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 m 3 /yr? (4 marks). 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 (2 marks)?

Fig. 1

Question 3. Calculate the volume (m 3 ), mean depth (m), and depth ratio of Elkwater lake. Plot the hypsographic curve that you create using the bathymetric map given in Fig. 2 (16 marks – include hypsograph). Question 4. According to the formula for maximum wave height and the description for determining maximum wavelength in Appendix 2 of: Rasmussen and Rowan. 1997. Wave velocity thresholds for fine sediment accumulation in lakes, and their effect on zoobenthic biomass and composition. J. N. Am. Benthol. Soc., 16(3):449-465., how big (height and length) would the largest waves on this lake be? From your lecture notes at what depth would you predict the depositional boundary to occur along the transect marked AB. Compare the value you obtain on the steep side (near B) to the less steep side (near A), explain why the values are different. Based on the findings in Rasmussen and Rowan (1997), how would you expect the zoobenthic communities to differ on the different sides of the lake (steep side near A and less steep side near B) in terms of biomass and species composition? (17 marks).

Question 5. Using the model found in your lecture notes, estimate the depth at which the thermocline should occur in Elkwater Lake. Would you expect this lake to be deep enough to stratify thermally? Why? 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 percent 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 percent of the lake’s volume can support photosynthesis? (15 marks)

Question 6. Assuming that the lake’s volume does not change significantly from year to year estimate the mean flushing rate in a year of average runoff. Now calculate the lake’s water residence time for an average year. 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) (5 marks).

Question 7. 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 m 3 /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 (12 marks).

A B Fig. 2