Hydrologic Applications: Drought, Snow Cover and Flooding¶
Contents
This entire section has dealt with water - sensing its presence and its activities using spaceborne sensors. Water is ubiquitous - occurring in all the major spheres of the Earth: its outer rock shell; its surface; in its oceans and rivers, in the atmosphere, and in living matter. We close with an examination of some of the practical uses of remote sensing in finding and tracking the movements of water on the land. The first examples examine drought-driven changes in water volume and extent of large lakes. Attention is then focused on water as snow and water in streams. Satellite data are excellent forecasters of floods and imagery is capable of seeing the extent of flooding over wide areas. Both metsat and Landsat type imagery are effective monitors of snow cover and rivers out of their banks - the wide areal coverage suits this kind of use.
Hydrologic Applications: Drought, Snow Cover and Flooding¶
These views of frozen water carry us naturally into the last topic - hydrology - in this Section. In this context, hydrology refers to water distribution and management on land surfaces. We discuss here three of the four principal uses of satellites for hydrologic applications: drought assessment, runoff prediction, and flood monitoring/damage assessment. The fourth topic - drainage basin characterization - we do not consider.
First, several statistics: 1) There are approximately 2 million cubic miles of freshwater throughout the world, mostly in the top 173 m (570 ft) of the crust (both in and on); 2) most freshwater (70%) resides in Antarctic and Greenland ice; 3) there is ~30 times more freshwater as groundwater than in rivers and lakes; and 4) Lake Baikal (see below) has 20% of all lake freshwater. One added fact: water consumption (humans and animals) is about 400 billion gallons/day.
We touched upon the use of space imagery to monitor vegetation stress and other effects of drought on page page 3-4. These tend to be seasonal or short-termed over years, with eventual reversal of the climatic conditions that bring about water shortages which threaten crops and habitats.
As discussed on page 3-4, the North American continent has been experiencing droughts of varied severities in the 1990s into the 21st Century. That occurring in the Summer of 2002 has been the most severe and widepread. Much of the East and a large percentage of the West were most seriously affected. One consequence: a large number of wildfires burning over 4 million acres. This image shows satellite microwave measurements converted into surface wetness across the U.S. for a 6 day span in August. This does not disclose the full extent of the drought (see page 3-4) because it reflects rainfall over just one week but it does help to assess the patterns of soil moisture that will influence longer term conditions that contribute to the water deficit underlying the on-going drought. For this week, the rain shortfall has been greatest in the eastern half of the country but in prior weeks the West has had similar deficiencies.
One significant long-term change is the gradual reduction or disappearance of lakes. Lakes tend to be ephemeral when considered at geologic time scales. However, notable shifts in climate - such as is now much talked about in terms of global warming - can sometimes destroy major bodies of water in a century or less. In the U.S., the Great Salt Lake (see Overview) has experienced some fluctuations of size and water level in the 20th Century. Drought often accompanies increased aridity and desertification which pose a threat to lakes that have survived intact since their host areas were first settled. Two drastic examples will illustrate this.
The first is at the southern end of the Sahara Desert which now appears to be enlarging at the expense of once vegetated central African lands. Lake Chad, which lies at the triple junction borders of the nations of Chad, Niger, and Nigeria, was once the 6th largest freshwater lake in the world. This AVHRR image locates it within the yellow brackets.
This body of water has been essential to nomadic peoples living around its shores. But its area and volume have been rapidly diminishing over the last half century, as the vegetation line pushes south. This image - actually a photograph taken by astronauts - shows its state in 1963. Below that are two Landsat images - the top acquired in 1973 and the bottom in 1997.
These views almost speak for themselves. In 1973, the southern part of the lake began converting to marshlands. By 1997, almost all of the open waters of Lake Chad had disappeared as both marsh and landfill replaced them. A time-sequential plot of water levels over a 120 year span, made from data processed by the U.S. Geological Survey, shows (unfortunately, with information in red too small to be legible here) the pronounced changes since 1960. Unless there is a miraculous reversal in climate and rainfall, the future for this lake is obvious: Chad has be Had.
A comparable situation faces the Aral Sea in southern Uzbekistan. This so-called sea is actually a freshwater lake. It is seen in this AVHRR image which also contains the larger, somewhat saltier Caspian Sea (the larger body of water near the left edge of the view below) that abuts parts of southern Russia. The Caspian Sea holds the distinction of being the largest lake containing near-freshwater in the world. Lake Baikal in southeastern Siberia is the deepest (1.7 km or 5712 ft).(
The Caspian Sea is slowly losing water. The best indication of this is the Kara-Bogas Gol (Gulf) located on its eastern shore near the center. It is nearly filled in the AVHRR image. But this is now a rapid process. Two Landsat subscenes, in 1972 and 1987, show its progression in recent years, as salt is filling its interior.:
The Aral Sea has lost 50% of its area and 75% of its water volume since 1960. Some of this loss may be due to decrease in annual rainfall in that part of Asia, but the diminution also is caused by diversion of water from rivers feeding this inland lake for use in agriculture elsewhere. This has impacted the means of living by residents around the sea: loss of suitable conditions for cotton growing and great dropoff in fish supply. The changes this has wrought are evident in the next two views made by Russian satellites.
The reduction in water-covered area is obviously matched by a gain in land surface. The next three images, taken in 1973, 1987, and 2000 by Landsats, clearly reveals the growth in land area along a part of the Aral Sea. Note the decrease in vegetation as the region experiences increasing desertification, owing in part to the waning influence in causing rainfall as the Aral water source diminishes.
|Landsat views of changes around part of the Aral Sea betwen 1973 and 2000. |
Further east, along the border between Afghanistan and Iran, the Hamound Wetlands were still in good shape in the early 1970s, with natural vegetation and some cropland benefiting from the water carried in by the Helmand River. The extended drought in that part of the world converted this wetland to a wasteland of mud and salt. These Landsat images show this abrupt change:
Parts of the (48 contiguous) United States has been experiencing significant and worrisome drought conditions since the 1990s, largely caused by reduced rainfall and warmer temperatures. In much of the northeastern region, this drought has become troublesome and serious to severe after the turn of the millenium (the writer [NMS] is directly affected by reduced pressure in his home water as the level in his well has dropped). New York City and the surrounding metropolitan areas have been forced to apply use restrictions as their major sources - reservoirs - have shrunk to as low as 10% capacity by volume. The next pair of images, acquired by the ASTER sensor on NASA’s Terra, shows a reservoir in the Catskills to the northwest on September 18, 2000 and again on February 3, 2002, when the volume has been reduced to 58% of normal.
However, we must determine thickness variations and packing densities from onsite ground measurements, in order to estimate the volume of runoff. This information is important not only for flood warnings and control but also to estimate water supply from reservoir fillings, river channeling, and aqueduct retrieval.
` <>`__14-39: Make an estimate, using the above Landsat image pair, of the amount of increased water one could get from the melting of the 1975 snowpack in the Sierra Nevada compared with 1977, assuming the snow is melted entirely. **ANSWER**
Its damaging effects struck home with the writer very quickly. A combination of ice, followed by more than 50 cm (20 in) of snow, and then a rapid thaw led to basement flooding in his house (located on the side of a hill, away from any floodplain). While the above images didn’t play a role in this instance, it was almost “comforting” to see afterwards how violent this storm was and how vast its snow output.
` <>`__14-40: Below (south) of the edge of the snow cover, the land is shown as green. What happened to it during the storm? `ANSWER <Sect14_answers.html#14-40>`__
The MODIS sensor on Terra caught an unusual snow pattern in the eastern half of the U.S. following a “clipper” storm on December 6, 2002. The snow follows a narrow band that swept rapidly across central Kansas and Missouri and southern Illinois and Indiana. This resulted when cold polar air met warm Gulf air (moisture-laden), making snow, with the front being guided by the jet stream moving eastward.
