ASSIGNMENT 2 – NASE 328
Mono Lake Water and Salt Balance
Water budgets take into account inputs and outputs of water into a watershed or specific water body. When considering a particular water body, such as a lake, if water outputs exceed inputs, then the volume of water in the lake will decrease. In this exercise, we will be considering the water budget for Mono Lake in California.
Let’s begin with a simple model. Inputs and outputs of water are called fluxes. A flux is quantified as energy, mass, or volume per time. When working with hydrologic models, we typically quantify fluxes in terms of volume per time (e.g., cubic feet per second, gallons per hour, cubic meters per year, etc.).
Read through the State of the Lake, paying careful attention to the section “Why is Mono Lake rising more slowly than expected?” https://www.monolake.org/learn/stateofthelake/
What are the four components that most strongly influence the volume of water in Mono Lake? [12 pts.] The Sierra Nevada runoff, basin precipitation, evaporation and DWP stream diversions.
Add these components to Fig. 1 with arrows representing fluxes of water into or out of Mono Lake (as exemplified by the groundwater inputs and outputs already included).
Figure 1. Simplified water budget model for Mono Lake
The change in the volume of water stored (ΔS) in Mono Lake for a given period of time is equal to volumetric inputs minus outputs and can be calculated as follows:
Using the fluxes drawn on Fig. 1, modify the right-hand side of this equation to explicitly name the inputs and outputs, including the proper +/- sign for each flux; e.g., GW_net=GW_in-GW_out. [12 pts.]
Let’s begin by calculating the change of water volume in Mono Lake over the course of a typical year in the 1960s.
The average annual precipitation for Mono Lake 10,590 Mgal/yr (million gallons per year), the rate of evaporation was 45,440 Mgal/yr, runoff from snowmelt provided 42,380, and 35,270 Mgal/year was diverted by Los Angeles Department of Water and Power (LA DWP). Assuming that groundwater inputs were equal to groundwater outputs, what volume of water was lost from Mono Lake in one year? [12 pts.]
Use Fig. 2 to calculate the annual change in water level throughout the 1960s. [12 pts.]
Figure 2. Mono Lake elevation, 1938-2021
Discuss how altering the magnitude of each flux into and out of Mono Lake would impact the lake’s salinity. Consider sources and losses of salinity here as well. Hint: You might want to search for images of Mono Lake and the surrounding area and conduct some more research here. Be sure to cite any sources you use. [40 pts.]