1. Water Issues Salton Sea is located in Southern California and was formed in 1
ID: 160156 • Letter: 1
Question
1. Water Issues
Salton Sea is located in Southern California and was formed in 1905 when the Colorado River breached an irrigation channel and flooded the Salton Basin. The breached bank was repaired in 1907 but the water in the basin was maintained for agricultural purposes. This “manmade” lake is designated with a primary purpose to retain runoffs from agricultural activities. Over the years, the Sea has supported recreational activities, fishing and served as a sanctuary for migratory birds. See http://www.parks.ca.gov/?page_id=639.
Salton Sea is a terminal lake –water enters the lake by precipitation and flows from streams and groundwater. Water leaves only by evaporation. The lake level fluctuates with climatic changes. In the last 4 decades, water from the Colorado River is being diverted from agricultural purposes to satisfy population growth of the urban coastal areas of California. This has resulted lower runoffs from agricultural fields to the Salton Sea.
Questions:
At a water elevation of 228 feet, the Salton Sea has a volume of 7,625,000 acft and a surface are of 230,000 ac. (An acrefoot is the amount of water that will cover an acre of land to a depth of 1 foot (43,560 ft3 )). This is a measure of the volume of water commonly used in Western US). Annual inputs to the lake include 2.52 ins. of direct precipitation, agricultural runoffs from gauged streams of 1,110,000 acft per year, and ungauged streams of 250,000 acft per year. Evaporation is 73.2 ins. per year.
(a) Make a water budget showing inputs and outputs (in acft per year) and indicate whether the input balances with the output?
(b) If the input does not balance with the output (in (a)), by how much will the lake level rise or fall from the 228 ft elevation assuming that the surface area is the same (i.e., at 228 feet) for any changes in the water level?
(c) What is the residence time (average time it takes for the volume of water to be exchanged once) for the water in Salton Sea when the water surface is at 228 ft?
(d) If the water elevation falls below 230 feet, the water quality and ecosystem of Salton Sea will be impacted. Estimate the amount of the agricultural runoff (gauged and ungauged runoff and groundwater) that can be reduced from the 228 feet level to obtain this water level. Assume the surface area at 230 feet is 190,000 ac. (Note that evaporation and precipitation are proportional to the surface area of the Sea)
Explanation / Answer
Given data is
Solution:
First convert all measurements to units of acre-feet per year
Precipitation= 2.52 in/yr× 0.083 ft/in = 0.209 ft/yr
= 0.209 ft/yr × 230,000 ac = 48106.8 ac-ft/yr
Evaporation = 73.2 in/yr × 0.083 ft/in = 6.07 ft/yr
= 6.07 ft/yr × 230,000 ac = 1397388 ac-ft/yr
(A) INPUTS](ac-ft/yr)
Precipitation 48106
Stream flow 111,0000
Ground water inflow 250,000
Total 1408106
OUTFLOWS (ac-ft/yr)
Evaporation 1397388
Total 1397388
Input is greater than output
(B) The lake level will rise over the long term, as inputs are greater than the only output, which is evaporation.
(C) In order for the inputs and outputs to be equal, the lake will rise so that the surface area is greater and more water evaporates as evaporation as well as the precipitation input is a function of lake surface area.
The excess input at an area of 230,000 acre surface area is 1408106 ac-ft/yr – 1397388 ac-ft/yr or 10718 ac-ft/yr.
The net evaporation is total evaporation less precipitation
Net evaporation = 73.2 in/yr – 2.52 in/yr = 70.68 in/yr = 5.866 ft/yr
Increase in area = (10718 ac-ft/yr) / (5.866 ft/yr) = 1827.13 ac
Total area of lake = 1827.13 ac + 230,000 ac = 231827.13 ac = 231827 ac
(D) The residence time = (7625,000 ac-ft) / (1397388 ac-ft/yr) = 5.45 yr
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