As water demands for agricultural use increase, changes in inland water bodies can be seen. In some instances we can see the depletion of these natural resources and in others we can observe the creation and pollution of water bodies. Three examples of these are the Salton Sea, Lake Chad, and the Aral Sea. With the help of Remote Sensing and GIS Applications we can monitor many aspects of these changes.
Remote sensing can be used to monitor many aspects of inland water bodies. The surface area of water bodies can be measured, which will help monitor the recession or increases of lakes over time. The best wavelengths for monitoring surface area are the near-infrared and middle-infrared bands. Water depth can also be measured with remote sensing. This allows users to show the bathymetry of the lakes being studied. The amount, type, and distribution of suspended sediment in water bodies may also be monitored by using remote sensing. This will help researchers monitor sedimentation of lakes, rivers, and ponds to estimate their lifespan. Sediment is also a carrier of pesticides and other agents from agricultural practices. Chlorophyll in water will change the spectral reflectance of the water. This would allow researchers to help predict algal blooms in water bodies. By using thermal infrared remote sensing we can also monitor the surface temperature of the water. The amount of cloud cover that an area receives throughout the year can be monitored with remote sensing. This will help scientists determine if their is climate change in the area over time. These remote sensing techniques can be used to monitor the inland water bodies described below and help with any remediation that may begin in the future.Return to the top.
Lake Chad is located in central of Africa (Fig. 1). It is bordered by four West African countries: Nigeria, Niger, Chad, and Cameroon.
At one time Lake Chad was the sixth-largest lake (land area) in the world. The levels of the lake have decreased due to climate change and the demands of humans for fresh water. The lake is now 1/20th the size it was 35 years ago, as shown in Figures 2-5 (NASA, 2001). The surface area of Lake Chad was measured in 1963 and estimated at 10,000 square miles. Today the lake surface is only 839 square miles (CNN, 2001). The lake is known for its variation in size due to the rainfall in the area. This area of Africa has seen a decrease in the amount of rainfall since the 1960s. Because of this, the amount of water used for irrigation quadrupled between 1983 and 1994. The four African countries surrounding the lake also compete for the water for irrigation needs. This has accounted for 30 percent of the decrease in the size of the lake since the early 1960s. Agriculture in the area includes farming the lake bottom and the seasonal recessional lands.
|Figure 2: 1963-Argon Satellite Photo||Figure 3: 1973-Landsat 1 MSS, bands 4,2,1|
|Figure 4: 1987-Landsat 5, bands 4,2,1||Figure 5: 1997-NOAA 14 AVHRR, bands 2,1,1|
The only water coming into the lake is from the annual monsoons and the Chari River. The amount of water flowing in from the Chari River had decreased 75 percent over the last 40 years (Devitt, 2001). With the change in climate there also comes a higher demand for water. The future of Lake Chad does not look very promising. Unless rainfall increases in the area or irrigation practices are monitored and controlled, Lake Chad may become a swamp instead of a lake.Return to the top.
The Salton Sea is an inland saline lake located in the Sonoran Desert of southeastern California near the Mexico border (Fig. 6). The lake formed in the Salton Basin, which over the last 2000 years has held at least three different large lakes and many smaller ones. Some of these lakes lasted for a hundred years or more before evaporating away in the desert.
The current lake was formed between 1905 and 1907 when an irrigation canal broke due to flooding and nearly the entire flow of the Colorado River spilled into the Salton Basin. The river flowed into the basin for more than a year before it was stopped. The current size of the Salton Sea is approximately 35 miles by 15 miles with an average depth of about 30 feet. The lake is a terminal lake and is sustained by agricultural runoff from the Imperial and Coachella valleys. As you can see from Figure 7, the Salton Sea is located in a desert region.
If it were not for the agricultural runoff in this region, the lake would have dried up in a number of years. The Salton Sea was originally a freshwater lake, but is now 25 percent saltier than the Pacific Ocean. The salinity of the lake is increasing over time due to the agricultural runoff from the flushing of irrigated farmlands in the area. The soils in the area are known to contain salt, therefore flushing removes the salt, selenium, and pesticides which is then carried into the Salton Sea. The salinity of the lake is increasing by 1.2 percent per year. The agriculture is sustaining the lake, but it is also causing the salinity to rise to levels that are affecting the wildlife of the region. The lake is located on the Pacific Flyway and is a resting stop for many species of birds during migration. If the levels of salt continue to rise, there will be a decrease of food and habitat for the birds during migration and will be too salty for the fish and other organisms within the lake.
|Figure 8: Salton Sea agriculture, 1973.||Figure 9: Salton Sea agriculture 1992.|
You can see from Figures 8 and 9 that the agricultural lands have not changed much over time. This is due to the scarcity of water for irrigation in this region. In these images you can see the U.S./Mexico border near the bottom center of the image by the change in agriculture. The Salton Sea is a highly eutrophic lake which often has algal blooms which sometimes cause fish kills.Return to the top.
The Aral Sea is a terminal lake in the central portion of the Turanian Plain in Central Asia. It was the fourth largest lake in surface are, but has now shrunk to nearly half its original size. Many say that the decline of the Aral Sea is one of the Earth's largest environmental disasters.
Two rivers flow into the Aral Sea, these are the Amu Darya and the Syr Darya (Fig. 10). Nearly all of the water from these two rivers was eventually diverted for agriculture after 1960. The drainage basin of these two rivers affects many independent states in this region. These include: Afghanistan, Kazakhstan, Kyrgyzstan, Tadjikistan, and Turkmenistan. The lake has been receding since 1960. Since then, the lake surface area has decreased from 64,500 square kilometers to less than 30,000 square kilometers and the lake's salinity tripled (oneworld). The water diverted from these two river systems is used to irrigate cotton fields and rice paddies. The efficiency of the irrigation practices is said to be only 40-50 percent.
|Figure 11: Satellite image of Aral Sea, 1976.||Figure 12: Satellite image of Aral Sea, 1997.|
The environment of the area is slowly deteriorating. The use of herbicides, pesticides, and other chemicals in the area is slowly destroying what little remains. During 1970 in Uzbekistan, 44,000 tons of chlorine-organic fertilizers, 6,000 tons of phosphor-organic fertilizers, and 15,400 tons of other chemicals were used on agricultural lands. The drinking water in the area is contaminated with heavy metals, salts, organochlorine pesticides (DDT) and other toxic substances which are causing many health problems. During the last ten to fifteen years kidney and liver diseases have increased 30-40 times, arthritic disease by 60 times, and chronic bronchitis by 30 times (Hinrichsen, 1995).
As the lake declines the climate of the region has been changing. The area is now experiencing more continentality than before. The summers are shorter and hotter, with less precipitation and the winters are longer and colder, with less precipitation. The growing season has also decreased from 200 days to approximately 170 days, which is not suitable for the growing of cotton (one world). Dust storms are scattering some 75 million metric tons of salt and contaminated dust as far away as Belarus, which is 1000 km northwest (Hinrichsen, 1995).Return to the top.
By reviewing the damages caused to these three inland water bodies by agricultural uses, we can see that remote sensing can play a role in the future monitoring and remediation efforts. Although agriculture is a necessity in many areas, we need to place restrictions on the usage of the water in these areas. Crops grown in these areas should be suited for the climate of the region. Remote sensing should be able to help not only these areas, but other areas across the globe.
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This page was created on 12/11/2001.
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