Assignment 2 -Lake Watersheds and morphometry
Elkwater lake is a small lake situated near Cypress Hills Provincial Park. The
watershed is mostly forested however it contains several roads and much of
the land near the north side of the lake is cleared for agriculture. The area is
semi-arid and water in the region is in short supply. Intensive cattle
operations near the lake have been proposed, as have irrigation schemes
involving allocation of water from the lake. In this assignment we will explore
this lake and its watershed and some proposed water allocation scenarios.
Question 1. Using the area measuring tool in Adobe, measure the area of the
lake’s watershed whose boundaries are outlined by the purple line in Fig. 1.
Consider only the land draining to the lake and do not include the lake itself.
The runoff coefficient for this region is only 0.075 m. What should be the
discharge into this lake in m3/yr.
Question 2. If the mean annual precipitation rate in this region, averaged over 30
yr is 0.570 m, and no significant long-term changes have occurred in the
watershed storage pools, what should be the average rate of evapotranspiration
from this watershed?
Fig. 1
Question3. Calculate the volume, surface area and mean depth, and depth
ratio of Elkwater lake by measuring the area under the hypsographic curve
that you create using the bathymetric map given in Fig. 2. According to
section 12.3 in your text how big (height and length) would the largest waves
on this lake be. From Eq. 20.1 in your text what depth would you predict that
the depositional boundary to occur at along the transect marked AB. Compare
the value you obtain on the steep side (near B) to the less steep side (near A).
Question 4. Using the first regression model on Table 11-2 in your text
estimate the depth at which the thermocline should occur in a lake of this
surface area. Would you expect this lake be deep enough to stratify
thermally? The light extinction coefficient for this lake is 0.85/m. Outline on
the map the area that you think should be covered by rooted aquatic
macrophytes, what fraction of the lake’s area is this. Assume that
macrophytes can colonize areas that receive 10% or more of incident light.
Assuming that phytoplankton photosynthesis can go on down to the 1% light
level, What fraction of the lake’s volume can support photosynthesis?
Question 5, Assuming that the lake’s volume does not change significantly from
year to year estimate the water turnover rate in a year of average runoff, a year
with twice the average runoff, and a year with half the average runoff. Now
calculate the lake’s water residence time for these years. The mean annual lake
surface evaporation rate estimated from the Canadian Hydrological Atlas for
lakes of this area is 0.770 m (assume it to be constant).
Question 6. How much of the lake’s water can be allocated if the constraint is (a)
for the lake’s level to remain stable in the average year, (b) for the lake’s outflow
Ross Creek, (upon which many downstream users depend for water) to remain at
or above 1 million m3/yr in an average yr (c ) 0.5 million m3/yr. How would these
allocation limits change for a year with half the average runoff? Discuss in general
terms the wisdom of allocating water from this lake.
A
B