In this article we will discuss about:- 1. Hydrologic Cycle Reservoirs 2. The Mechanics of the Hydrologic Cycle 3. Alteration.
Hydrologic Cycle Reservoirs:
In its simplest form, the hydrologic cycle is the movement of water throughout the environment, back and forth, from the atmosphere to the earth; essentially, the global recycling of water. Water is held by the atmosphere and released to the earth as precipitation. It infiltrates into the soil, with some losses to evaporation and uptake by plants, and flows as groundwater eventually to surface waters. From the surface waters, principally the lakes and oceans, it is evaporated back to the atmosphere and precipitated again.
As the water goes through this cycle, it undergoes not only the physical changes of state (i.e., gas, liquid, solid), but also chemical changes. Reactions take place with the gases in the atmosphere, the earth it contacts, and the plants it falls upon. In fact, it not only undergoes change itself but causes change by its nature and composition.
Because man’s activities pollute the atmosphere, land, and water, this anthropocentric influence can be profound. If we consider the impact of the stack gases from man’s industrial complex, we see the acidification of rainwater. This in turn can deface our buildings, kill vegetation, and alter the quality of runoff to surface waters to an extent sufficient to permanently change the ecology of the receiving aquatic system.
Water does not remain permanently in any one place but continually moves following the cyclic pattern we have defined as the hydrologic cycle shown in. A more specific hydrologic cycle for Long Island is presented in Figure 2.6.
Places where water “rests” for a while can be called reservoirs or reserves. These would include, but not be limited to, the oceans, lakes, ice caps, and groundwater.
Table 2.1 presents an inventory of the earth’s water reserves. Although the rate of water movement in certain segments of the cycle fluctuates, the total volume of water has been constant for millions of years, perhaps as long as one-half billion years. The origins of water on the earth are not exactly known. The oldest rocks so far identified date back about 3.8 billion years. They include sedimentary rocks which are under water, so we know of water back at least this far in the earth’s history.
All of the water on earth is not moving all of the time, but may be stored in both short- and long-term reservoirs. Water may be stored for centuries in glaciers. Connate water can become trapped in geologic deposits when they are formed and remain there for thousands to millions of years. The Ogallala Aquifer covers an eight-state region in the central United States with water that seeped into the ground more than 3 million years ago.
In comparison to the glacial water and connate reserves, the residence time of water in the atmosphere is relatively short. At any instant, only about 0.005% of the total water in the hydrologic cycle reservoirs may be in motion. Although water is the most abundant substance on earth, it is not equally distributed.
Variations in latitude, rainfall patterns, topography, etc. vary the distribution significantly. Man tends to live near coastal waters and other surface water bodies and does not exhibit an even distribution of population. To accommodate himself and all of his needs, man expends considerable efforts in redistributing water.
In addition, he contaminates the atmosphere and uses ground and surface water for a variety of purposes that result in its degradation. He can also indirectly affect redistribution, for example, of rainfall. In industrial areas, rainfall has increased due to the water droplets condensing more quickly around minute mineral particles in the air from man’s activities. All of the man-made systems and effects function within the larger and very complex hydrologic cycle.
The Mechanics of the Hydrologic Cycle:
The engine behind the water movement of the hydrologic cycle is the sun’s energy and the earth’s gravity.
At the heart of the system are the two processes driven by the sun’s energy:
i. Evaporation and
i. Evaporation:
Of all the sunlight that reaches the surface of the earth (referred to as insolation) less than 1% is used by plants for photosynthesis; about 20% is used in evaporation. Water evaporated from the oceans and the land surface remains in the atmosphere for a relatively short time, averaging about 10 days before being returned to the earth’s surface as precipitation.
More water evaporates from surface water bodies than is returned to them by precipitation. On land there is more precipitation than evaporation. The differential is returned to surface waters via runoff and subsurface discharges or is used by man.
As water is evaporated, it generally is purified with most contaminants being left behind. Some exceptions are DDT and the contaminants related to acid rain. About two-thirds of the precipitation on land is evaporated, the rest can be considered a harvestable or minable resource without causing depletion.
In some areas, land development is restricted based on the available ground water. In Florida, the South Florida Water Management District controls land development by calculation of a “water crop.” Basically this is the open recharge area, multiplied by the net precipitation (annual precipitation minus evaporation and other losses) divided by water demand for that particular location.
The water demand would be based on consumption for a metered customer. This in turn would consist of the gallons per capita per day (GPCD) usage in that geographical location, times the average number of people per dwelling, plus the fire protection needs. The available water crop can then be used to restrict the type of development or control the number of allowable housing units per acre.
The term “evapotranspiration” refers to the total loss of water from vegetation as well as from on or beneath the land surface by its return to the atmosphere as water vapor. The term transpiration is the process by which water stored in plant tissues, e.g., leaves, diffuses through the plant’s membranes and enters the atmosphere as water vapor.
At different times of the year this can be significant enough to alter the groundwater table elevation. One estimate places the volume of water transpired by an acre of corn at 400,000 gallons in a growing season. A more appropriate way to examine the rate at which plants transpire water is by calculation of the “transpiration ratio.” This is the weighted value in pounds of water per pound of crop product, or for natural plants per pound of dry matter. For corn this would be 821 to 1998 with a mean of 1405.
Of all the condensed water vapor or clouds that pass over the United States in the course of a year, only about 10% falls as precipitation. About three times as much precipitation falls over the ocean as compared to the land.
When precipitation occurs the following happens:
(i) It immediately re-evaporates (simultaneous evaporation),
(ii) It falls directly on a water reservoir, e.g., the ocean, or
(iii) It falls on the land-
(a) Becomes runoff,
(b) Infiltrates as soil pore water, or
(c) Percolates to groundwater.
It is important to remember that sediments, nutrients, organic and inorganic contaminants, heat, etc. are constantly and naturally entering receiving water bodies as a consequence of precipitation events. The biotic and abiotic elements of any given ecosystem have the capacity to handle certain amounts of these adverse factors.
This capacity, of course, varies by the type of receiving system, geographic location, time of the year and many other factors. However, if man inputs large amounts of these substances or creates a severe enough condition in a relatively short period of time, the receiving system may not be able to handle the burden. The receiving system could then suffer immediate detrimental effects or ultimately be destroyed. This is really where the term “pollution” applies.
When precipitation falls on land, we should give careful consideration to two special types of terrestrial environments that are worthy of our highest degree of protection.
The first is the “watershed.” Simply stated, it is a waterway’s (e.g., a river) drainage basin. More precisely, it is a topographically defined area of land drained by a single stream or a system of connecting streams, such that the entire outflow is discharged through a single outlet (e.g., a river). Therefore, it is easy to see how protection of this environment is important to us whether the river is a potable water supply or simply is a wildlife habitat or it has some other significant resource value.
Our perceptions of the value of watersheds and wetlands have changed over the years. In 1954, the Watershed Protection and Flood Prevention Act were passed. This authorized the Soil Conservation Service (SCS) to drain wetlands along rivers to make more farmland and decrease flooding. More than 8,000 miles of rivers and streams have been “channelized” as a result.
This has sometimes resulted in disastrous environmental effects and has had questionable results in terms of flood prevention. The loss of habitat on the Kissimmee River in central Florida, and the downstream flood threat and alteration of the Louisiana Delta System due to channelization of the Mississippi are just two examples. Fortunately, efforts to stem the destruction of watersheds and wetlands in a more appropriate manner have been slowing this destructive trend.
The Coastal Zone Management Act (1972), the formation of the National Wildlife Refuge System (1966), the Wild and Scenic Rivers Act (1968), and a whole host of state and local regulations throughout the U.S. have begun to afford protection in a more appropriate way.
The second special environment we want to address here is known as the “recharge zone.” When groundwater is used as a potable water supply, production wells are installed in an “aquifer” or water-bearing geologic strata. Most shallow geologic strata have some water within them. What we commonly refer to as an aquifer are those with suitable “yield” and acceptable quality characteristics.
These aquifers are typically composed of unconsolidated sands and gravels. At some point these strata may be exposed on the land surface or be connected through special geologic features to the surface. Here precipitation can enter or “recharge” the aquifer. An example of this type of environment is the Pine Barrens areas in Suffolk County on Long Island.
In fact, Long Island is divided into eight hydrogeologic zones as first identified in the L.I. Comprehensive Waste Treatment Plan (Nassau-Suffolk Section 208 Plan). Zones I, II, and III are deep-recharge areas, zones IV to VIII are shallow-flow areas. Like watershed areas, it is also easy to see why recharge areas demand special protection.
In some places where the land has been developed and covered with concrete/asphalt roads and parking lots, etc., structures to collect the precipitation that would have percolated into the ground are constructed. They are called recharge basins and you can see them throughout Long Island and elsewhere.
Protection of these two special environments that are part of the hydrologic cycle is very difficult. Efforts to do so must advance on many fronts. The precipitation that is the origin of the water supply condenses and falls through an atmosphere laden with a whole host of problems.
With man polluting the land and air that is part of the hydrologic cycle, we see both direct and indirect impacts on aquatic systems. Precipitation acquiring its air pollutant load falls on ground that is polluted. The acidic rainwater can leach pollutants from the soil down into the subsurface aquifers. Alternately, it can “run off’ the land to surface waters, acquiring the sediments, fertilizers, pesticides, toxins, and metals resulting from man’s past and present activities.
So, as the hydrologic cycle moves water from one reservoir to another, anthropocentric activities degrade its quality. The effects are both acute and chronic. The impact can result not only in damage to the ecosystem but sometimes destruction of life itself, including threatening men.
Alteration of the Hydrologic Reserves:
Demand as the Source of Impact:
The human population is not evenly distributed over the earth. Man tends to congregate in coastal zone areas or around lakes and rivers or other surface water bodies. In New York State, more than half the people who are dependent upon groundwater reside on less than 3% of the total land area.
Water resources are also not evenly distributed over the earth. Some areas are prone to flooding; other areas suffer from chronic or acute shortages. In fact, there probably are sufficient water resources for all man’s needs. However, the problem is that they are not usually present in the quality or quantity where man needs them. As the human population grows, our interaction with the water reserves grows, and the hydrologic cycle is affected both in the quality of the water and its quantity.
In the United States between 1950 and 1985, the withdrawal of water from surface and subsurface supplies increased about 150%, from about 150 to 450 billion gpd as shown in Figure 2.8. Domestic use of water directly by the growing population does not really cause a significant impact on water resources.
However, the industrial and agricultural demands on water resources to support the growing population are very significant. To resolve the quality and quantity problems, man has attempted to manage the water resources in a number of ways that include modifying the structure of surface water bodies (e.g., dams), tapping new underground reserves, and redistribution of supplies over great distances.
All of these efforts have had some ecological impact; in some cases it has been severe.
Man builds dams creating reservoirs for a number of reasons. In some cases it is to equalize supply, in others it is for flood control, and still others for hydroelectric power production. Often multiple purposes and needs are addressed by dam construction.
However, a whole host of environmental and, in some cases, even cultural problems can result. We will list just a few of them here.
Loss of Habitat:
The reservoir created by a dam will flood surrounding lands submerging both naturally evolved ecosystems and displacing plant and animal habitats including even humans. China’s enormous Three Gorges Dam to be constructed on the Yangtze River is estimated to potentially displace two million people and eliminate almost 100,000 acres of farmland. The falling water levels downstream of a dam can also cause loss of habitat that survives based on occasional river flooding. These are some of the terrestrial environment’s most productive areas.
Loss of Silt:
Nutrient-rich silt that would normally have been deposited on river banks will be eliminated.
Potential Increase in Disease-Causing Vectors:
Due to the artificially created habitat, the potential exists to improve the viability of parasites or the hosts of parasites. An example is the blood fluke that causes schistosomiasis, which in Egypt has an infection rate of perhaps 80% of the population living near reservoirs and associated irrigation canals.
Interruption of Life Cycles:
Perhaps one of the most notable impacts of a dam is the inability of migrating species to follow their instinctive spawning cycles. This affects anadromous species like salmon, shad, and some trout. It could affect catadromous species as well, such as the freshwater eel. Fish ladders and other assist devices or structures have had only limited success.
Nitrogen Supersaturation:
It has been found that where a series of dams is in place, atmospheric nitrogen may be driven into the water through the action of the water falling over the spillway. If the supersaturated water eventually flows to a lake, it may have acquired nitrogen to such a level as to kill fish by giving them the “bends.”
Increased Downstream Erosion:
Due to both the altered river level as well as the lack of sediment deposition originating from upstream erosion, erosion downstream is accelerated.
Sediment Buildup:
All lakes and reservoirs behind a dam eventually become filled with sediments and silts that would have been borne by the previously existing flow system. The sediments drop out of suspension in the same way as they would in a treatment plant’s settling tank. Besides being periodically unsightly when exposed, they present a formidable logistic problem for their removal as well as high cost. It was reported that the storage capacity of Lake Austin in Texas was reduced 95% within 13 years due to siltation.
Special Quality and Quantity Problems:
Because reservoirs behind dams have poor circulation and are deep, anoxic conditions can develop. With continuous sediment deposition containing some organic matter, eventually these conditions can result in periodic degradation of the withdrawn or downstream water quality. It also should be noted that quantity can be an issue in some locations. Created by the Aswan Dam, Lake Wasser is estimated to lose 10% of its total water supply annually due to evaporation.
Aesthetics:
We should acknowledge the significant loss of pristine and wild environments. This may also include areas with special historical value or scientific interest.
It is estimated that 65% to 80% of the total water withdrawal for man’s usage comes from surface waters and the remainder from groundwater. However, this resource is very important because its quality can be very high. Also, the human populations living away from potable water distribution systems may rely on potable supply wells for domestic and agricultural use.
Groundwater does provide about half the supply in households in the United States. It is also estimated that 56% of the United States population relies on groundwater as a source of drinking water. This resource requires tremendous efforts in protecting it, monitoring it, and managing it. Some of the more common problems are delineated as follows.
Overdrafting:
Groundwater moves and recharges very slowly and so it can be quite easy to deplete the reserve. When groundwater withdrawal exceeds recharge, there is a net decrease in the available reserve, which is sometimes referred to as groundwater mining. This will result in a depressed water table elevation and a decreased sustainable yield. In some places, this is accompanied by a decrease in quality. In coastal Florida during high summer usage, hydrogen sulfide, for example, can become a very disagreeable side effect.
The Ogallala Aquifer, covering an eight-state region from Texas to South Dakota, contains connate water. The huge pumping of this vast reserve now has many wells running dry with an accompanying loss of valuable farmlands. In Lubbock, Texas, the water elevation has dropped 200 feet since 1940; and in Kansas, some areas as much as 100 feet.
Land Subsidence:
Soil compaction and subsequent land subsidence has resulted from over-drafting, extreme pumping, and the associated lowering of the water table. In Houston, several feet of subsidence has resulted in homes flooding with saltwater from Galveston Bay.
Fifty years of pumping in the San Joaquin Valley has caused the land elevation to drop 30 feet. This problem has been exacerbated because the compact soils won’t facilitate recharge. In Florida, groundwater in Karst formation aquifers has been so severely withdrawn that the ground has collapsed with homes disappearing into “sink holes.”
Saltwater Intrusion:
As groundwater is over-pumped in coastal regions, saltwater begins to push back or intrude into the freshwater aquifer’s domain. Due to the different density and other characteristics, saltwater and freshwater do not mix as readily as you might think. It takes a column of freshwater 41 feet high to hold back a column of 40 feet of salt water as shown in Figure 2.9. Figure 2.10 shows the static balance of salt and fresh waters surrounding and under Long Island.
In the past, water was withdrawn from the shallow Glacial Aquifer, used, and discharged back to the adjacent ground where it could directly recharge. In the second stage, to meet the quality and quantity of a growing population, the Magothy Aquifer was tapped, and without direct recharge to it the saltwater front moved in. In the final stage, with more and more wells, and most water discharged to the sea after treatment, less and less recharging is occurring accompanied by a further expansion inward of the saltwater front.
Contamination:
As man’s population grows, and the industry and agriculture to support it, much pollution and many problems have resulted. The self-cleansing mechanisms of the subsurface system are extremely slow. Additionally, groundwater moves very slowly.
When contamination from leaching of pollutants from the land above the groundwater, septic tanks, illegal dry wells, underground leaking tanks, deep-well injection, illegal dumping, and many other sources can contribute to the degradation and even potential danger to us from groundwater contamination. This problem is enhanced since as a well is pumped it creates a cone of depression that draws the contaminants into it.
Irrigation and the Buildup of Salts:
Water naturally acquires a wide variety of dissolved minerals and substances as it passes through soil and rocks. This includes ions such as Ca, Mg, Na, So4, CI, bicarbonate, and many salts of these ions. As water is withdrawn for irrigation and spread on the land, it is evaporated and transpired, leaving the salts behind. As the process continues, the salts build up to higher and higher concentrations. Eventually, if not controlled, crops will be damaged or die. The situation may become so bad that the land may be unsuitable for farming.
Water with more than 700 ppm of salt cannot be used for agriculture. Additionally, runoff from these lands will pollute surface waters, which will become more and more salty. The Colorado River headwaters have an average salinity less than 50 ppm. By the time the water reaches the Imperial Dam just north of the Mexican border, its salinity is in the range of 750 to 1100 ppm. The downstream salinity of the river resulted in the destruction of Mexico’s richest cotton-growing land.
One of the most dramatic ways in which man can influence the hydrologic cycle is through his efforts to take water from a reserve and move it vast distances to serve his purposes. Many cities, including New York, import water through diversion systems. Some of these systems are large, some are small. Water diversion is based on the concept that with interconnections and integrated system management, adequate supplies of water for future use can be obtained.
However, some systems have been poorly designed or have become so large that they have the potential to cause major environmental impacts. For example, one conceptual plan called for the diversion of water from Alaska to Mexico. So much water has been diverted from the Colorado River, one of our nation’s longest, that it disappears upon arriving in Mexico.
The Southwest Sewer District in Suffolk County, N.Y., has diverted so much freshwater from the Peconic Bay that it has gotten progressively more salty and has caused a loss of the harvesting of shellfish, particularly for bay scallops. Besides the continuous battles between constituencies over their water rights, environmental consequences can be the most important. Diversion projects have resulted in loss of wildlife habitat, destruction of life cycles, pollution, and loss of recreation areas.