3,5)
(3,3)
Normal annual precipitation
(cm)
28
15
30
25
Precipitation
(cm)
25
10
25
?
(1,5)
(3,5)
(4,4)
(3,3)
(1,2.5)
(2.5,1.5)
(1,1)
(4,1)
3. Use the inverse distance method to solve exercise 2, and compare the result with that of the normal
ratio method.
4. Figure 1 shows a 1-km2 basin having three rain guages .Estimate the mean areal rainfall by the
Thiesson method for the storm whose rainfall depths are given on the map.The areas of the polygons
around guages A,B, and C ,respectively,are 0.2,0.4,0.4 km2.Compare it with the arithmetic mean
rainfall.
Figure 1. A watershed showing rain-guage locations.
5. Consider the probable maximum precipitation (PMP) for Isfahan.
a. Compute the PMP for a one-hour storm using the approximate formula for worldrecord precipitation amounts.
b. Describe engineering situations (if any) in which you would need to use the PMP
for Isfahan.